SIST ISO 6145-1:2004
(Main)Gas analysis -- Preparation of calibration gas mixtures using dynamic volumetric methods -- Part 1: Methods of calibration
Gas analysis -- Preparation of calibration gas mixtures using dynamic volumetric methods -- Part 1: Methods of calibration
ISO 6145-1:2003 specifies the calibration methods involved in the preparation of gas mixtures by dynamic volumetric techniques. It also gives a brief presentation of a non-exhaustive list of examples of dynamic volumetric techniques which are described in more detail in other parts of ISO 6145.
Analyse des gaz -- Préparation des mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques dynamiques -- Partie 1: Méthodes d'étalonnage
L'ISO 6145-1:2003 sp�cifie les m�thodes d'�talonnage impliqu�es dans la pr�paration des m�langes de gaz par des techniques volum�triques dynamiques. Elle pr�sente �galement bri�vement une liste non exhaustive d'exemples de techniques volum�triques dynamiques d�crites plus en d�tails dans d'autres parties de l'ISO 6145.
Analiza plinov – Priprava kalibrirnih plinskih zmesi z dinamičnimi volumetrijskimi metodami – 1. del: Kalibracijske metode
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INTERNATIONAL ISO
STANDARD 6145-1
Second edition
2003-11-15
Gas analysis — Preparation of calibration
gas mixtures using dynamic volumetric
methods —
Part 1:
Methods of calibration
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage
à l'aide de méthodes volumétriques dynamiques —
Partie 1: Méthodes d'étalonnage
Reference number
ISO 6145-1:2003(E)
©
ISO 2003
---------------------- Page: 1 ----------------------
ISO 6145-1:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved
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ISO 6145-1:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Calibration methods . 2
4.1 General. 2
4.2 Description of primary or potentially primary measuring devices . 4
4.3 Measurements on the final mixture. 12
5 Techniques for preparation of gas mixtures calibrated by the methods described in
Clause 4. 13
5.1 General. 13
[3]
5.2 Volumetric pumps (see ISO 6145-2 ) . 15
[4]
5.3 Continuous injection (see ISO 6145-4 ). 15
[5]
5.4 Capillary (see ISO 6145-5 ). 15
[6]
5.5 Critical orifices (see ISO 6145-6 ). 16
[7]
5.6 Thermal mass flow controllers (see ISO 6145-7 ). 16
[8]
5.7 Diffusion (see ISO 6145-8 ) . 16
[9]
5.8 Saturation (see ISO 6145-9 ). 17
[10]
5.9 Permeation (see ISO 6145-10 ). 17
Annex A (normative) Volume measurement by weighing the water content. 19
Annex B (informative) Description of secondary devices which need calibration against primary
devices . 23
Bibliography . 32
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ISO 6145-1:2003(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6145-1 was prepared by Technical Committee ISO/TC 158, Analysis of gases.
This second edition cancels and replaces the first edition (ISO 6145-1:1986), in which the estimated
uncertainties in the calibration methods and techniques have now been combined in a square-root sum-of-
squares manner to form the relative combined standard uncertainty. In comparison with the previous edition
the periodic injection has been deleted (limited application).
ISO 6145 consists of the following parts, under the general title Gas analysis — Preparation of calibration gas
mixtures using dynamic volumetric methods:
Part 1: Methods of calibration
— Part 2: Volumetric pumps
— Part 4: Continuous injection methods
— Part 5: Capillary calibration devices
— Part 6: Critical orifices
— Part 7: Thermal mass-flow controllers
— Part 9: Saturation method
— Part 10: Permeation method
Diffusion will be the subject of a future Part 8 to ISO 6145. Part 3 to ISO 6145, entitled Periodic injections into
a flowing gas, has been withdrawn.
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ISO 6145-1:2003(E)
Introduction
This part of ISO 6145 is one of a series of standards which describes the various dynamic volumetric methods
used for the preparation of calibration gas mixtures.
In dynamic volumetric methods a gas, A, is introduced at volume or mass flow rate q into a constant flow rate
A
q of a complementary gas B. Gas A can be either a pure calibration component, i, or a mixture of i in A.
B
The volume fraction, ϕ of i in the final calibration gas mixture is given in the following equation:
i,M
q
A
ϕϕ=
ii,M ,A
qq+
AB
where ϕ is the volume or mass fraction of component, i, in the pre-mixed gas A, and is already known from
i,A
its method of preparation. It is assumed that in this equation, ϕ , the concentration of component, i, in gas B,
i,B
is zero.
The introduction of gas A can be continuous (e.g. permeation tube) or pseudo-continuous (e.g. volumetric
pump). A mixing chamber should be inserted in the system before the analyser and is particularly essential in
the case of pseudo-continuous introduction. The flow rate of component A is measured either directly in terms
of volume or mass, or indirectly by measuring the variation of a physical property.
The dynamic volumetric preparation techniques produce a continuous flow rate of calibration gas mixtures into
the analyser but do not generally allow the build-up of a reserve by storage under pressure.
The main techniques used for the preparation of the mixtures are:
a) volumetric pumps;
b) continuous injection;
c) capillary;
d) critical orifices;
e) thermal mass-flow controllers;
f) diffusion;
g) saturation;
h) permeation;
i) electrochemical generation.
In all cases, and most particularly if very dilute mixtures are concerned, the materials used for the apparatus
are chosen as a function of their resistance to corrosion and low absorption capacity (usually glass, PTFE or
stainless steel). It should, however, be pointed out that the phenomena are less important for dynamic
volumetric methods than for static methods.
Numerous variants or combinations of the main techniques can be considered and mixtures of several
constituents can also be prepared by successive operations.
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ISO 6145-1:2003(E)
Some of these techniques allow calculation of the final concentration of the gas mixture from basic physical
information (e.g. mass rates of diffusion, flow through capillaries). However, since all techniques are dynamic
and rely on stable flow rates, this part of ISO 6145 emphasizes calibration of the techniques by measurement
of the individual flow rates or their ratios, or by determination of the composition of the final mixture.
The uncertainty of the composition of the calibration gas mixture is best determined by comparison with a gas
mixture traceable to international standards. Certain of the techniques which may be used to prepare a range
of calibration gas mixtures may require several such traceable gas mixtures to verify their performance over
that range. The dynamic volumetric technique used has a level of uncertainty associated with it. Information
on the final mixture composition depends both on the calibration method and on the preparation technique.
vi © ISO 2003 — All rights reserved
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INTERNATIONAL STANDARD ISO 6145-1:2003(E)
Gas analysis — Preparation of calibration gas mixtures using
dynamic volumetric methods —
Part 1:
Methods of calibration
1 Scope
This part of ISO 6145 specifies the calibration methods involved in the preparation of gas mixtures by dynamic
volumetric techniques. It also gives a brief presentation of a non-exhaustive list of examples of dynamic
volumetric techniques which are described in more detail in other parts of ISO 6145.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6142, Gas analysis — Preparation of calibration gas mixtures — Gravimetric method
ISO 6143, Gas analysis — Comparison methods for determining and checking the composition of calibration
gas mixtures
ISO 7504, Gas analysis — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7504 and the following apply.
3.1
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
NOTE 1 Values of the individual statistical uncertainties found in some methods and techniques in this part of
ISO 6145 are combined with the values of systematic uncertainties that also occur in a square-root sum-of-squares
manner to provide a relative combined uncertainty, or in some cases as a relative expanded uncertainty by application of
the coverage factor “2”.
NOTE 2 In keeping with Reference [1] of the Bibliography, the uncertainty of the composition of a mixture is expressed
as a relative expanded uncertainty.
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ISO 6145-1:2003(E)
4 Calibration methods
4.1 General
4.1.1 The uncertainty in the composition i,M of a component i of a calibration mixture M depends at any
time on
a) the uncertainty of the calibration method,
b) the frequency with which it is applied,
c) the stability of the control devices involved in the dynamic preparation technique.
To assess the uncertainty of the whole procedure, possible instantaneous variations and drift of the principle
parameters of the technique during the calibration procedure shall be considered.
According to the preparation technique for the gas mixtures used, calibration can be carried out by one of the
following methods:
measurement of flow rate (mass or volume);
comparison method;
tracer method;
direct chemical analysis.
Table 1 shows the applicability of each calibration method to the different preparation techniques.
