Gas analysis — Preparation of calibration gas mixtures using dynamic methods — Part 2: Piston pumps

ISO 6145 comprises a series of International Standards dealing with various dynamic methods used for the preparation of calibration gas mixtures. ISO 6145-2:2014 describes a method and preparation system using piston pumps. The mixture composition and its associated uncertainty are based on calibration of the piston pumps by dimensional measurements. The calibration gas mixtures prepared using this method consist of two or more components, prepared from pure gases or other gas mixtures using gas-mixing pumps. Such gas-mixing pumps contain at least two piston pumps, each driven with a defined ratio of strokes, and appropriate accessories for gas feeding and mixture homogenization. ISO 6145-2:2014 is applicable only to mixtures of gaseous or totally vaporized components including corrosive gases, as long as these components neither react with each other nor with the wetted surfaces of the mixing pump. The use of gas mixtures as parent gases is covered as well. Multi-component gas mixtures and multi-step dilution procedures are included in this International Standard as they are considered to be special cases of the preparation of two-component mixtures. ISO 6145-2:2014 describes a method of preparing calibration gas mixtures whose composition is expressed in volume fractions. The necessary equations and associated uncertainty evaluation to express the gas composition in amount?of?substance fractions are given in Annex A. With this method, provided that sufficient quality assurance and control measures are taken, calibration gas mixtures can be prepared with a relative expanded uncertainty of 0,5 % (coverage factor k = 2) in the volume fraction. Numerical examples showing that under specified conditions smaller uncertainties are attainable are given in Annexes B through D. Using this method, dilution ratios of 1:10 000 can be achieved in discrete increments. Lower fractions (down to 1 × 10−8) can be achieved by multi-stage dilution or by the use of gas mixtures as input gases. Final mixture flow rates of 5 l/h to 500 l/h can be realized depending on the equipment used.

Analyse des gaz — Préparation des mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques dynamiques — Partie 2: Pompes à piston

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Status
Published
Publication Date
17-Jul-2014
Current Stage
9093 - International Standard confirmed
Completion Date
17-Dec-2019
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INTERNATIONAL ISO
STANDARD 6145-2
Second edition
2014-08-15
Gas analysis — Preparation of
calibration gas mixtures using
dynamic methods —
Part 2:
Piston pumps
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage
à l’aide de méthodes volumétriques dynamiques —
Partie 2: Pompes à piston
Reference number
ISO 6145-2:2014(E)
©
ISO 2014

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ISO 6145-2:2014(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
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Published in Switzerland
ii © ISO 2014 – All rights reserved

