SIST EN ISO 6145-10:2008

Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 10: Permeation method (ISO 6145-10:2002)

This part of ISO 6145 specifies a dynamic method using permeation membranes for the preparation of calibration gas mixtures containing component mole fractions ranging from 10 9 and 10 6. A relative expanded uncertainty of 2,5 % of the component mole fraction can be achieved using this method. In the mole fraction range considered, it is difficult to maintain some gas mixtures, for example in cylinders, in a stable state. It is therefore desirable to prepare the calibration gas immediately before use, and to transfer it by the shortest possible path to the place where it is to be used. This technique has been successfully applied in generating low content calibration gas mixtures of, for example, sulfur dioxide (SO2), nitrogen dioxide (NO2) and benzene (C6H6) in air. If the carrier gas flow is measured as a gas mass-flow, the preparation of calibration gas mixtures using permeation tubes is a dynamic-gravimetric method which gives contents in mole fractions.

Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-volumetrischen Verfahren - Teil 10: Permeationsverfahren (ISO 6145-10:2002)

Dieser Teil der ISO 6145 legt ein dynamisches Verfahren unter Verwendung von Permeationsmembranen zur
Herstellung von Kalibriergasgemischen mit Stoffmengenanteilen der Komponenten im Bereich von 10–9 bis
10–6 fest. Mit diesem Verfahren lässt sich eine relative erweiterte Messunsicherheit von 2,5 % des Stoffmengenanteils
der Komponente erreichen. In dem betrachteten Bereich des Stoffmengenanteils ist es bei
manchen Gasgemischen schwierig, sie beispielsweise in Druckgasflaschen in einem stabilen Zustand zu
bewahren. Daher ist es erstrebenswert, solche Gasgemische unmittelbar vor dem Gebrauch herzustellen und
sie auf dem kürzestmöglichen Weg zur Verbrauchsstelle zu leiten. Diese Technik wurde erfolgreich eingesetzt
bei der Herstellung von Kalibriergasgemischen niedrigen Gehalts von beispielsweise Schwefeldioxid (SO2),
Stickstoffdioxid (NO2) und Benzol (C6H6) in Luft.
Wird der Grundgasstrom als Massenstrom gemessen, dann handelt es sich bei der Herstellung von Kalibriergasgemischen
mit Hilfe von Permeationsröhrchen um ein dynamisch-gravimetrisches Verfahren, das
Gehalte in Stoffmengenanteilen liefert.

Analyse des gaz - Préparation des mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques dynamiques - Partie 10: Méthode par perméation (ISO 6145-10:2002)

La présente partie de l'ISO 6145 spécifie une méthode dynamique utilisant des membranes de perméation pour la
préparation de mélanges de gaz pour étalonnage, dont la teneur en constituants est en général comprise entre
10−9 et 10−6 en fraction molaire. En appliquant cette méthode, on obtient une incertitude relative étendue de 2,5 %
de la fraction molaire du constituant. Dans la gamme des fractions molaires considérée, il est difficile de maintenir
stables certains mélanges de gaz (notamment en bouteilles). Aussi est-il nécessaire de préparer le gaz pour
étalonnage immédiatement avant emploi et de le transférer par le plus court chemin sur son lieu d'utilisation. Cette
technique a notamment été appliquée avec succès pour générer des mélanges de gaz à faible teneur pour
étalonnage, par exemple, de dioxyde de soufre (SO2), de dioxyde d'azote (NO2) et de benzène (C6H6) dans l'air.
Si le débit de gaz vecteur est mesuré sous forme de débit massique, la préparation des mélanges de gaz pour
étalonnage à l'aide de tubes à perméation est une méthode gravimétrique dynamique qui donne les teneurs en
fractions molaires.