Table 1 — Calibration methods applicable to the preparation techniques
Calibration methods
Preparation techniques
Comparison with Flow rate
a
Tracer Direct analysis
a a
ISO 6143 measurement
Volumetric pumps + — +
Continuous injection + — +
Capillary + + +
May be applicable;
Critical orifice + + +
depends on nature
Thermal mass flow controllers + + +
of components
Diffusion + — —
Saturation + — —
Permeation + — —
a
The pluses refer to the measurement of a volume flow. In principle, flow rate measurement can also be performed for continuous
injection methods, diffusion methods and permeation methods. Here, mass flows are measured rather than volume flows. For diffusion
and permeation tubes the mass flow may be measured continuously using a suspension balance.
4.1.2 In general, the principles of the methods fall into two categories, as follows.
Principles in which the flow rates of component gases are measured either by volume or by mass and in
which the concentration in the final mixture is calculated from the flow rate. Different techniques may be
used for the individual components of a mixture and these may be calibrated by different methods. The
principle of measurements of individual flow rates, however, remains.
Principles which operate directly on the final mixtures.
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ISO 6145-1:2003(E)
Since different principles are involved, they are given separately under each individual method.
Since the calibration methods rely upon different principles and the equipment used for the realization of the
gas flow rates is different, different units can be used to express the contents.
For calibrations using the comparison method, the content is expressed as a mole fraction or mole
concentration because most of the calibration gas mixtures used for the comparison, if possible, are described
in this way.
Using techniques based on volume flow rate leads in the first instance to volume fractions. Recalculation of
these data to mole fractions is possible but leads to an increase in the uncertainty because of the uncertainty
of the density and molar-volume data. In this case, the expression in volume fractions is preferred.
Calibration by the gravimetric method gives mass fractions for the contents of components in gas mixtures.
These can be recalculated to mole fractions by dividing by the respective atomic or molar masses. Expression
in mole fraction is therefore preferred.
Under some circumstances, the total flow rate cannot be taken as the sum of two individual flow rates q and
A
q which have been measured separately. These problems can be caused by deviations from the ideal gas
B
laws or by changes in conditions such as backpressure or viscosity resulting from the blending of the two flow
rates. Deviations from ideal behaviour can be predicted with reasonable accuracy and other uncertainties can
be minimized by careful attention to apparatus design.
4.1.3 Flow rate measurement is normally carried out using one of the following:
a) primary devices, based on absolute principles, for example:
gravimetric method;
b) methods which may be considered as potentially primary when the volume of the device is determined by
weighing the relevant volume of water, or another suitable liquid of higher density:
mercury-sealed piston,
bell-prover;
c) many other devices available for the measurement of volume flow, some of which are listed below
(calibration of these devices is carried out by using one of the above primary or potentially primary
methods):
soap-film meter,
wet-gas meter,
thermal mass flow sensor,
variable area flow meter.
The soap-film and mercury-sealed piston flow meters share a common principle, i.e. that of timing the travel of
a soap bubble or piston between carefully defined points either electronically or by observation, for example
by means of a cathetometer. The volume between these points can be determined by filling with water, which
is subsequently weighed (see Annex A).
The wet-gas meter is an integrating device which indicates the total volume of gas that has been passed
through it (the dry-gas meter, familiar from the domestic environment, has a similar integrating property but
has not been included because it is less accurate). The variable area flow meter is a continuously indicating
device. The thermal mass flow sensor measures mass flow rate as a function of heat flux.
NOTE These devices are fully described in Annex B.
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ISO 6145-1:2003(E)
4.1.4 Calibration of these flow-rate measuring devices is carried out using one of the primary or potentially
primary methods:
a) gravimetric method;
b) mercury-sealed piston;
c) bell prover.
The gravimetric method measures the mass of gas, which has flowed at a constant rate for a defined time
through the device to be calibrated. The mercury-sealed piston drives a defined volume of gas over a
measured time period into the device to be calibrated. The bell prover is a device for creating a constant and
defined flow rate of gas, acting as a mechanically driven gasholder.
The bell prover and the gravimetric method may be used directly, where appropriate, to calibrate the various
preparation techniques, but the information is more commonly transferred via one of the flow-rate measuring
devices.
4.2 Description of primary or potentially primary measuring devices
4.2.1 Gravimetric method
4.2.1.1 Principle
Gas from a cylinder flows at a constant rate through the device to be calibrated. This is continued for a
sufficiently long period for the loss of mass from the cylinder to be accurately measured. The procedure
provides data in terms of mass flow, which can then be converted to molar flow rate or, with assessed
uncertainty, to a volume flow rate.
The gas cylinder and flow-rate measuring device are set up as shown in Figure 1. The cylinder (1) is fitted with
a pressure regulator (2) on the outlet of which a precision needle valve (3) and shut-off valve (4) lead to the
device to calibrated (5). The dead volume between the needle valve outlet and the shutoff valve is minimized
by using the smallest size of tubing and fittings commensurate with the desired gas flow rate. The temperature
and pressure of the gas are measured at the inlet to the device to be calibrated.
The cylinder valve is opened, the pressure regulator is set to a value of, e.g. 100 kPa (1 bar) gauge, and the
needle valve is adjusted to the desired flow rate. When conditions are seen to be steady, the shut-off valve is
closed and the pipe-work is disconnected at the outlet of this valve. The cylinder, regulator, needle valve and
shut-off valve are weighed as a single unit. The pipe-work is reconnected and the shut-off valve is opened to
re-start the flow at the same rate. After the gas has flowed for a period long enough for the mass used to be
measured accurately, the shut-off valve is closed and the cylinder, regulator, needle valve and shut-off valve
weighed as before. During this period, the gas flow is accurately measured by first calculating the volume of
gas from the change in mass, then the flow rate from the volume and the time.
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ISO 6145-1:2003(E)
Key
1 cylinder
2 pressure regulator
3 needle valve
4 shut-off valve
5 device to be calibrated
a
To vent.
Figure 1 — Gravimetric method
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ISO 6145-1:2003(E)
4.2.1.2 Uncertainty of measurement
4.2.1.2.1 Uncertainty of weighing
Gravimetric preparation of mixtures is described in ISO 6142. Using the procedures given in ISO 6142, it can
−4
be assumed that the mass of gas used in a test can be weighed to a relative standard uncertainty of 2 × 10
(i.e. 20 g of gas taken from a 10 kg cylinder whose mass before and after the test can be measured with an
−3
−4
uncertainty of 2 mg, giving a relative standard uncertainty of 22/20 ×10 , i.e. 1,4 × 10 ).
4.2.1.2.2 Uncertainty with unstable flows
This uncertainty can be neglected provided the cylinder and its flow-rate control devices are both pressurized
with gas to the same degree for both weighings. However, when the gas is shut off before weighing, the pipe-
work between the needle valve and the shutoff valve becomes pressurized to the value set on the regulator,
and this will cause a surge when the gas flow rate restarts. The uncertainty caused by this surge is the
amount of gas required to pressurize the volume between the needle valve and the shut-off valve relative to
the amount of gas having flowed. If 2 ml of dead-space is pressurized to 1 bar gauge in a test in which 20 g of
−5
methane flows, the standard uncertainty is 7 × 10 .
To reduce pressure surge effects which can cause oscillations of flow, stabilize the gas flow before taking any
readings. This avoids any uncertainty.
4.2.1.2.3 Uncertainty on conversion of mass to volume
The temperature, pressure, compression (Z) factor and molar mass of the gas, all affect the uncertainty on
conversion of mass to volume. Measurement of temperature with an uncertainty of 0,05 °C and pressure to
−4 −4
10 Pa (0,1 mbar) represents relative standard uncertainties of 1,7 × 10 and 10 , respectively. Compression
−4
factors are commonly quoted to four decimal places, which implies an uncertainty of 10 , and molar masses
are known with sufficient accuracy not to contribute significantly. The relative standard uncertainty is therefore
−4
not greater than 2,2 × 10 .
4.2.1.2.4 Uncertainty due to flow rate variation
If the device to be calibrated measures either instantaneous flow rates or volumes which are small by
comparison with the volume taken from the cylinder, then variations in flow rate are a contribution to the
uncertainty.
A high quality pressure regulator and needle valve should ensure a flow rate constancy of 0,2 % relative, apart
from the initial flow surge (see 4.2.1.2.2), but should be checked for each installation. This level of flow-rate
−3
control represents a relative standard uncertainty of 2 × 10 .
