SIST EN ISO 6145-8:2008

Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 8: Diffusion method (ISO 6145-8:2005)

This part of ISO 6145 specifies a dynamic method using diffusion for the preparation of calibration gas mixtures containing component mole fractions ranging from 10 9 to 10 3. A relative expanded uncertainty of measurement, U, obtained by multiplying the relative combined standard uncertainty by a coverage factor k = 2, of not greater than 2 % can be achieved by using this method. By keeping the path between the diffusion source and place of use as short as possible, the method can be applied for the generation of low-concentration calibration gases of organic components that are liquid at room temperature, with boiling points ranging from about 40 °C to 160 °C. This part of ISO 6145 is applicable not only for the generation of calibration gas mixtures of a wide range of hydrocarbons at ambient and indoor air concentration levels, but also for the generation of low-concentration gas mixtures of water.

Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-volumetrischen Verfahren - Teil 8: Diffusionsverfahren (ISO 6145-8:2005)

Dieser Teil der ISO 6145 legt ein dynamisches Verfahren zur Herstellung von Kalibriergasgemischen mit
Stoffmengenanteilen der Komponenten im Bereich von 10-9 bis 10-3 fest. Mit diesem Verfahren lässt sich eine
relative erweiterte Messunsicherheit U, bestimmt durch Multiplikation der relativen kombinierten Standardunsicherheit
mit einem Überdeckungsfaktor k = 2, von 2 % oder besser erreichen.
Wird der Weg zwischen der Diffusionsquelle und der Kalibriergasaufgabe so kurz wie möglich gehalten, so
kann das Verfahren zur Erzeugung von Kalibriergasen im niedrigen Konzentrationsbereich mit organischen
Komponenten, die bei Raumtemperatur flüssig sind, mit Siedepunkten von etwa 40 ºC bis 160 ºC verwendet
werden.
Der Anwendungsbereich dieses Teils der ISO 6145 umfasst nicht nur die Herstellung von Kalibriergasgemischen
mit einer Vielzahl von Kohlenwasserstoffen auf Außenluft- und Innenraumluft-Konzentrationsniveau
sondern auch die Erzeugung von Gasgemischen mit Wasser in niedrigen Konzentrationsbereichen.

Analyse des gaz - Préparation de mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques - Partie 8: Méthode par diffusion (ISO 6145-8:2005)

L'ISO 6145-8:2005 spécifie une méthode dynamique par diffusion pour la préparation de mélanges de gaz pour étalonnage, dont la teneur en fractions molaires est comprise entre 10-9 et 10-3. En appliquant cette méthode, il est possible d'obtenir une incertitude relative élargie du mesurage, U, obtenue par multiplication de l'incertitude type relative composée par un facteur d'élargissement k = 2, de ± 2 % au maximum.
En maintenant la distance entre la source de diffusion et le lieu d'utilisation aussi courte que possible, la méthode peut être appliquée pour générer des gaz pour étalonnage à faible concentration de composants organiques liquides à température ambiante, les points d'ébullition étant compris entre 40 °C et 160 °C environ.
L'ISO 6145-8:2005 ne s'applique pas uniquement à la génération des mélanges de gaz pour étalonnage d'un large éventail d'hydrocarbures aux niveaux de concentration de l'air ambiant et d'intérieur, mais également à la génération de mélanges de gaz de faible concentration en eau.

Analiza plinov - Priprava kalibracijske plinske zmesi z uporabo dinamičnih volumetričnih metod - 8. del: Difuzijska metoda (ISO 6145-8:2005)

General Information

• Status: Published
• Publication Date: 04-Sep-2008
• ICS: 71.040.40 - Chemical analysis
• Technical Committee: NAD - Petroleum products, lubricants and related products
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 29-Aug-2008
• Due Date: 03-Nov-2008
• Completion Date: 05-Sep-2008
• Ref Project: EN ISO 6145-8:2008 - Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 8: Diffusion method (ISO 6145-8:2005)

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 6145-8:2008
01-oktober-2008
$QDOL]DSOLQRY3ULSUDYDNDOLEUDFLMVNHSOLQVNH]PHVL]XSRUDERGLQDPLþQLK
YROXPHWULþQLKPHWRGGHO'LIX]LMVNDPHWRGD,62
Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods
- Part 8: Diffusion method (ISO 6145-8:2005)
Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-
volumetrischen Verfahren - Teil 8: Diffusionsverfahren (ISO 6145-8:2005)
Analyse des gaz - Préparation de mélanges de gaz pour étalonnage à l'aide de
méthodes volumétriques - Partie 8: Méthode par diffusion (ISO 6145-8:2005)
Ta slovenski standard je istoveten z: EN ISO 6145-8:2008
ICS:
71.040.40 Kemijska analiza Chemical analysis
SIST EN ISO 6145-8:2008 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 6145-8:2008