Analiza plinov - Priprava kalibracijske plinske zmesi z uporabo dinamičnih volumetričnih metod - 10. del: Metoda pronicanja (ISO 6145-10:2002)

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 6145-10:2008 - Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 10: Permeation method (ISO 6145-10:2002)

Relations

• Revised: SIST ISO 6349:1995 - Gas analysis -- Preparation of calibration gas mixtures -- Permeation method
• Effective Date: 01-Oct-2008

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 6145-10:2008
01-oktober-2008
$QDOL]DSOLQRY3ULSUDYDNDOLEUDFLMVNHSOLQVNH]PHVL]XSRUDERGLQDPLþQLK
YROXPHWULþQLKPHWRGGHO0HWRGDSURQLFDQMD,62
Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods
- Part 10: Permeation method (ISO 6145-10:2002)
Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-
volumetrischen Verfahren - Teil 10: Permeationsverfahren (ISO 6145-10:2002)
Analyse des gaz - Préparation des mélanges de gaz pour étalonnage à l'aide de
méthodes volumétriques dynamiques - Partie 10: Méthode par perméation (ISO 6145-
10:2002)
Ta slovenski standard je istoveten z: EN ISO 6145-10:2008
ICS:
71.040.40 Kemijska analiza Chemical analysis
SIST EN ISO 6145-10:2008 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 6145-10:2008

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SIST EN ISO 6145-10:2008
EUROPEAN STANDARD
EN ISO 6145-10
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2008
ICS 71.040.40

English Version
Gas analysis - Preparation of calibration gas mixtures using
dynamic volumetric methods - Part 10: Permeation method (ISO
6145-10:2002)
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 10:
dynamiques - Partie 10: Méthode par perméation (ISO Permeationsverfahren (ISO 6145-10:2002)
6145-10:2002)
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 6145-10:2008: E
worldwide for CEN national Members.

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SIST EN ISO 6145-10:2008
EN ISO 6145-10:2008 (E)
Contents Page
Foreword.3

2

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SIST EN ISO 6145-10:2008
EN ISO 6145-10:2008 (E)
Foreword
The text of ISO 6145-10:2002 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 6145-10:2008 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 6145-10:2002 has been approved by CEN as a EN ISO 6145-10:2008 without any
modification.

3

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SIST EN ISO 6145-10:2008

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SIST EN ISO 6145-10:2008

INTERNATIONAL ISO
STANDARD 6145-10
First edition
2002-02-01


Gas analysis — Preparation of calibration
gas mixtures using dynamic volumetric
methods —
Part 10:
Permeation method
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage à
l'aide de méthodes volumétriques dynamiques —
Partie 10: Méthode par perméation




Reference number
ISO 6145-10:2002(E)
©
 ISO 2002

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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
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©  ISO 2002
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
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ii © ISO 2002 – All rights reserved

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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative reference.1
3 Principle.1
4 Reagents and materials .2
5 Apparatus .2
6 Procedure .5
6.1 Preliminary checks and operating conditions.5
6.2 Determination of mass loss.6
7 Expression of results .7
7.1 Calculation .7
7.2 Sources of uncertainty.8
7.3 Estimation of uncertainties.10
7.4 Example calculation of uncertainties .13
Annex A (informative) Example of uncertainty calculation for a two-pan continuous weighing system.14
Bibliography.16


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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(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 3.
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 part of ISO 6145 may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6145-10 was prepared by Technical Committee ISO/TC 158, Analysis of gases.
It cancels and replaces ISO 6349:1979 which has been technically revised.
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 method
 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 stream, has been withdrawn by Technical Committee ISO/TC 158, Analysis of gases.
Annex A of this part of ISO 6145 is for information only.
iv © ISO 2002 – All rights reserved

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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
Introduction
This part of ISO 6145 is one of a series of standards dealing with various dynamic volumetric methods used for the
preparation of calibration gas mixtures.
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SIST EN ISO 6145-10:2008

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SIST EN ISO 6145-10:2008
INTERNATIONAL STANDARD ISO 6145-10:2002(E)