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ISO 6145-2:2014(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 2
5 Principle and equipment . 3
5.1 Principle . 3
5.2 Equipment . 3
6 Calibration gas mixture preparation . 5
6.1 Safety issues . 5
6.2 Mixture feasibility . 6
6.3 Preparation system and setting-up of mixture composition . 7
6.4 Input pressure control . 7
6.5 Temperature control . 7
6.6 Homogenization . 7
6.7 Stability . 8
6.8 Output pressure and flow pulsation . 8
6.9 Composition of the parent gases . 8
7 Calculation of volume fractions and associated uncertainty evaluation .9
7.1 Calculation method A . 9
7.2 Calculation method B .10
8 Gas mixture composition verification .12
Annex A (normative) Amount-of-substance fractions .13
Annex B (informative) Uncertainty evaluation of the gas mixture composition .15
Annex C (informative) Gas mixture verification .21
Annex D (informative) Numerical example .25
Bibliography
.30
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ISO 6145-2:2014(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2. www.iso.org/directives
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. Details of any
patent rights identified during the development of the document will be in the Introduction and/or on
the ISO list of patent declarations received. www.iso.org/patents
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 158, Analysis of gases.
This second edition cancels and replaces the first edition (ISO 6145-2:2001), which has been technically
revised. The main objective of this revision is to extend the first edition for calculating the composition
in volume and amount–of–substance fractions from the displacement volumes of piston pumps.
Appropriate measurement functions and guidance on uncertainty evaluation are given for the mixing of
real gases at unequal operational conditions of the piston pumps.
ISO 6145 consists of the following parts, under the general title Gas analysis — Preparation of calibration
gas mixtures using dynamic methods:
— Part 1: Methods of calibration
— Part 2: Volumetric pumps
— Part 4: Continuous syringe injection method
— Part 5: Capillary calibration devices
— Part 6: Critical orifices
— Part 7: Thermal mass-flow controllers
— Part 8: Diffusion method
— Part 9: Saturation method
— Part 10: Permeation method
— Part 11: Electrochemical generation
ISO 6145-3, entitled Periodic injections into a flowing gas stream, has been withdrawn.
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INTERNATIONAL STANDARD ISO 6145-2:2014(E)
Gas analysis — Preparation of calibration gas mixtures
using dynamic methods —
Part 2:
Piston pumps
1 Scope
ISO 6145 comprises a series of International Standards dealing with various dynamic methods used for
the preparation of calibration gas mixtures. This part of ISO 6145 describes a method and preparation
system using piston pumps. The mixture composition and its associated uncertainty are based on
calibration of the piston pumps by dimensional measurements.
The calibration gas mixtures prepared using this method consist of two or more components, prepared
from pure gases or other gas mixtures using gas-mixing pumps. Such gas-mixing pumps contain at
least two piston pumps, each driven with a defined ratio of strokes, and appropriate accessories for gas
feeding and mixture homogenization.
This part of ISO 6145 is applicable only to mixtures of gaseous or totally vaporized components including
corrosive gases, as long as these components neither react with each other nor with the wetted surfaces
of the mixing pump. The use of gas mixtures as parent gases is covered as well. Multi-component gas
mixtures and multi-step dilution procedures are included in this International Standard as they are
considered to be special cases of the preparation of two-component mixtures.
This part of ISO 6145 describes a method of preparing calibration gas mixtures whose composition
is expressed in volume fractions. The necessary equations and associated uncertainty evaluation to
express the gas composition in amount–of–substance fractions are given in Annex A.
With this method, provided that sufficient quality assurance and control measures are taken, calibration
gas mixtures can be prepared with a relative expanded uncertainty of 0,5 % (coverage factor k = 2) in
the volume fraction. Numerical examples showing that under specified conditions smaller uncertainties
are attainable are given in Annexes B through D.
Using this method, dilution ratios of 1:10 000 can be achieved in discrete increments. Lower fractions
−8
(down to 1 × 10 ) can be achieved by multi-stage dilution or by the use of gas mixtures as input gases.
Final mixture flow rates of 5 l/h to 500 l/h can be realized depending on the equipment used.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 7504, Gas analysis — Vocabulary
ISO 14912, Gas analysis — Conversion of gas mixture composition data
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
ISO/IEC Guide 99, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
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ISO 6145-2:2014(E)

3 Terms and definitions
For the purposes of this document, terms and definitions given in ISO/IEC Guide 99, ISO/IEC Guide 98-3,
ISO 14912, ISO 7504, and the following apply.
3.1
operational conditions
pressure and temperature in the piston pumps at which the gas mixture is prepared
3.2
parent gas
pure gas or gas mixture used for preparation of a gas mixture
3.3
piston pump
gas forwarding system comprising cylinder, piston, steering plate, and eccentric driving disk mounted
on a common plate
3.4
reduction gear ratio
quotient of the number of strokes and the maximum number of strokes of the piston pump that can be
set in distinct steps by the switch gear
3.5
reference conditions
pressure and temperature to which volume fractions refer
3.6
stroke volume
forwarding geometric displacement volume per stroke of a piston pump
4 Symbols
Symbol Quantity Unit
−1
B’ second virial coefficient (virial equation–of–state in pressure) Pa
i index of a component
k,l index of a piston pump; index of a parent gas
L gear ratio
N number of strokes (in a given period of time)
N maximum number of strokes (in a given period of time)
max
n amount–of–substance mol
n total amount–of–substance of a mixture mol
mix
p pressure Pa
−1 −1
R ideal gas constant J mol K
T temperature K
u standard uncertainty
3
V gas volume m
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ISO 6145-2:2014(E)