4.2.1.2.5 Uncertainty of time measurement
The time during which the gas flows from the cylinder can be measured by an electronic timer with a relative
−4
standard uncertainty of 2 × 10 .
NOTE The uncertainty of the time measurement generally depends on the discharge time. The timer can be very
accurate, but if "hand" clocking is used to start and stop the timer the uncertainty in the time measurement is in the order
of ± 0,2 s, requiring a 1 000 s discharge time to reach the stated relative uncertainty.
4.2.1.2.6 Relative combined standard uncertainty
The combination of the standard uncertainties described in 4.2.1.2.1 to 4.2.1.2.5 is as follows:
−4
weighing 2 × 10
−5
flow transients 7 × 10
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ISO 6145-1:2003(E)
−4
mass to volume 2,2 × 10
−3
flow rate variation 2 × 10
−4
timing 2 × 10
−3
relative combined standard uncertainty 2,0 × 10
4.2.2 Mercury-sealed piston flow meter
4.2.2.1 Principle
A glass measuring tube (see Figure 2) of known diameter and uniformity is set vertically in an insulated box
fitted with temperature control. The temperature is maintained constant to within ± 0,02 °C.
The measuring tube is divided into a number of sections by photoelectric cells serving as sensors, and the
actual volume between two adjacent photoelectric cells is determined by filling with water and weighing (see
Annex A). Greater accuracy is achieved in the calibration if a liquid of higher density is used.
A constant flow moves a frictionless piston with a constant speed upwards. The displaced volume can be
estimated from the dimensions of the tube or measured with reference to the water calibration.
The piston, made of plastics (e.g. PVC) or glass contains a horizontal, circular groove, filled with mercury. The
purity of the mercury is such as to ensure that the piston does not stick in operation. The use of triple distilled
mercury is recommended.
The piston is allowed to attain a constant speed before time measurement is started at Sensor 1.
Depending on the flow rate and the tube size, time measurement is stopped when the piston passes Sensor 2
or Sensor 3. Sensors may be of the reflection type because of the high reflectance of the mercury ring.
Because of a high back-pressure caused by the weight of the piston, the measured pressure difference is
approximately from 0,1 kPa (1 mbar) up to 1 kPa (10 mbar).
The measuring sequence starts by closing Side A of the 3-way valve (see Figure 2). As soon as the piston
passes Sensor 1, time measurement starts; it stops after the piston passes the next sensor. The three-way
valve resets its position and the piston falls down on the spring. The flow meter is then ready to restart.
4.2.2.2 Uncertainty of measurement
4.2.2.2.1 Influence of temperature variation
−6 −1
The measuring tube is made of borosilicate glass having a coefficient of linear expansion of 3,3 × 10 K .
The result is that, taking into account the control of temperature to ± 0,02 °C, there are relative standard
−7 −5
uncertainties in the volume of the tube of approximately 2 × 10 and in the volume of gas of 7 × 10 .
NOTE The user should be aware that there can be a temperature gradient if flow sensors are heated to operate (e.g.
MFCs) in the upstream system. The expansion effects on glass can be neglected.
4.2.2.2.2 Correction for pressure differences and piston pressure
Correction for pressure differences of the flow device between calibration (p ) and use (p ) is made using
cal use
the factor (p + p ) / p , in which the piston pressure, p , generally takes values between 0,1 kPa
cal piston use piston
and 1 kPa.
Assuming the absolute pressure to be measurable with a relative uncertainty of ± 0,1 %, and the piston
pressure to be measurable with an uncertainty of less than ± 10 Pa, then the relative uncertainty of the
−3
pressure correction is 1,4 × 10 .
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ISO 6145-1:2003(E)
Key
1 photoelectric cell Sensor 2 (first volume)
2 photoelectric cell Sensor 3 (second volume)
3 piston
4 photoelectric cell Sensor 1 (start counting)
5 pressure sensor
6 spring
7 3-way valve (Sides A, B, C)
a
Flow in.
b
To vent.
Figure 2 — Mercury-sealed piston flow meter
4.2.2.2.3 Diffusion across the piston
The construction of the mercury-sealed piston does not provide for the possibility of keeping the same
composition of the gas on both sides. Although diffusion along the mercury seal is still possible, the effect is
considered negligible in general practice.
4.2.2.2.4 Relative combined standard uncertainty
The combination of the standard uncertainties described in parts 4.2.2.2.1. to 4.2.2.2.3 is as follows:
−5
temperature 7 × 10
−3
pressure 1,4 × 10
diffusion across piston 0
−3
relative combined standard uncertainty 1,4 × 10
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ISO 6145-1:2003(E)
4.2.3 Bell prover
4.2.3.1 General
A gas flow measurement shall be provided by displacing a defined volume of gas at constant flow from the
holder of a bell prover within a measured time period.
4.2.3.2 Principle
A schematic diagram of a bell prover is gi
...
SLOVENSKI STANDARD
SIST ISO 6145-1:2004
01-oktober-2004
$QDOL]DSOLQRY±3ULSUDYDNDOLEULUQLKSOLQVNLK]PHVL]GLQDPLþQLPLYROXPHWULMVNLPL
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Gas analysis -- Preparation of calibration gas mixtures using dynamic volumetric
methods -- Part 1: Methods of calibration
Analyse des gaz -- Préparation des mélanges de gaz pour étalonnage à l'aide de
méthodes volumétriques dynamiques -- Partie 1: Méthodes d'étalonnage
Ta slovenski standard je istoveten z: ISO 6145-1:2003
ICS:
71.040.40 Kemijska analiza Chemical analysis
SIST ISO 6145-1:2004 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 6145-1:2004
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SIST ISO 6145-1:2004
INTERNATIONAL ISO
STANDARD 6145-1
Second edition
2003-11-15
Gas analysis — Preparation of calibration
gas mixtures using dynamic volumetric
methods —
Part 1:
Methods of calibration
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage
à l'aide de méthodes volumétriques dynamiques —
Partie 1: Méthodes d'étalonnage
Reference number
ISO 6145-1:2003(E)
©
ISO 2003
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SIST ISO 6145-1:2004
ISO 6145-1:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 6145-1:2004
ISO 6145-1:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Calibration methods . 2
4.1 General. 2
4.2 Description of primary or potentially primary measuring devices . 4
4.3 Measurements on the final mixture. 12
5 Techniques for preparation of gas mixtures calibrated by the methods described in
Clause 4. 13
5.1 General. 13
[3]
5.2 Volumetric pumps (see ISO 6145-2 ) . 15
[4]
5.3 Continuous injection (see ISO 6145-4 ). 15
[5]
5.4 Capillary (see ISO 6145-5 ). 15
[6]
5.5 Critical orifices (see ISO 6145-6 ). 16
[7]
5.6 Thermal mass flow controllers (see ISO 6145-7 ). 16
[8]
5.7 Diffusion (see ISO 6145-8 ) . 16
[9]
5.8 Saturation (see ISO 6145-9 ). 17
[10]
5.9 Permeation (see ISO 6145-10 ). 17
Annex A (normative) Volume measurement by weighing the water content. 19
Annex B (informative) Description of secondary devices which need calibration against primary
devices . 23
Bibliography . 32
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6145-1 was prepared by Technical Committee ISO/TC 158, Analysis of gases.
This second edition cancels and replaces the first edition (ISO 6145-1:1986), in which the estimated
uncertainties in the calibration methods and techniques have now been combined in a square-root sum-of-
squares manner to form the relative combined standard uncertainty. In comparison with the previous edition
the periodic injection has been deleted (limited application).
ISO 6145 consists of the following parts, under the general title Gas analysis — Preparation of calibration gas
mixtures using dynamic volumetric methods:
Part 1: Methods of calibration
— Part 2: Volumetric pumps
— Part 4: Continuous injection methods
— Part 5: Capillary calibration devices
— Part 6: Critical orifices
— Part 7: Thermal mass-flow controllers
— Part 9: Saturation method
— Part 10: Permeation method
Diffusion will be the subject of a future Part 8 to ISO 6145. Part 3 to ISO 6145, entitled Periodic injections into
a flowing gas, has been withdrawn.
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Introduction
This part of ISO 6145 is one of a series of standards which describes the various dynamic volumetric methods
used for the preparation of calibration gas mixtures.