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SIST EN ISO 6145-8:2008
EUROPEAN STANDARD
EN ISO 6145-8
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2008
ICS 71.040.40

English Version
Gas analysis - Preparation of calibration gas mixtures using
dynamic volumetric methods - Part 8: Diffusion method (ISO
6145-8:2005)
Analyse des gaz - Préparation des mélanges de gaz pour Gasanalyse - Herstellung von Kalibriergasgemischen mit
étalonnage à l'aide de méthodes volumétriques Hilfe von dynamisch-volumetrischen Verfahren - Teil 8:
dynamiques - Partie 8: Méthode par diffusion (ISO 6145- Diffusionsverfahren (ISO 6145-8:2005)
8:2005)
This European Standard was approved by CEN on 30 July 2008.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2008 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6145-8:2008: E
worldwide for CEN national Members.

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SIST EN ISO 6145-8:2008
EN ISO 6145-8:2008 (E)
Contents Page
Foreword.3

2

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SIST EN ISO 6145-8:2008
EN ISO 6145-8:2008 (E)
Foreword
The text of ISO 6145-8:2005 has been prepared by Technical Committee ISO/TC 158 “Analysis of gases” of
the International Organization for Standardization (ISO) and has been taken over as EN ISO 6145-8:2008 by
Technical Committee CEN/SS N21 “Gaseous fuels and combustible gas” the secretariat of which is held by
CMC.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by February 2009, and conflicting national standards shall be withdrawn
at the latest by February 2009.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 6145-8:2005 has been approved by CEN as a EN ISO 6145-8:2008 without any modification.

3

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SIST EN ISO 6145-8:2008

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SIST EN ISO 6145-8:2008


INTERNATIONAL ISO
STANDARD 6145-8
First edition
2005-02-01

Gas analysis — Preparation of calibration
gas mixtures using dynamic volumetric
methods —
Part 8:
Diffusion method
Analyse des gaz — Préparation de mélanges de gaz pour étalonnage à
l'aide de méthodes volumétriques —
Partie 8: Méthode par diffusion




Reference number
ISO 6145-8:2005(E)
©
ISO 2005

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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
PDF disclaimer
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©  ISO 2005
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.
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Published in Switzerland

ii © ISO 2005 – All rights reserved

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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Principle . 1
4 Reagents and materials. 2
5 Apparatus. 3
6 Procedure. 4
6.1 Preliminary checks and operating conditions . 4
6.2 Determination of mass loss . 5
7 Expression of results. 6
7.1 Calculation. 6
7.2 Sources of uncertainty . 7
Annex A (informative) Practical example of a diffusion cell calibrator configured for evaluating
speed of response in a hygrometer . 10
Annex B (informative) Example of performances of diffusion cells for toluene and
trichloromethane. 13
Annex C (informative) Example of uncertainty calculations for a periodic weighing system. 15
Bibliography . 19

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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(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 6145-8 was prepared by Technical Committee ISO/TC 158, Analysis of gases.
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 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
iv © ISO 2005 – All rights reserved

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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
Introduction
This part of ISO 6145 is one of a series of International Standards that present various dynamic volumetric
methods used for the preparation of calibration gas mixtures. In the lower part of the mole fraction range
considered, it is difficult to prepare and maintain gas mixtures – for example of certain organic or reactive
components – in cylinders. This dynamic method has the advantage of a practically unlimited supply of
calibration component, whereby adsorption effects can be reduced or even eliminated.
If the complementary gas flow is measured as a gas mass flow, the preparation of calibration gas mixtures
using diffusion is a dynamic-gravimetric method which gives contents in mole fractions. Principles for the
measurement of the complementary gas flow are given in ISO 6145-1.