Gas analysis — Preparation of calibration gas mixtures using
dynamic volumetric methods —
Part 10:
Permeation method
1 Scope
This part of ISO 6145 specifies a dynamic method using permeation membranes for the preparation of calibration
−9 −6
gas mixtures containing component mole fractions ranging from 10 and 10 . A relative expanded uncertainty of
2,5 % of the component mole fraction can be achieved using this method. In the mole fraction range considered, it
is difficult to maintain some gas mixtures, for example in cylinders, in a stable state. It is therefore desirable to
prepare the calibration gas immediately before use, and to transfer it by the shortest possible path to the place
where it is to be used. This technique has been successfully applied in generating low content calibration gas
mixtures of, for example, sulfur dioxide (SO ), nitrogen dioxide (NO ) and benzene (C H ) in air.
2 2 6 6
If the carrier gas flow is measured as a gas mass-flow, the preparation of calibration gas mixtures using permeation
tubes is a dynamic-gravimetric method which gives contents in mole fractions.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this part of ISO 6145. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 6145 are encouraged to investigate the
possibility of applying the most recent edition of the normative document indicated below. For undated references,
the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO 6145-1, Gas analysis — Preparation of calibration gas mixtures using dynamic volumetric methods — Part 1:
Methods of calibration
3 Principle
The calibration component [for example SO , NO , ammonia (NH ), benzene, toluene, xylene] is permeated
2 2 3
through an appropriate membrane into the flow of a carrier gas, i.e. the complementary gas of the mixture obtained.
The calibration component, of known purity, is contained in a tube, which is itself contained in a temperature-
controlled vessel. This vessel is purged at a known and controlled flow rate by the carrier gas. The composition of
the mixture is determined from the permeation rate of the calibration component as well as the flow rate of the high
quality carrier gas, free from any trace of the calibration component and from any chemical interaction with the
material of the permeation tube.
The permeation rate of the calibration component through the membrane depends upon the component itself, the
chemical nature and structure of the membrane, its area and thickness, the temperature, and the partial pressure
gradient of the calibration component across the membrane. These factors can be kept constant by proper
operation of the system.
The permeation rate can be measured directly by mounting the tube on a microbalance and weighing the tube
either continuously or periodically.
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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
4 Reagents and materials
4.1 Permeating substances for calibration, of the highest possible purity so as to avoid any effect of impurities
on the permeation rate; if this is not possible, the nature and quantities of the impurities shall be known and
allowance made for their effect.
4.2 Carrier gas, of known purity, established by an appropriate analytical technique, for example, gas
chromatography (GC) and/or Fourier transform infrared (FTIR) spectrometry.
5 Apparatus
5.1 Permeation apparatus, typically consisting of one of two modes (5.1.1 and 5.1.2) of application of the
permeation method.
The materials of the permeation apparatus shall be chosen so as to avoid any effect on the content of the
calibration component by sorption (chemical or physical). The smaller the desired final content, the greater the
effect of adsorption phenomena. If possible, use glass as the housing of the temperature-controlled permeation
tube. Choose flexible and chemically inert tube materials and metals, especially having regard to the transfer of the
gas between the permeation apparatus and the analyser. Pay special attention to all junctions so as to keep them
free from leaks.
The flow range of the carrier gas is kept constant by a control system and is monitored by a flowmeter. The value of
the flow rate can, for example, be controlled by means of a mass flow controller and determined using a mass
flowmeter.
The existence of an outlet for surplus gas enables the analyser under calibration to take the gas flow rate
necessary for its proper operation, the remainder of the flow of gas being vented to atmosphere.
5.1.1 Periodic-weighing-mode permeation apparatus, consisting of a permeation tube kept in a temperature-
controlled enclosure, swept by carrier gas. The permeation tube is periodically removed from the enclosure to be
weighed.
Typical examples are given in Figures 1 and 2.
5.1.2 Continuous-weighing-mode permeation apparatus, consisting of a permeation tube kept in a
temperature-controlled enclosure, swept by carrier gas. The permeation tube is suspended from a weighing device
and weighed continuously.
A typical example is given in Figure 3.
2 © ISO 2002 – All rights reserved

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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)

Key
1 Flowmeter 5 Thermometer
2 Carrier gas 6 Permeation tube
3 Drier 7 Outlet for surplus gas
4 Filter 8 Analyser
Figure 1 — Example 1 of a periodic-weighing-mode permeation apparatus