In dynamic volumetric methods a gas, A, is introduced at volume or mass flow rate q into a constant flow rate
A
q of a complementary gas B. Gas A can be either a pure calibration component, i, or a mixture of i in A.
B
The volume fraction, ϕ of i in the final calibration gas mixture is given in the following equation:
i,M
q
A
ϕϕ=
ii,M ,A
qq+
AB
where ϕ is the volume or mass fraction of component, i, in the pre-mixed gas A, and is already known from
i,A
its method of preparation. It is assumed that in this equation, ϕ , the concentration of component, i, in gas B,
i,B
is zero.
The introduction of gas A can be continuous (e.g. permeation tube) or pseudo-continuous (e.g. volumetric
pump). A mixing chamber should be inserted in the system before the analyser and is particularly essential in
the case of pseudo-continuous introduction. The flow rate of component A is measured either directly in terms
of volume or mass, or indirectly by measuring the variation of a physical property.
The dynamic volumetric preparation techniques produce a continuous flow rate of calibration gas mixtures into
the analyser but do not generally allow the build-up of a reserve by storage under pressure.
The main techniques used for the preparation of the mixtures are:
a) volumetric pumps;
b) continuous injection;
c) capillary;
d) critical orifices;
e) thermal mass-flow controllers;
f) diffusion;
g) saturation;
h) permeation;
i) electrochemical generation.
In all cases, and most particularly if very dilute mixtures are concerned, the materials used for the apparatus
are chosen as a function of their resistance to corrosion and low absorption capacity (usually glass, PTFE or
stainless steel). It should, however, be pointed out that the phenomena are less important for dynamic
volumetric methods than for static methods.
Numerous variants or combinations of the main techniques can be considered and mixtures of several
constituents can also be prepared by successive operations.
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Some of these techniques allow calculation of the final concentration of the gas mixture from basic physical
information (e.g. mass rates of diffusion, flow through capillaries). However, since all techniques are dynamic
and rely on stable flow rates, this part of ISO 6145 emphasizes calibration of the techniques by measurement
of the individual flow rates or their ratios, or by determination of the composition of the final mixture.
The uncertainty of the composition of the calibration gas mixture is best determined by comparison with a gas
mixture traceable to international standards. Certain of the techniques which may be used to prepare a range
of calibration gas mixtures may require several such traceable gas mixtures to verify their performance over
that range. The dynamic volumetric technique used has a level of uncertainty associated with it. Information
on the final mixture composition depends both on the calibration method and on the preparation technique.
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SIST ISO 6145-1:2004
INTERNATIONAL STANDARD ISO 6145-1:2003(E)
Gas analysis — Preparation of calibration gas mixtures using
dynamic volumetric methods —
Part 1:
Methods of calibration
1 Scope
This part of ISO 6145 specifies the calibration methods involved in the preparation of gas mixtures by dynamic
volumetric techniques. It also gives a brief presentation of a non-exhaustive list of examples of dynamic
volumetric techniques which are described in more detail in other parts of ISO 6145.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6142, Gas analysis — Preparation of calibration gas mixtures — Gravimetric method
ISO 6143, Gas analysis — Comparison methods for determining and checking the composition of calibration
gas mixtures
ISO 7504, Gas analysis — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7504 and the following apply.
3.1
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
NOTE 1 Values of the individual statistical uncertainties found in some methods and techniques in this part of
ISO 6145 are combined with the values of systematic uncertainties that also occur in a square-root sum-of-squares
manner to provide a relative combined uncertainty, or in some cases as a relative expanded uncertainty by application of
the coverage factor “2”.
NOTE 2 In keeping with Reference [1] of the Bibliography, the uncertainty of the composition of a mixture is expressed
as a relative expanded uncertainty.
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4 Calibration methods
4.1 General
4.1.1 The uncertainty in the composition i,M of a component i of a calibration mixture M depends at any
time on
a) the uncertainty of the calibration method,
b) the frequency with which it is applied,
c) the stability of the control devices involved in the dynamic preparation technique.
To assess the uncertainty of the whole procedure, possible instantaneous variations and drift of the principle
parameters of the technique during the calibration procedure shall be considered.
According to the preparation technique for the gas mixtures used, calibration can be carried out by one of the
following methods:
measurement of flow rate (mass or volume);
comparison method;
tracer method;
direct chemical analysis.
Table 1 shows the applicability of each calibration method to the different preparation techniques.
Table 1 — Calibration methods applicable to the preparation techniques
Calibration methods
Preparation techniques
Comparison with Flow rate
a
Tracer Direct analysis
a a
ISO 6143 measurement
Volumetric pumps + — +
Continuous injection + — +
Capillary + + +
May be applicable;
Critical orifice + + +
depends on nature
Thermal mass flow controllers + + +
of components
Diffusion + — —
Saturation + — —
Permeation + — —
a
The pluses refer to the measurement of a volume flow. In principle, flow rate measurement can also be performed for continuous
injection methods, diffusion methods and permeation methods. Here, mass flows are measured rather than volume flows. For diffusion
and permeation tubes the mass flow may be measured continuously using a suspension balance.
4.1.2 In general, the principles of the methods fall into two categories, as follows.
Principles in which the flow rates of component gases are measured either by volume or by mass and in
which the concentration in the final mixture is calculated from the flow rate. Different techniques may be
used for the individual components of a mixture and these may be calibrated by different methods. The
principle of measurements of individual flow rates, however, remains.
Principles which operate directly on the final mixtures.
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Since different principles are involved, they are given separately under each individual method.
Since the calibration methods rely upon different principles and the equipment used for the realization of the
gas flow rates is different, different units can be used to express the contents.
For calibrations using the comparison method, the content is expressed as a mole fraction or mole
concentration because most of the calibration gas mixtures used for the comparison, if possible, are described
in this way.
Using techniques based on volume flow rate leads in the first instance to volume fractions. Recalculation of
these data to mole fractions is possible but leads to an increase in the uncertainty because of the uncertainty
of the density and molar-volume data. In this case, the expression in volume fractions is preferred.
Calibration by the gravimetric method gives mass fractions for the contents of components in gas mixtures.
These can be recalculated to mole fractions by dividing by the respective atomic or molar masses. Expression
in mole fraction is therefore preferred.
Under some circumstances, the total flow rate cannot be taken as the sum of two individual flow rates q and
A
q which have been measured separately. These problems can be caused by deviations from the ideal gas
B
laws or by changes in conditions such as backpressure or viscosity resulting from the blending of the two flow
rates. Deviations from ideal behaviour can be predicted with reasonable accuracy and other uncertainties can
be minimized by careful attention to apparatus design.
4.1.3 Flow rate measurement is normally carried out using one of the following:
a) primary devices, based on absolute principles, for example:
gravimetric method;
b) methods which may be considered as potentially primary when the volume of the device is determined by
weighing the relevant volume of water, or another suitable liquid of higher density:
mercury-sealed piston,
bell-prover;
c) many other devices available for the measurement of volume flow, some of which are listed below
(calibration of these devices is carried out by using one of the above primary or potentially primary
methods):
soap-film meter,
wet-gas meter,
thermal mass flow sensor,
variable area flow meter.
The soap-film and mercury-sealed piston flow meters share a common principle, i.e. that of timing the travel of
a soap bubble or piston between carefully defined points either electronically or by observation, for example
by means of a cathetometer. The volume between these points can be determined by filling with water, which
is subsequently weighed (see Annex A).
The wet-gas meter is an integrating device which indicates the total volume of gas that has been passed
through it (the dry-gas meter, familiar from the domestic environment, has a similar integrating property but
has not been included because it is less accurate). The variable area flow meter is a continuously indicating
device. The thermal mass flow sensor measures mass flow rate as a function of heat flux.
NOTE These devices are fully described in Annex B.
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4.1.4 Calibration of these flow-rate measuring devices is carried out using one of the primary or potentially
primary methods:
a) gravimetric method;
b) mercury-sealed piston;
c) bell prover.
The gravimetric method measures the mass of gas, which has flowed at a constant rate for a defined time
through the device to be calibrated. The mercury-sealed piston drives a defined volume of gas over a
measured time period into the device to be calibrated. The bell prover is a device for creating a constant and
defined flow rate of gas, acting as a mechanically driven gasholder.