Symbol Quantity Unit
3
V stroke volume m
geo
x amount–of–substance fraction (of a component in a parent gas) 1
y amount–of–substance fraction (of a component in the prepared gas mixture) 1
Z compressibility 1
ϕ volume fraction (of a component in the prepared gas mixture) 1
φ volume fraction (of a component in a parent gas) 1
5 Principle and equipment
5.1 Principle
The principle of the dynamic preparation method described in this part of ISO 6145 is based on the
displacement volume of piston pumps forwarding defined gas portions that are continuously merged and
homogenized for obtaining the required gas mixture. For pure gases, the volume fraction of component
i in the prepared gas mixture is approximately equal to the volume of component i divided by total
volume of all components, as given by Formula (1):
NV⋅
igeo,i
ϕ (1)

i
NV⋅
∑ kgeo,k
k
where ϕ denotes the volume fraction of component i at the operational conditions of the piston pumps.
i
These conditions may differ from the conditions at which the calibration gas mixture thus prepared is
going to be used.
The calculation of volume fractions is described in Clause 7, in two variants. Method A requires the
prepared gas mixture to be used at the operational conditions (7.1), whereas method B covers the
expression of the volume fractions at reference conditions (7.2). Depending on the situation, one of these
methods shall be used in applications where volume fractions are needed.
In applications, where amount–of–substance fractions are needed, these shall be calculated directly
from the displacement volumes. The necessary expressions and associated uncertainty evaluation are
given in Annex A.
5.2 Equipment
Calibration gas mixtures with defined composition are prepared using gas-mixing pumps containing
two or more piston pumps, pneumatically separated from each other. A common motor drives the piston
pumps via separate gear trains and individual switch gears. The number of strokes of the individual
piston pumps is defined by preset reduction gear ratios. The gas portions forwarded by each of the
piston pumps are quantified by the stroke volume V and by their individual number of strokes N
geo,k k
(see Figure 1).
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ISO 6145-2:2014(E)

Key
d diameter of cylinder
h height of piston stroke
Figure 1 — Principle of a piston pump
To achieve the required calibration gas mixture, separately forwarded gas portions are merged and
homogenized. Since the stroke volume V is constant, different gas compositions are prepared only
geo,k
by variation of the number of strokes N . The use of both quantities is sufficient for the calculation of
k
mixture composition when using pure gases.
The stroke volume of piston pump k is calculated from the diameter of its cylinder and the height of its
piston stroke
π
2
Vd=⋅ ⋅h (2)
geok, kk
4
The forwarded gas volume is usually given as a whole number times the stroke volume. The number of
strokes can be chosen in order to achieve the desired mixing ratio. The gear ratio L relates the stroke
k
number to the maximum number of strokes
NL=⋅N (3)
kk max
An example of realization of the described method is shown in Figure 2 for a gas-mixing pump comprising
two piston pumps 1 and 2 of the same size. Both piston pumps are driven by a common electrical motor,
key 8, via defined gear trains with switch gears key 2 and key 4, respectively. Gases 1 and 2 are fed to
the piston pumps via gas inlets figure footnotes a and b, respectively. Bubbling vessels (key 5) at the
gas inlets are used to control the input pressure to the piston pumps and to adjust a small excess gas
flow which is vented at ambient pressure. The temperature of each piston pump can be measured with
temperature sensors T and T that shall be integrated into the body of the piston pumps. The gases
1 2
forwarded by the piston pumps are merged and homogenized in mixing vessel (key 6). The final gas
mixture is provided at gas outlet (key 7) to the intended application. Details of the implementation
of temperature control of piston pumps and parent gases attaining reduced uncertainties is given in
Annex B.
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ISO 6145-2:2014(E)

Gas 1
+ Gas 2
2 4
1 2
7
8
1 3 6
T T
1 2
p p
2
5 1 5
a Gas 1 b
Gas 2
Key
p pressure, piston pump 1 4 switch gears 2
1
p pressure, piston pump 2 5 bubbling vessel
2
T temperature sensor, piston pump 1 6 mixing vessel
1
T temperature sensor, piston pump 2 7 gas outlet for gas mixture
2
1 piston pump 1 8 drive motor
a
2 switch gears 1 Gas inlet for gas 1.
b
3 piston pump 2 Gas inlet for gas 2.
NOTE Piston pumps 1 and 2 as shown in Figure 1.
Figure 2 — Example of realization for the dynamic preparation of two-component calibration
gas mixtures
6 Calibration gas mixture preparation
6.1 Safety issues
The possibility of dangerous reactions, such as explosions (e.g. mixtures containing flammable gases
and oxygen) or strongly exothermic polymerisations (e.g. hydrogen cyanide) and decompositions (e.g.
acetylene), shall be excluded for safety reasons. If there is the possibility of formation of hazardous
gas mixtures, all appropriate safety precautions shall be applied. Information on dangerous reactions
and dangerous combinations that shall be excluded for safety reasons is provided in dangerous goods
[1]
regulations and in gas supplier handbooks.
Safe discharge of toxic or flammable gases and gas mixtures shall be ensured. Contact with ignition
sources shall be avoided, if merging of the parent gases can form flammable mixtures. Short–term
concentration peaks can occur when the composition is changed.
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ISO 6145-2:2014(E)