Key
1 Outlet for surplus gas 5 Diluent gas 9 Water bath
2 Sampling system 6 Thermometer 10 Flowmeter 1
3 Mixing bulb 7 Permeation tube 11 Carrier gas
4 Flowmeter 2 8 Copper tubing 12 Drier
Figure 2 — Example 2 of a periodic-weighing-mode permeation apparatus
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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)

Permeation tube

Mass flow controller

Tare mass

Key
1 High purity air/N 5 Gas blender 9 Flow rate calibration facility
2
2 Temperature controller
6 RS232 link 10 Gas analyser
3 Water
7 PC (acquisition, analysis and diagnostics) 11 Stable mixture requiring certification
4 Microbalance controller
8 16-bit ADC
Figure 3 — Continuous-weighing-mode permeation apparatus
4 © ISO 2002 – All rights reserved

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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
5.2 Permeation membrane, made from polymers and having sufficient chemical and mechanical resistance,
e.g. suitable polytetrafluoroethylene (PTFE), polyethylene, polypropylene or a copolymer of tetrafluoroethylene and
hexafluoropropylene (FEP).
Take into account variations of the material characteristics which occur with a change of temperature.
5.3 Permeation tubes, or containers, made of stainless steel or glass, fitted with a permeation membrane (5.2)
and capable of holding the calibration component in the liquid phase and gaseous phase; the membrane through
which the permeation takes place may be in contact with the liquid phase only, or with the gaseous phase only, or
with both.
See examples given in Figure 4.
Before use, keep the permeation tube in an airtight container under an anhydrous atmosphere in a cold place (e.g.
in a refrigerator at approximately 5 °C) so as to maintain the diffusion rate as low as possible, hence to minimize
loss of the calibration component and avoid any condensation on the tube.