The bell prover and the gravimetric method may be used directly, where appropriate, to calibrate the various
preparation techniques, but the information is more commonly transferred via one of the flow-rate measuring
devices.
4.2 Description of primary or potentially primary measuring devices
4.2.1 Gravimetric method
4.2.1.1 Principle
Gas from a cylinder flows at a constant rate through the device to be calibrated. This is continued for a
sufficiently long period for the loss of mass from the cylinder to be accurately measured. The procedure
provides data in terms of mass flow, which can then be converted to molar flow rate or, with assessed
uncertainty, to a volume flow rate.
The gas cylinder and flow-rate measuring device are set up as shown in Figure 1. The cylinder (1) is fitted with
a pressure regulator (2) on the outlet of which a precision needle valve (3) and shut-off valve (4) lead to the
device to calibrated (5). The dead volume between the needle valve outlet and the shutoff valve is minimized
by using the smallest size of tubing and fittings commensurate with the desired gas flow rate. The temperature
and pressure of the gas are measured at the inlet to the device to be calibrated.
The cylinder valve is opened, the pressure regulator is set to a value of, e.g. 100 kPa (1 bar) gauge, and the
needle valve is adjusted to the desired flow rate. When conditions are seen to be steady, the shut-off valve is
closed and the pipe-work is disconnected at the outlet of this valve. The cylinder, regulator, needle valve and
shut-off valve are weighed as a single unit. The pipe-work is reconnected and the shut-off valve is opened to
re-start the flow at the same rate. After the gas has flowed for a period long enough for the mass used to be
measured accurately, the shut-off valve is closed and the cylinder, regulator, needle valve and shut-off valve
weighed as before. During this period, the gas flow is accurately measured by first calculating the volume of
gas from the change in mass, then the flow rate from the volume and the time.
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Key
1 cylinder
2 pressure regulator
3 needle valve
4 shut-off valve
5 device to be calibrated
a
To vent.
Figure 1 — Gravimetric method
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4.2.1.2 Uncertainty of measurement
4.2.1.2.1 Uncertainty of weighing
Gravimetric preparation of mixtures is described in ISO 6142. Using the procedures given in ISO 6142, it can
−4
be assumed that the mass of gas used in a test can be weighed to a relative standard uncertainty of 2 × 10
(i.e. 20 g of gas taken from a 10 kg cylinder whose mass before and after the test can be measured with an
−3
−4
uncertainty of 2 mg, giving a relative standard uncertainty of 22/20 ×10 , i.e. 1,4 × 10 ).
4.2.1.2.2 Uncertainty with unstable flows
This uncertainty can be neglected provided the cylinder and its flow-rate control devices are both pressurized
with gas to the same degree for both weighings. However, when the gas is shut off before weighing, the pipe-
work between the needle valve and the shutoff valve becomes pressurized to the value set on the regulator,
and this will cause a surge when the gas flow rate restarts. The uncertainty caused by this surge is the
amount of gas required to pressurize the volume between the needle valve and the shut-off valve relative to
the amount of gas having flowed. If 2 ml of dead-space is pressurized to 1 bar gauge in a test in which 20 g of
−5
methane flows, the standard uncertainty is 7 × 10 .
To reduce pressure surge effects which can cause oscillations of flow, stabilize the gas flow before taking any
readings. This avoids any uncertainty.
4.2.1.2.3 Uncertainty on conversion of mass to volume
The temperature, pressure, compression (Z) factor and molar mass of the gas, all affect the uncertainty on
conversion of mass to volume. Measurement of temperature with an uncertainty of 0,05 °C and pressure to
−4 −4
10 Pa (0,1 mbar) represents relative standard uncertainties of 1,7 × 10 and 10 , respectively. Compression
−4
factors are commonly quoted to four decimal places, which implies an uncertainty of 10 , and molar masses
are known with sufficient accuracy not to contribute significantly. The relative standard uncertainty is therefore
−4
not greater than 2,2 × 10 .
4.2.1.2.4 Uncertainty due to flow rate variation
If the device to be calibrated measures either instantaneous flow rates or volumes which are small by
comparison with the volume taken from the cylinder, then variations in flow rate are a contribution to the
uncertainty.
A high quality pressure regulator and needle valve should ensure a flow rate constancy of 0,2 % relative, apart
from the initial flow surge (see 4.2.1.2.2), but should be checked for each installation. This level of flow-rate
−3
control represents a relative standard uncertainty of 2 × 10 .
4.2.1.2.5 Uncertainty of time measurement
The time during which the gas flows from the cylinder can be measured by an electronic timer with a relative
−4
standard uncertainty of 2 × 10 .
NOTE The uncertainty of the time measurement generally depends on the discharge time. The timer can be very
accurate, but if "hand" clocking is used to start and stop the timer the uncertainty in the time measurement is in the order
of ± 0,2 s, requiring a 1 000 s discharge time to reach the stated relative uncertainty.
4.2.1.2.6 Relative combined standard uncertainty
The combination of the standard uncertainties described in 4.2.1.2.1 to 4.2.1.2.5 is as follows:
−4
weighing 2 × 10
−5
flow transients 7 × 10
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−4
mass to volume 2,2 × 10
−3
flow rate variation 2 × 10
−4
timing 2 × 10
−3
relative combined standard uncertainty 2,0 × 10
4.2.2 Mercury-sealed piston flow meter
4.2.2.1 Principle
A glass measuring tube (see Figure 2) of known diameter and uniformity is set vertically in an insulated box
fitted with temperature control. The temperature is maintained constant to within ± 0,02 °C.
The measuring tube is divided into a number of sections by photoelectric cells serving as sensors, and the
actual volume between two adjacent photoelectric cells is determined by filling with water and weighing (see
Annex A). Greater accuracy is achieved in the calibration if a liquid of higher density is used.
A constant flow moves a frictionless piston with a constant speed upwards. The displaced volume can be
estimated from the dimensions of the tube or measured with reference to the water calibration.
The piston, made of plastics (e.g. PVC) or glass contains a horizontal, circular groove, filled with mercury. The
purity of the mercury is such as to ensure that the piston does not stick in operation. The use of triple distilled
mercury is recommended.
The piston is allowed to attain a constant speed before time measurement is started at Sensor 1.
Depending on the flow rate and the tube size, time measurement is stopped when the piston passes Sensor 2
or Sensor 3. Sensors may be of the reflection type because of the high reflectance of the mercury ring.
Because of a high back-pressure caused by the weight of the piston, the measured pressure difference is
approximately from 0,1 kPa (1 mbar) up to 1 kPa (10 mbar).
The measuring sequence starts by closing Side A of the 3-way valve (see Figure 2). As soon as the piston
passes Sensor 1, time measurement starts; it stops after the piston passes the next sensor. The three-way
valve resets its position and the piston falls down on the spring. The flow meter is then ready to restart.
4.2.2.2 Uncertainty of measurement
4.2.2.2.1 Influence of temperature variation
−6 −1
The measuring tube is made of borosilicate glass having a coefficient of linear expansion of 3,3 × 10 K .
The result is that, taking into account the control of temperature to ± 0,02 °C, there are relative standard
−7 −5
uncertainties in the volume of the tube of approximately 2 × 10 and in the volume of gas of 7 × 10 .
NOTE The user should be aware that there can be a temperature gradient if flow sensors are heated to operate (e.g.
MFCs) in the upstream system. The expansion effects on glass can be neglected.
4.2.2.2.2 Correction for pressure differences and piston pressure
Correction for pressure differences of the flow device between calibration (p ) and use (p ) is made using
cal use
the factor (p + p ) / p , in which the piston pressure, p , generally takes values between 0,1 kPa
cal piston use piston
and 1 kPa.
Assuming the absolute pressure to be measurable with a relative uncertainty of ± 0,1 %, and the piston
pressure to be measurable with an uncertainty of less than ± 10 Pa, then the relative uncertainty of the
−3
pressure correction is 1,4 × 10 .
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ISO 6145-1:2003(E)
Key
1 photoelectric cell Sensor 2 (first volume)
2 photoelectric cell Sensor 3 (second volum
...