Precautions shall be taken during feeding the parent gases to the piston pumps, handling the
intermediate gas mixtures and final mixtures. The compliance with applicable safety instructions of
the mixing pumps, pressurization pumps, and filling reservoirs shall be confirmed before beginning the
preparation.
6.2 Mixture feasibility
The choice of appropriate set-up and suitable procedure for dynamic preparation of gas mixtures can be
a complex procedure. At first, all requirements concerning the intended application of the prepared gas
mixture shall be defined. Then, the properties of available gases and gas mixtures, possible reactions
between gas components and the wetted material of the pumps and peripherals, and the purity and
impurities of the mixed gases shall be considered. Further, the characteristics of the applied gas-mixing
pumps and the blending method shall be considered.
The following phenomena shall be taken into account when considering the feasibility of preparing the
required gas mixture:
a) reactions between mixture components;
b) reactions with piston pump, pressurization pump, and container material;
c) reactions with elastomers and greases (e.g. in the piston pumps, the pressurization pump, the valve
seat and seals).
Reactions with elastomers and greases should be prevented by using only materials that are inert to
all components of the mixture. If this is not possible, measures should be taken to minimize corrosive
attack on the materials with which the gases will make contact such that there is no significant effect on
mixture composition and no danger in storage and use.
When choosing a suitable preparation procedure, a number of considerations should be made to ensure
that the most appropriate method is used. The following parameters should be considered:
a) number of components in the final mixture;
b) range of fractions of each component of the final mixture;
c) flow rate of the final mixture;
d) flow rate of the parent gases;
e) established composition of each parent gas mixture used;
f) blending method: parallel method — serial method — multiple dilution;
g) mechanical characteristics of the piston pumps to be used;
h) performance characteristics of the mixing pump to be used;
i) pressure to which the final gas mixture has to be delivered;
j) characteristics of the pressurization pump (if necessary);
k) possibility of condensation;
l) requirements on the preparation tolerance.
Using Formula (1), the target composition can be calculated and a preparation procedure selected.
The final mixture composition is calculated using the expressions given in Clause 7 for volume fractions
and Annex A for amount–of–substance fractions.
In principle, it is recommended to use gas mixing pumps at those operating conditions where the
influence of the sources listed in Table B.1 can be considered not significant. If this is not possible, the
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ISO 6145-2:2014(E)