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SIST EN ISO 6145-8:2008

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SIST EN ISO 6145-8:2008
INTERNATIONAL STANDARD ISO 6145-8:2005(E)

Gas analysis — Preparation of calibration gas mixtures using
dynamic volumetric methods —
Part 8:
Diffusion method
1 Scope
This part of ISO 6145 specifies a dynamic method using diffusion for the preparation of calibration gas
−9 −3
mixtures containing component mole fractions ranging from 10 to 10 . A relative expanded uncertainty of
measurement, U, obtained by multiplying the relative combined standard uncertainty by a coverage factor
k = 2, of not greater than ± 2 % can be achieved by using this method.
By keeping the path between the diffusion source and place of use as short as possible, the method can be
applied for the generation of low-concentration calibration gases of organic components that are liquid at room
temperature, with boiling points ranging from about 40 °C to 160 °C.
This part of ISO 6145 is applicable not only for the generation of calibration gas mixtures of a wide range of
hydrocarbons at ambient and indoor air concentration levels, but also for the generation of low-concentration
gas mixtures of water.
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 6145-7, Gas analysis — Preparation of calibration gas mixtures using dynamic volumetric methods —
Part 7: Thermal mass-flow controllers
3 Principle
The calibration component migrates by diffusion through a diffusion tube of suitable dimensions (length,
diameter) into a flow of a complementary gas, i.e. the complementary gas of the mixture prepared. The liquid
calibration component, of a known high purity, is contained in a reservoir that acts as the source of the
component vapour. The reservoir is provided with a vertically placed diffusion tube. This assembly (the
diffusion cell) is placed in a temperature-controlled containment that is purged at a known and constant flow
rate by a high-purity complementary gas (see Figure 1). The composition of the mixture is determined from
the diffusion mass flow of the calibration component and the flow rate of the complementary gas.
The diffusion mass flow rate of the calibration component in principle depends on
 its diffusion coefficient in the complementary gas,
 its vapour pressure at the temperature of the containment,
 the dimensions of the diffusion tube.
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
Accurate determination of the mass flow rate is achieved by either continuous weighing, after mounting the
tube in a suspension microbalance, or by periodic weighing. The method of determination affects the
uncertainty of the (momentary) mass flow of the calibration component (see 7.2).

Key
1 complementary gas inlet
2 containment
3 diffusion tube
4 liquid reservoir
5 calibration gas outlet
Figure 1 — Schematic of diffusion apparatus
4 Reagents and materials
4.1 Liquid substances to be used as calibration component, of the highest possible purity so as to
avoid any effects on the diffusion mass flow.
If possible, the nature and quantities of the impurities should be known and allowance made for their effects.
4.2 Complementary gas, of known purity, established by appropriate analytical techniques, e.g. Fourier-
transform infrared spectrometry or gas chromatography.
The nature of the complementary gas shall be adapted to the substance to be used as the calibration
component. For example, air shall not be used as complementary gas for the preparation by diffusion of
calibration gas mixtures of oxidizable substances.
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
5 Apparatus
5.1 Diffusion apparatus
5.1.1 Materials
The materials of the diffusion apparatus shall be chosen so as to avoid effects of physical or chemical sorption
or desorption on the content of the calibration component. The smaller the desired content, the greater the
effect of sorption/desorption phenomena.
Diffusion reservoirs and tubes, as well as temperature containments and blending apparatus, should
preferably be manufactured out of borosilicate glass. Choose chemically inert, flexible tube materials for the
supply of complementary gas and transport of calibration gas mixture. Pay special attention to all junctions as
possible sources of leaks.
5.1.2 Complementary gas flow configuration
Before the complementary gas reaches the diffusion cell, it is essential that its temperature be controlled to
that of the diffusion cell containment. In order to achieve the uncertainty stated in Clause 1, the temperature in
the containment should be controlled to within ± 0,15 K.
The minimum flow rate of the complementary gas should be sufficient to remove all component vapour without
saturation. The maximum allowable rate should be low enough to avoid convective transport of the calibration
component vapour inside the diffusion tube. This maximum flow rate is dependent upon the geometry of the
diffusion apparatus. It is recommended to keep the Reynolds number of the complementary gas flow in the
diffusion cell below 100. At a temperature of 25 °C, the following condition should approximately be fulfilled:
−3
vd⋅< 1, 6× 10
where
v is the average linear velocity of the complementary gas, in metres per second;
d is the diameter, in metres, of the tubing through which the complementary gas flows.
5.1.3 Choice of temperature
The choice of temperature depends on the diffusion cell characteristics and the diffusion mass flow rate
required. To carry out temperature control, establish thermal equilibrium within the diffusion cell at a value
close to ambient temperature or at a temperature sufficiently above ambient so as to avoid effects of ambient
conditions on temperature control. The use of a temperature slightly above ambient has two advantages:
 accurate control of temperature can more easily be achieved near ambient temperature,
 the temperature of the complementary gas can more easily be controlled.
5.2 Diffusion cells, consisting of a borosilicate glass reservoir capable of holding a sufficiently large
quantity of the liquid calibration component, fitted with a diffusion tube. Several design examples are given in
Reference [1].
[1]
In principle, Equation (1) can be applied for the prediction of diffusion volume flow rates and, conversely, for
the calculation of approximate dimensions and temperatures of diffusion tubes and containments necessary
for the generation of a given mass flow rate of the calibration component.