NORME ISO
INTERNATIONALE 6145-1
Deuxième édition
2003-11-15
Analyse des gaz — Préparation des
mélanges de gaz pour étalonnage à l'aide
de méthodes volumétriques
dynamiques —
Partie 1:
Méthodes d'étalonnage
Gas analysis — Preparation of calibration mixtures using dynamic
volumetric methods —
Part 1: Methods of calibration
Numéro de référence
ISO 6145-1:2003(F)
©
ISO 2003
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ISO 6145-1:2003(F)
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ii © ISO 2003 – Tous droits réservés
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ISO 6145-1:2003(F)
Sommaire Page
Avant-propos. iv
Introduction . v
1 Domaine d'application.1
2 Références normatives .1
3 Termes et définitions.1
4 Méthodes d'étalonnage .2
4.1 Généralités .2
4.2 Description des dispositifs de mesure primaires ou potentiellement primaires.4
4.3 Mesures sur le mélange final.12
5 Techniques de préparation des mélanges de gaz étalonnés par les méthodes décrites
dans l'Article 4 .14
5.1 Généralités .14
[3]
5.2 Pompes volumétriques (voir l'ISO 6145-2 ).15
[4]
5.3 Injection continue (voir l'ISO 6145-4 ).15
[5]
5.4 Capillaire (voir l'ISO 6145-5 ) .15
[6]
5.5 Orifices critiques (voir l'ISO 6145-6 ) .16
[7]
5.6 Régulateurs thermiques de débit-masse (voir l'ISO 6145-7 ).17
[8]
5.7 Diffusion (voir l'ISO 6145-8 ).17
[9]
5.8 Saturation (voir l'ISO 6145-9 ).17
[10]
5.9 Perméation (voir l'ISO 6145-10 ) .18
Annexe A (normative) Mesure du volume par pesée de la teneur en eau.19
Annexe B (informative) Description des dispositifs secondaires à étalonner par rapport
aux dispositifs primaires .23
Bibliographie .32
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ISO 6145-1:2003(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiée
aux comités techniques de l'ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du
comité technique créé à cet effet. Les organisations internationales, gouvernementales et non
gouvernementales, en liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec
la Commission électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI,
Partie 2.
La tâche principale des comités techniques est d'élaborer les Normes internationales. Les projets de Normes
internationales adoptés par les comités techniques sont soumis aux comités membres pour vote. Leur
publication comme Normes internationales requiert l'approbation de 75 % au moins des comités membres
votants.
L'attention est appelée sur le fait que certains des éléments du présent document peuvent faire l'objet de
droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour responsable de ne
pas avoir identifié de tels droits de propriété et averti de leur existence.
L'ISO 6145-1 a été élaborée par le comité technique ISO/TC 158, Analyse des gaz.
Cette deuxième édition annule et remplace la première édition (ISO 6145-1:1986), dans laquelle les
incertitudes évaluées dans les méthodes et les techniques d'étalonnage sont à présent associées par la
racine carrée de la somme des carrés, de manière à former une incertitude type relative combinée. Par
rapport à l'édition précédente, la partie relative aux injections périodiques a été retirée (application limitée).
L'ISO 6145 comprend les parties suivantes, présentées sous le titre général Analyse des gaz — Préparation
des mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques dynamiques:
⎯ Partie 1: Méthodes d'étalonnage
⎯ Partie 2: Pompes volumétriques
⎯ Partie 4: Méthode d'injection continue
⎯ Partie 5: Dispositifs d'étalonnage par capillaires
⎯ Partie 6: Orifices critiques
⎯ Partie 7: Régulateurs thermiques de débit-masse
⎯ Partie 9: Méthode par saturation
⎯ Partie 10: Méthode par perméation
La diffusion fera l'objet d'une future Partie 8 de l'ISO 6145. La Partie 3 de l'ISO 6145, intitulée Injections
périodiques dans un flux gazeux, a été retirée.
iv © ISO 2003 – Tous droits réservés
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ISO 6145-1:2003(F)
Introduction
La présente partie de l'ISO 6145 fait partie d'une série de normes présentant différentes méthodes
volumétriques dynamiques, utilisées pour préparer des mélanges de gaz pour étalonnage.
Dans les méthodes volumétriques dynamiques, un gaz A est introduit au volume ou débit massique, q dans
A
un gaz de complément B à débit constant, q . Le gaz A peut être un constituant pour étalonnage pur, i, ou un
B
mélange de i dans A.
La fraction volumique, ϕ , de i dans le mélange final de gaz pour étalonnage est donnée par l'équation
i,M
suivante:
⎛⎞q
A
ϕϕ=
⎜⎟
ii,M ,A
qq+
⎝⎠AB
où ϕ est la fraction volumique ou massique du composant, i, dans le prémélange de gaz A et est déjà
i,A
connue d’après sa méthode de préparation. Dans cette équation, il est admis que, ϕ , la concentration du
i,B
composant, i, dans le gaz B, est nulle.
L'introduction du gaz A peut être continue (par tube à perméation, par exemple) ou pseudo-continue (pompe
volumétrique, par exemple). Il convient d'insérer une chambre de mélange dans le système, avant l'analyseur.
Elle est particulièrement importante dans le cas d'une introduction pseudo-continue. Le débit du composant A
est mesuré directement en termes de volume ou de masse, ou indirectement par la mesure de la variation
d'une propriété physique.
Les techniques de préparation volumétriques dynamiques produisent un débit continu de mélanges de gaz
pour étalonnage dans l'analyseur, mais ne permettent en général pas la constitution d'une réserve par
stockage sous pression.
Les principales techniques de préparation des mélanges sont les suivantes:
a) pompes volumétriques;
b) injection continue;
c) capillaire;
d) orifices critiques;
e) régulateurs thermique de débit-masse;
f) diffusion;
g) saturation;
h) perméation;
i) génération électrochimique.
Dans tous les cas, et plus particulièrement si des mélanges très dilués sont concernés, les matériaux de
l'appareillage sont choisis en fonction de leur résistance à la corrosion et de leur faible capacité d'absorption
(du verre, du PTFE ou de l'acier inoxydable, en général). Néanmoins, il convient de souligner que ces
phénomènes ont moins d'importance pour les méthodes volumétriques dynamiques que pour les méthodes
statiques.
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ISO 6145-1:2003(F)
De nombreuses variantes ou combinaisons des principales techniques peuvent être considérées, et des
mélanges de plusieurs constituants peuvent également être préparés par opérations successives.
Certaines de ces techniques permettent de calculer la concentration finale du mélange de gaz à partir de
données physiques de base (les débits massiques de diffusion ou le débit au travers de capillaires, par
exemple). Toutefois, étant donné que toutes les techniques sont dynamiques et reposent sur des débits
stables, la présente partie de l'ISO 6145 met l'accent sur l'étalonnage des techniques par mesurage des
débits individuels ou de leurs rapports, ou par détermination de la composition du mélange final.
La méthode la plus efficace pour déterminer l’incertitude de la composition du mélange de gaz pour
étalonnage est la comparaison avec un mélange de gaz certifié et traçable suivant les normes internationales.
Certaines des techniques qui peuvent être utilisées pour préparer une gamme de mélanges de gaz pour
étalonnage peuvent nécessiter plusieurs mélanges de gaz étalons pour vérifier leur performance sur toute la
gamme. La technique volumétrique dynamique utilisée comporte un niveau d'incertitude. Les informations
relatives à la composition du mélange final dépendent de la méthode d'étalonnage et de la technique de
préparation.
vi © ISO 2003 – Tous droits réservés
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NORME INTERNATIONALE ISO 6145-1:2003(F)
Analyse des gaz — Préparation des mélanges de gaz pour
étalonnage à l'aide de méthodes volumétriques dynamiques —
Partie 1:
Méthodes d'étalonnage
1 Domaine d'application
La présente partie de l'ISO 6145 spécifie les méthodes d'étalonnage impliquées dans la préparation des
mélanges de gaz par des techniques volumétriques dynamiques. Elle présente également brièvement une
liste non exhaustive d'exemples de techniques volumétriques dynamiques, décrites plus en détails dans
d'autres parties de l'ISO 6145.
2 Références normatives
Les documents de référence suivants sont indispensables pour l'application du présent document. Pour les
références datées, seule l'édition citée s'applique. Pour les références non datées, la dernière édition du
document de référence s'applique (y compris les éventuels amendements).