corresponding influences have to be considered by an appropriate uncertainty contribution. Table B.1
and further information about potential sources of uncertainty are listed in Annex B. An example for a
set-up with reduced contribution of uncertainty of temperature is given in B.4.
6.3 Preparation system and setting-up of mixture composition
Examples for the setup of a complete system for the dynamic preparation of calibration gas mixtures
according to the volumetric method described in this part of ISO 6145 are shown schematically in
Figure 2, and in B.4 for high-end applications (i.e. applications with reduced uncertainty).
Gas containing the component(s) of interest and matrix gases shall be fed to piston pumps with slight
excess gas flow at a pressure slightly above ambient pressure and at constant temperature (preferably
at ambient temperature). The requirement of gas intake at the desired pressure can be met by use of
bubblers in a by-pass flow at the gas inlets. This excess gas flow also inhibits the leakage of air into the
pump.
Before starting preparation of calibration gas mixtures, the entire flow system external to the mixing
pump itself shall be checked for leak tightness and contaminations of gas conduits.
Gas portions forwarded by piston pumps are preset by positioning the reduction gear ratio in a way that
the target composition of the calibration gas mixture is obtained as described in Clause 7.
NOTE Expressions for the calibration gas mixture in terms of amount–of–substance fractions are given in
Annex A.
6.4 Input pressure control
Elimination of differences between input pressures of piston pumps is highly relevant for the performance
of the method. Pressures at the inputs of the gas-mixing pump shall be maintained as close as possible
to the same values for all pistons. For this purpose, the level of sealing liquid in the bubblers shall be the
same.
The pressure of gases taken from gas cylinders shall be reduced to the appropriate pressure using 1- or
2-stage pressure reducers. The use of precisely adjustable needle valves or diaphragm pressure reducing
valves is recommended to reduce the gas consumption. The regulators shall be directly connected to
the gas inputs of the gas-mixing pump. A slight excess of gas is conducted to ambient pressure via a by-
pass that is equipped with a bubbling vessel. The use of appropriate bubbling vessels allows reducing
pressure difference in the pumps to 10 Pa or less between the gas inlets.
6.5 Temperature control
Elimination of temperature differences between the piston pumps is highly relevant for the performance
of the method. The exposure of the mixing pump to all kinds of radiation and air flow shall be avoided.
The Joule-Thomson effect of expanding gases due to pressure reduction or evaporation in case of liquids
shall be minimized. If large gas volumes from compressed gas-cylinders are used, reheating of the gases
can be necessary.
For optimal performance, it is recommended to measure the temperatures using calibrated temperature
sensors using calibrated temperature sensors that are introduced into the sockets integrated into each
piston pump and to take temperature differences into account as necessary.
NOTE The operation of the piston pumps and the gas feed at a constant temperature reduces the uncertainty
associated with the composition, an example of its realization is given in B.4.
6.6 Homogenization
The gases forwarded by the piston pumps shall be merged and homogenized in a flow process. For this
purpose, the gas mixture is continuously conducted through one or more appropriate mixing vessels.
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ISO 6145-2:2014(E)

The design and volume of the mixing vessels shall be adapted to the stroke volume of the piston pumps,
their capacities being preferably 10 to 15 times the stroke volume of the largest piston pump.
NOTE An improvement of gas mixture homogeneity is attained by cascading two or three mixing vessels of
the same capacity rather than using only one mixing vessel with increased capacity.
The efficiency of the homogenization equipment, e.g. the mixing vessels, shall be verified by appropriate
methods for different gas mixtures and intended applications.
6.7 Stability
Calibration gas mixtures prepared according to this part of ISO 6145 are intended for direct use. If the
safety and mixture feasibility conditions are met, no degradation of the mixture composition occurs.
The reproducibility of a gas mixture generated by the preparation system is ensured under the condition
that the influencing factors, in particular temperature and pressure, are kept constant.
6.8 Output pressure and flow pulsation
After homogenization, the gas mixtures are available at the gas outlet at approximately ambient
pressure. Diameter and length of connecting tubes shall be of appropriate dimensions to avoid an
unacceptably high back pressure caused by flow resistance. Tubes with sufficiently wide inner diameter
are preferably used. Possible flow restrictions of connected analysers or other devices and apparatus
shall be minimized to the extent possible. If so necessary, an outlet pressure above ambient shall be
generated separately using suitable compression pumps with appropriate accessories.
Owing to the forwarding principle of piston pumps, the gas flow at the outlet is subject to pulsations in
flow. The influence of these pulsations can be minimized by technical means, e.g. appropriate diameter
of conduits, reduced flow resistance inside connected units, inserted buffers, or by-pass installations.
6.9 Composition of the parent gases
If pure gases are used, then the volume or amount–of–substance fraction of the main component shall be
corrected for the presence of impurities. In case of composition data in terms of amount–of–substance
fractions, the amount–of–substance fraction of the main component can be calculated using Formula
(4):
J
xx=−1 (4)
1 ∑ j
j=2
The standard uncertainty associated with the amount–of–substance fraction of the main component
(x ) is calculated using Formula (5)
1
J
2 2
ux()= ux() (5)
1 ∑ j
j=2
If the gas composition data are expressed as volume fractions, the volume fraction of the main component
is calculated using Formula (6)
J
φφ=−1 (6)
1 ∑ j
j=2
It is important to verify that the conditions (p and T) at which the volume fractions are given match
those at which the gases are mixed and used, respectively. Otherwise, the volume fraction shall be
converted from the stated conditions to the required conditions, using, e.g. the method of ISO 14912.
More guidance in converting volume fractions is given in 7.2.
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ISO 6145-2:2014(E)

The standard uncertainty associated with the volume fraction of the main component (φ ) is calculated
1
using Formula (7)
J
2 2
uu()φφ= () (7)
1 ∑ j
...

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