Ap
qD(A)=⋅ln (1)

V
L pp−
v
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
where
q (A) is the volume flow rate of component A, in cubic metres per second;
V
A is the cross-sectional area of the diffusion tube, in square metres;
L is the length of the diffusion tube, in metres;
D is the diffusion coefficient, in square metres per second;
p is the pressure in the diffusion cell, in pascals or kilopascals;
p is the partial pressure of the calibration component, in pascals or kilopascals.
v
If no data for the diffusion coefficients exist, methods for their calculation are given in the literature. The
[2]
method of Fuller, Schettler and Giddings is the most successful, but errors of up to 25 % can easily occur.
Data for the atomic and structural volume increments applicable to calibration component and complementary
gases and vapours are given in Reference [4].
To achieve the best performance, diffusion tubes should remain within the following dimensional ranges:
 L > 0,03 m;
 ratio of L to diameter of diffusion tube > 3;
 diameter: 0,001 m to 0,02 m.
NOTE Units which operate on the diffusion principle are commercially available and provide calibration gas mixtures
containing highly adsorptive vapours. An example of one such unit for the preparation of reference standards of humidity
–9
in volume fractions of 10 and its performance details are given in Annex A.
6 Procedure
6.1 Preliminary checks and operating conditions
Before assembling or filling a diffusion cell, the purity of the substance to be used as calibration component is
to be assessed using an appropriate analytical technique (e.g. Fourier-transform infrared spectrometry or gas
chromatography) so as to quantify any likely major contaminants.
Periodically check the diffusion mass flow at a known, fixed temperature and complementary gas flow rate as
an indication of stability of the calibration compound in the reservoir. If the diffusion mass flow drifts by more
than 1 % per month, this may be an indication of the presence of impurities. In that case, the contents of the
diffusion cell should be replaced.
When first placing the diffusion cell in its containment, allow the system to equilibrate before performing the
first weighing so as to ensure constancy of the diffusion mass flow. Generally, a period of 24 h is sufficient.
To change the content of the calibration gas mixture, adjust the complementary gas flow rate. Alternatively,
the calibration gas mixture can be further diluted, and its contents adjusted, by application of a secondary flow
of a diluent gas. Changing the temperature of the diffusion-cell containment for adjustment of the content of
the calibration gas mixture is not recommended.
During the period of use, maintain the diffusion cell at constant temperature in order to avoid delay due to the
time needed to restore equilibrium.
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
6.2 Determination of mass loss
6.2.1 Handling the diffusion cell
Ensure that all weighing is performed with extreme cleanliness and avoid direct contact of the diffusion cell
with hands. Use gloves and clean pliers or tweezers. If appropriate, depending on the type used, close the
diffusion cell before weighing.
6.2.2 Periodic-weighing mode
The temperature and relative humidity in the weighing room should be controlled and kept constant during
successive readings. The cell is periodically removed from the enclosure, weighed, and returned immediately
to the enclosure. In a given time interval, the diffusion cell will decrease in mass. The measurement of this
change in mass will have an associated measurement uncertainty. Therefore, the choice of the time interval
over which the weighings are made depends on the required uncertainty. Choose the time interval such that
the weighing uncertainty is a small fraction (e.g. < 1%) of the mass loss of the diffusion cell during this interval.
Determine the diffusion rate by calculation of the mass difference between the periodic weighings and the time
interval between them.
Because of the dependence of the diffusion mass flow rate on ambient pressure, a correction to standard
pressure (usually 101,325 kPa) may be applied as follows
∆mp
q (A) = (2)
m
∆tp
0
where
q (A) is the average mass flow rate of component A from the diffusion cell over time period ∆t, in grams
m
per minute;
∆m is the mass difference, in grams, between consecutive weighings;
∆t is the time interval, in minutes, between consecutive weighings;
p is the average air pressure, in kilopascals, over the interval between weighings;
p is the standard pressure for correction (usually 101,325 kPa).
0
The actual momentary mass flow is then calculated from q (A) by applying a reverse correction for actual
m
pressure
p
0
qq()A(= A) (3)
mm
p
where p is the actual air pressure, in kilopascals.
NOTE An example of the mass flows of diffusion cells for toluene and for trichloromethane as a function of time is