ISO 6142, Analyse des gaz — Préparation des mélanges de gaz pour étalonnage — Méthode gravimétrique
ISO 6143, Analyse des gaz — Méthodes comparatives pour la détermination et la vérification de la
composition des mélanges de gaz pour étalonnage
ISO 7504, Analyse des gaz — Vocabulaire
3 Termes et définitions
Pour les besoins du présent document, les termes et définitions donnés dans l'ISO 7504 ainsi que les
suivants s'appliquent.
3.1
incertitude de mesure
paramètre, associé au résultat d'une mesure, caractérisant la dispersion des valeurs qui pourraient être
raisonnablement attribuées au mesurande
NOTE 1 Les valeurs des incertitudes statistiques individuelles présentes dans certaines méthodes et techniques de la
présente partie de l'ISO 6145 sont combinées aux valeurs des incertitudes systématiques d'une racine carrée de la
somme des carrés pour fournir une incertitude combinée relative ou, dans les cas d'incertitude relative élargie, par
application du coefficient d’élargissement «2».
NOTE 2 À l'instar de la Référence [1] de la Bibliographie, l'incertitude de la composition d'un mélange est exprimée en
tant qu'incertitude relative élargie.
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ISO 6145-1:2003(F)
4 Méthodes d'étalonnage
4.1 Généralités
4.1.1 L'incertitude de la composition i,M du composant i d'un mélange pour étalonnage M dépend souvent
de
a) l'incertitude de la méthode d'étalonnage,
b) la fréquence à laquelle elle est appliquée,
c) la stabilité des dispositifs de contrôle impliqués dans la technique de préparation dynamique.
Pour évaluer l'incertitude de l'ensemble du mode opératoire, les variations instantanées et les dérives
possibles des principaux paramètres de la technique lors de l'étalonnage doivent être prises en compte.
Selon la technique de préparation des mélanges de gaz utilisés, l'étalonnage peut être réalisé au moyen de
l'une des méthodes suivantes:
⎯ mesure du débit (massique ou volumique);
⎯ méthode de comparaison;
⎯ méthode du traceur;
⎯ analyse chimique directe.
Le Tableau 1 illustre l'applicabilité de chaque méthode d'étalonnage en fonction des différentes techniques de
préparation.
Tableau 1 — Méthodes d'étalonnage applicables aux techniques de préparation
Méthodes d'étalonnage
Techniques
de
Comparaison avec
a a
Mesure du débit Traceur Analyse directe
préparation
a
l'ISO 6143
Pompes
+ — +
volumétriques
Injection
+ — +
continue
Capillaire + + +
Orifices
+ + +
Peut être applicable;
critiques
selon la nature des
Régulateurs
composants
thermiques
+ + +
de débit-
masse
Diffusion + — —
Saturation + — —
Perméation + — —
a
Les signes plus (+) font référence à la mesure d'un débit volumique. En principe, la mesure du débit peut également être réalisée
pour les méthodes d'injection continue, les méthodes de diffusion et les méthodes de perméation. Ici, les débits-masses sont mesurés
plutôt que les débits volumiques. Pour les tubes à diffusion et à perméation, le débit-masse peut être mesuré de manière continue à
l'aide d'une balance à suspension.
2 © ISO 2003 – Tous droits réservés
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ISO 6145-1:2003(F)
4.1.2 D'une manière générale, les deux catégories ci-dessous caractérisent les principes des méthodes.
⎯ Principes dans lesquels les débits des composants des gaz sont mesurés par volume ou par masse, et la
concentration dans le mélange final est calculée à partir du débit. Différentes techniques peuvent être
utilisées pour les composants individuels d'un mélange, qui peuvent être étalonnées par différentes
méthodes. Toutefois, le principe des mesures des débits individuels est toujours en vigueur.
⎯ Principes fonctionnant directement sur les mélanges finals.
Étant donné que différents principes sont impliqués, ils sont donnés séparément dans chaque méthode
individuelle.
Étant donné que les méthodes d'étalonnage reposent sur différents principes et que l'appareillage utilisé pour
la réalisation des débits de gaz est différent, les teneurs peuvent être exprimées en unités différentes.
Pour les étalonnages utilisant la méthode de comparaison, la teneur est exprimée en fraction molaire ou en
concentration molaire, car la plupart des mélanges de gaz pour étalonnage, utilisés dans le cadre de la
comparaison, si possible, sont décrits de cette manière.
L'utilisation des techniques reposant sur le débit volumique donne, dans le premier exemple, des fractions
volumiques. Il est possible de recalculer ces données en fractions molaires, mais cela augmente l'incertitude
à cause de l'incertitude des données de masse volumique et des données de volume molaire. Dans ce cas,
l'expression en fractions volumiques est préférée.
L'étalonnage par la méthode gravimétrique permet d'obtenir la fraction massique de chacun des composants
dans les mélanges gazeux. Il est possible de recalculer ces fractions en fractions molaires en divisant par les
masses atomiques ou masses molaires respectives. L'expression en fraction molaire est alors préférée.
Dans certaines circonstances, le débit total ne peut pas être considéré comme la somme de deux débits
individuels q et q , qui ont été mesurés séparément. Ces problèmes peuvent être le résultat d'écarts par
A B
rapport aux lois des gaz parfaits ou de modifications des conditions telles que contre-pression ou viscosité
résultant du mélange de deux débits, par exemple. Il est possible de prévoir assez précisément les écarts par
rapport au comportement parfait, et les autres incertitudes peuvent être limitées en apportant une attention
particulière à la conception de l'appareillage.
4.1.3 La mesure de débit est normalement réalisée par l’une des méthodes suivantes:
a) dispositifs primaires, reposant sur des principes absolus, par exemple:
⎯ méthode gravimétrique;
b) méthodes pouvant être considérées comme potentiellement primaires lorsque le volume du dispositif est
déterminé en pesant le volume d'eau approprié ou de tout autre liquide adéquat de masse volumique
plus élevée:
⎯ piston scellé au mercure,
⎯ clepsydre;
c) de nombreux autres dispositifs disponibles pour mesurer le débit volumique, dont certains sont
répertoriés ci-dessous (ces dispositifs sont étalonnés à l'aide de l'une des méthodes primaires ou
potentiellement primaires ci-dessus):
⎯ débitmètre à film de savon,
⎯ compteur à gaz humide,
⎯ capteurs thermiques de débit-masse,
⎯ débitmètre à flotteur.
© ISO 2003 – Tous droits réservés 3
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ISO 6145-1:2003(F)
Les débitmètres à film de savon et à piston scellé au mercure utilisent un principe commun, celui de mesurer
le temps le déplacement d'une bulle de savon ou d'un piston entre deux points définis avec précaution soit
électroniquement, soit par observation, au moyen d'un cathétomètre, par exemple. Le volume entre ces points
peut être déterminé par remplissage avec de l'eau, qui est ensuite pesée (voir Annexe A).
Le compteur à gaz humide est un dispositif d'intégration indiquant le volume total de gaz qui le traverse (le
compteur à gaz par voie sèche, fréquent en environnement domestique, comporte une propriété d'intégration
analogue, mais il n'a pas été retenu, car moins précis). Le débitmètre à flotteur est un dispositif d'indication
continue. Le capteur thermique de débit-masse mesure le débit massique en fonction d’un flux de chaleur.
NOTE Ces dispositifs sont décrits en détail dans l'Annexe B.
4.1.4 Ces dispositifs de mesure du débit sont étalonnés à l'aide de l'une des méthodes primaires ou
potentiellement primaires suivantes:
a) méthode gravimétrique;
b) piston scellé au mercure;
c) clepsydre.
La méthode gravimétrique permet de mesurer la masse du gaz, passant à débit constant par le dispositif à
étalonner pendant une durée déterminée. Le piston scellé au mercure injecte un volume déterminé de gaz
pendant une durée mesurée dans le dispositif à étalonner. La clepsydre est un dispositif permettant de créer
un débit constant et déterminé de gaz, agissant comme un gazomètre mécanique.
La clepsydre et la méthode gravimétrique peuvent être utilisées directement, le cas échéant, pour étalonner
les diverses techniques de préparation, mais les informations sont plus communément transférées par
l'intermédiaire de l'un des dispositifs de mesure du débit.
4.2 Description des dispositifs de mesure primaires ou potentiellement primaires
4.2.1 Méthode gravimétrique
4.2.1.1 Principe
Le gaz s'écoule d'une bouteille à débit constant au travers du dispositif à étalonner. L'écoulement dure
suffisamment longtemps pour que la perte de masse de la bouteille soit mesurée précisément. Le mode
opératoire fournit des données en termes de débit-masse, qui peuvent ensuite être converties en débit
molaire ou, avec l'incertitude évaluée, en débit volumique.
La bouteille de gaz et le dispositif de mesure du débit sont configurés comme cela est illustré à la Figure 1. La
bouteille (1) est équipée d'un régulateur de pression (2), à la sortie duquel une vanne pointeau de
précision (3) et une vanne d’arrêt (4) sont reliées au dispositif à étalonner (5). Le volume mort entre la vanne
pointeau et la vanne d’arrêt est réduit au minimum par l’utilisation de tubes et de garnitures de diamètre
interne aussi faible que possible, tout en restant compatible avec les débits utilisés. La température et la
pression du gaz sont mesurées à l'entrée du dispositif à étalonner.
La vanne de la bouteille est ouverte, le régulateur de pression réglé à 100 kPa (1 bar), par exemple, et la
vanne pointeau ajustée sur le débit souhaité. Lorsque les conditions sont jugées stables, la vanne d’arrêt est
fermée et la canalisation déconnectée à la sortie de cette vanne. La bouteille, le régulateur, la vanne pointeau
et la vanne d’arrêt sont pesés comme un seul élément. La canalisation est reconnectée et ouverte pour
instaurer le même débit. Une fois le gaz écoulé pendant une période suffisamment longue pour mesurer
précisément la masse utilisée, la vanne d’arrêt est fermée et la bouteille, le régulateur, la vanne pointeau et la
vanne d’arrêt sont pesés dans les mêmes conditions que précédemment. Au cours de cette période, le débit
de gaz est mesuré précisément en calculant en premier lieu le volume de gaz en fonction du changement de
masse, puis le débit en fonction du volume et du temps.
4 © ISO 2003 – Tous droits réservés
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ISO 6145-1:2003(F)
Légende
1 bouteille
2 régulateur de pression
3 vanne pointeau
4 vanne d’arrêt
5 dispositif à étalonner
a
Vers l'évent.
Figure 1 — Méthode gravimétrique
© ISO 2003 – Tous droits réservés 5
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ISO 6145-1:2003(F)
4.2.1.2 Incertitude de mesure
4.2.1.2.1 Incertitude de pesage
La préparation gravimétrique des mélanges est décrite dans l'ISO 6142. Selon les modes opératoires de
l’ISO 6142, il est possible de supposer que la masse de gaz utilisée dans un essai peut être pesée en fonction
−4
d'une incertitude type relative de 2 × 10 (c'est-à-dire 20 g de gaz prélevé d'une bouteille de 10 kg dont la
masse avant et après l'essai peut être mesurée avec une incertitude de 2 mg, donnant une incertitude type
−3
−4
relative de 22/20 ×10 , c'est-à-dire 1,4 × 10 ).
4.2.1.2.2 Incertitude des flux instables
Cette incertitude peut être ignorée, à condition que la bouteille et ses dispositifs de contrôle du débit soient
sous des conditions de pression de gaz identiques au même degré pour les deux pesées. Toutefois, si le
débit de gaz est stoppé avant pesée, la canalisation entre la vanne pointeau et la vanne d’arrêt se trouve mise
à la pression définie par le régulateur; cela peut créer un à-coup de surpression à la reprise du débit de gaz.
L'incertitude provoquée par cette surpression est la quantité de gaz nécessaire à la pressurisation du volume
entre la vanne pointeau et la vanne d’arrêt par rapport à la quantité de gaz écoulé. Si 2 ml d'espace libre sont
mis sous pression à 1 bar dans un essai au cours duquel 20 g de méthane s'écoulent, l'incertitude type est de
−5
7 × 10 .
Pour réduire les effets de la surpression, qui peuvent être à l'origine d'oscillations du flux, stabiliser le débit de
gaz avant de procéder aux lectures. Cela permet d'éviter les incertitudes.
4.2.1.2.3 Incertitude de conversion de la masse en volume
La température, la pression, le coefficient de compression (Z) et la masse molaire du gaz ont un impact sur
l'incertitude de conversion de la masse en volume. La mesure de la température avec une incertitude de
0,05 °C et de la pression jusqu'à 10 Pa (0,1 mbar) représente respectivement des incertitudes types relatives
−4 −4
de 1,7 × 10 et de 10 . Les coefficients de compression sont habituellement indiqués à quatre décimales, ce
−4
qui implique une incertitude de 10 , et les masses molaires sont connues avec une exactitude suffisante
−4
pour ne pas avoir d’impact significatif. L'incertitude type relative n'est donc pas supérieure à 2,2 × 10 .
4.2.1.2.4 Incertitude due à la variation du débit
Si le dispositif à étalonner mesure de petits débits ou volumes instantanés comparés au volume prélevé de la
bouteille, les variations du débit contribuent à l'incertitude.
La qualité du régulateur de pression et celle de la vanne pointeau doivent garantir une précision du débit à
0,2 %, outre la surpression du débit initial (voir 4.2.1.2.2), mais il convient de les contrôler pour chaque
−3
installation. Ce niveau de contrôle du débit représente une incertitude type relative 2 × 10 .
4.2.1.2.5 Incertitude de mesure du temps
La durée pendant laquelle le gaz s’écoule à partir de la bouteille peut être mesurée à l'aide d'un chronomètre
−4
électronique, avec une incertitude type relative de 2 × 10 .
NOTE L'incertitude de la mesure du temps dépend généralement de la durée de déchargement. Le chronomètre
peut être très précis, mais si le déclenchement et l’arrêt sont manuels, l'incertitude de mesure du temps est de l'ordre de
± 0,2 s, ce qui nécessite un temps de déchargement de 1 000 s pour atteindre l'incertitude type stipulée.
4.2.1.2.6 Incertitude type relative combinée
La combinaison des incertitudes types décrites de 4.2.1.2.1 à 4.2.1.2.5 se présente comme suit:
−4
⎯ pesage 2 × 10
−5
⎯ transitoires de flux 7 × 10
6 © ISO 2003 – Tous droits réservés
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ISO 6145-1:2003(F)
−4
⎯ masse en volume 2,2 × 10
−3
⎯ variation du débit 2 × 10
−4
⎯ chronométrage 2 × 10
−3
⎯ incertitude type relative combinée 2,0 × 10
4.2.2 Débitmètre à piston scellé au mercure
4.2.2.1 Principe
Un tube de mesure en verre (voir Figure 2) de diamètre et d'uniformité connus est placé verticalement dans
une boîte isotherme avec régulation de température. La température reste constante à ± 0,02 °C près.
Légende
1 cellule photo-électrique capteur 2 (premier volume) 5 capteur de pression
2 cellule photo-électrique capteur 3 (second volume) 6 ressort
3 piston 7 valve à 3 voies (côtés A, B, C)
4 cellule photo-électrique capteur 1 (début du comptage)
a
Flux entrant.
b
Vers l'évent.
Figure 2 — Débitmètre à piston scellé au mercure
Le tube de mesure est divisé en un certain nombre de sections par des cellules photo-électriques faisant
office de capteurs, et le volume réel entre deux cellules photoélectriques adjacentes est déterminé par
remplissage avec de l'eau, puis pesage (voir Annexe A). Il est possible d'obtenir une plus grande exactitude
de l'étalonnage à l'aide d'un liquide de masse volumique plus élevée.
© ISO 2003 – Tous droits réservés 7
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ISO 6145-1:2003(F)
Un débit constant déplace un piston sans frottement vers le haut à vitesse constante. Le volume déplacé peut
être évalué à partir des dimensions du tube ou mesuré par rapport à l'étalonnage à l'eau.
Le piston, en plastique (du PVC, par exemple) ou en verre, comporte une rainure horizontale et circulaire
remplie de mercure. La pureté du mercure est telle que le piston ne colle pas en cours de fonctionnement. Il
est recommandé d'utiliser du mercure distillé trois fois.
On laisse le piston atteindre une vitesse constante avant que la mesure du temps ne soit établie au niveau du
capteur 1.
Selon le débit et la dimension du tube, la mesure du temps est interrompue lorsque le piston passe le
capteur 2 ou 3. Il peut s'agir de capteurs à réflexion, car l'anneau de mercure offre un facteur de réflexion
élevé
...
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