given in Annex B.
6.2.3 Continuous-weighing mode
The diffusion cell is weighed continuously on a load cell that transmits its readings to a computer (acquisition
analysis diagnostics). Choose the frequency at which weighings are to be recorded to be as close as possible
to the value obtained by dividing the diffusion rate by the accuracy of the weighing system. This will indicate
systematic deviations from a constant mass-loss rate.
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
−6 −1 −6
EXAMPLE A diffusion rate of 2,0 × 10 g⋅min and a weighing system accuracy of 1 × 10 g would suggest a
−1
sampling rate of 2 min .
7 Expression of results
7.1 Calculation
The mass concentration of the calibration component A in the resulting gas mixture, β(A), is given by:
q A
()
m
β()A = (4)
q
V
where
−1
q (A) is the diffusion rate (mass flow) of the calibration component A having dimensions M⋅T and, for
m
−1
example, expressed in micrograms per minute (µg⋅min );
q is the total volume flow rate of the complementary gas plus the flow rate of the component gas,
V
3 −1 −1
having dimensions L ⋅T and expressed, for example, in litres per minute (l⋅min ).
For practical purposes, the flow rate q of the component can be neglected. In the case of a two-stage
V
dilution procedure the flow rate q is the sum of the flow rates of the complementary gas and the diluent gas.
V
−3
The above calculation then gives the mass concentration of the gas mixture, β(A), in dimensions of M⋅L , in
−3
units for example of micrograms per cubic metre (µg⋅m ). Note that in this case the concentration is
dependent on the pressure and temperature conditions.
The calculated concentration can be converted into a mole fraction, x(A), by taking into account the molar
mass, M(A), of the component gas and the molar mass, M , of the sum of the gases under measurement
tot
conditions. The mass flow rate of the mixture can be calculated from the multiplication of volume flow rate,
q , and the density, ρ , of the mixture under measurement conditions; the molar mass flow rate is then
V,tot tot
obtained by dividing (q × ρ ) by M . For practical purposes the density and the molar mass of the
V,tot tot tot
complementary gas under measurement conditions can be used:
q A
( ) M
m
tot
x A=× (5)
()
MqA ⋅ ρ
()
V,tot tot
Combination of Equations (4) and (5) gives:
β A
( ) M
tot
x A=× (6)
()
M A ρ
()
tot
Alternatively, if the complementary gas flow is measured as a mass flow of gas, q , the mole fraction of the
m, cg
resulting gas mixture can be calculated by taking into account the molar mass of the component gas, M(A),
and that of the complementary gas, M . The component gas flow can usually be neglected in the sum of the
cg
mass flow so that:
M
q A
()
m cg
x()A=× (7)
Mq()A
m,cg
The results may be expressed in any appropriate units.
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SIST EN ISO 6145-8:2008
ISO 6145-8:2005(E)
NOTE Equations (4), (6) and (7) are related by constants which have a negligible uncertainty associated with them
−5
(the typical relative uncertainty in a molar mass is ± 1 × 10 ). Therefore, the relative uncertainty associated with the mole
fraction is the same as that associated with the mass concentration.
7.2 Sources of uncertainty
7.2.1 General
There are several sources of uncertainty, the principal ones of which are identified below:
a) measurement of mass flow from diffusion cell:
 balance,
 buoyancy effects,
 purity of calibration component,
 stability of calibration component,
 effects of sorption or desorption;
b) short-term fluctuations in mass flow of the calibration component;
c) measurement of time;
d) measurement of flow rate of complementary gas and optional diluent gas:
 flow meter,
 purity of complementary gas and optional diluent gas;
e) short-term fluctuations in complementary and diluent gas flows.
An example of an uncertainty evaluation of the generation of a calibration gas mixture by diffusion, based on
periodic weighing, is given in Annex C.
7.2.2 Measurement of the mass flow from the diffusion cell
7.2.2.1 Balance
Uncertainties in the mass measurement usually result from deficiencies in the calibration of the weighing
device and/or from the limited sensitivity of the balance. Weighing devices shall be traceably calibrated. The
intervals between subsequent weighings shall be sufficiently large so as to minimize the contribution of
balance resolution to the combined uncertainty.
7.2.2.2 Buoyancy effects
The mass of air displaced by the diffusion cell during weighing affects the apparent mass. Compensation for
this can be made by calculation of the magnitude of this buoyancy change. The true mass loss, ∆m, of the
diffusion cell is calculated according to Equation (8):
∆=mm −m +ρρ− V (8)
( )
12 1 2
where
m is the apparent mass of the diffusion cell at t
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