a) Cylindrical tube fitted with a b) Tube fitted with a membrane in c) Container fitted with a membrane
membrane in contact with both contact with only the liquid phase in contact with only the gaseous
phases phase
Key
1 Membrane
2 Stainless steel
3 Liquid level
4 Glass
Figure 4 — Examples of permeation tubes and container
6 Procedure
6.1 Preliminary checks and operating conditions
6.1.1 Permeation tube
Before use, assess the purity of the product of the permeation tube by collecting a sample of the permeated gas for
analysis by an appropriate analytical technique [e.g. GC or FTIR] so as to quantify any likely major contaminants.
This information may be provided by the suppliers of the tube and, if so, a certificate of analysis by an accredited
body shall be provided.
Periodically check the permeation rate of the tube at a known, fixed temperature by measuring the mass loss. This
gives a good indication as to the purity of the permeated gas. If the permeation rate changes by more than 10 % at
the known, fixed temperature, discard the permeation tube.
When first using the permeation tube, allow the system to reach a state of equilibrium before carrying out the first
weighing so as to ensure that the permeation rate is well stabilized at the constant value. The time needed to reach
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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
equilibrium is dependant on the component contained within the permeation tube, but a value of 72 h is applicable
to most species.
For most applications, it is essential to control the temperature of the enclosure to within 0,1 K because of the very
strong dependence of the permeation rate upon temperature. The tube diffusion rate may, for example, double for
an increase in temperature of approximately 7 K. Under certain circumstances, in which the diffusing gas is highly
soluble in the membrane polymer, an increase in temperature may reduce the permeation rate.
During the period of use, maintain the permeation tube at constant temperature, principally to avoid the delay,
sometimes very lengthy, which is necessary to restore equilibrium. Avoid any rapid changes in temperature.
If the operating conditions change significantly (e.g. a change in operating temperature), allow a period of 72 h for
the permeation tube to re-equilibrate before resuming measurements.
6.1.2 Carrier gas flow configuration
Before the carrier gas reaches the tube, it is essential that its temperature be controlled at that of the permeation
tube. Any system which enables the carrier gas to remain in the temperature-controlled enclosure for a sufficient
period of time is satisfactory.
To change the content of the calibration mixture, adjust the carrier gas flow rate and the diluent gas flow rate
(avoiding any change of the tube permeation rate as a result of temperature change); in this case, equilibrium is
rapidly obtained. Refer to Figures 1 and 2 for the distinction between carrier gas flow and diluent gas flow. The
dilution system shall have one or two stages, the first to carry gas away from the tube, the second to achieve the
required concentration. Figure 1 shows an example of a single-stage dilution and Figures 2 and 3 shows examples
of a two-stage dilution.
In the two-stage dilution procedure, establish the carrier gas flow at a suitable flow rate until temperature stability is
attained. The desired content of the calibration component is then achieved by adjustment of the diluent gas flow
rate, thus avoiding any disturbance to the thermal equilibrium of the permeation tube.
6.1.3 Choice of temperature
The choice of temperature depends on the tube characteristics and the permeation rate required. To carry out
temperature control, establish thermal equilibrium within the permeation apparatus at a value close to the ambient
value, or at a temperature sufficiently above the ambient value so as to ensure that no effect results from variations
in the latter.
The choice of a temperature close to ambient temperature has two advantages:
a) accurate control of temperature can be achieved more easily near ambient temperature;
b) the temperature of the carrier gas can be more easily controlled.
6.1.4 Handling the tube
Ensure that all weighing is performed with extreme cleanliness and avoid any direct contact with the operator’s
hands. Use gloves and clean tweezers.
6.2 Determination of mass loss
Make sure the temperature and relative humidity of the air in the weighing room are controlled and kept constant
during successive weighings. Weigh the tube and return it to the temperature-controlled environment after the
weighing procedure. Keep the time that the permeation tube spends outside the temperature controlled
environment to a minimum. Do not remove the permeation tube from the weighing enclosure if a continuous
weighing procedure is used.
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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
In a given time interval, the permeation device 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 weighings are
made depends on the required accuracy, expressed as a fraction of the total mass loss. Choose the time interval
such that the weighing uncertainty is a small fraction (e.g. < 1 %) of the mass loss of the permeation tube during
this interval. In the case of the continuous weighing mode, choose the rate of frequency at which weighings are to
be recorded to be as close as possible to the value obtained by dividing the permeation rate by the precision of the
weighing balance. This will indicate systematic deviations from a constant mass loss rate. For example, a
−6 −6
permeation rate of 2,0 × 10 g/min and a balance resolution of 1 × 10 g would suggest a 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, is given by:
qA()
m
b = (1)
q
V
where
−1
q (A) is the permeation rate (mass flow) of the calibration component A having dimensions MT and, for
m
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, having
V
3 −1 3
dimensions L T and, for example, expressed in cubic metres per minute (m /min).
For practical purposes, the flow rate q of the component can be neglected. In the case of the two-stage dilution
V
procedure the flow rate q is the sum of the flow rates of the carrier gas and the diluent gas.
V
−3
The above calculation then gives the mass concentration of the gas mixture, β, in dimensions of ML , for example
3
in units of micrograms per cubic metre (µg/m ). Note, 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 conditions. The
tot
mass flow rate of the mixture can be calculated from the multiplication of volume flow rate, q , and the density,
V,tot
ρ , of the mixture under measurement conditions; the molar mass flow rate is then obtained by dividing q ◊ρ
tot V,tot tot
by M . For practical purposes the density and the molar mass of the carrier gas under measurement conditions
tot
can be used:
qA()/M(A) qA( )◊M
mm tot
xA()== (2)
qM◊◊rr/(q◊MA)
VV,tot tot tot ,tot tot
where q is total volume flow rate of the mixture.
V,tot
Equations (1) and (2) give:
b()AM◊
tot
xA() = (3)
r ◊ M()A
tot
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SIST EN ISO 6145-10:2008
ISO 6145-10:2002(E)
Alternatively, if the carrier gas flow is measured as a mass flow of gas, q , the mole fraction of the resultant gas
m,cg
mixture can be calculated by taking into account the molar mass, M(A), of the calibration component and that, M ,
cg
of the carrier gas. The flow of the calibration component can usually be neglected in the sum of the mass flow so
that:
qA()/M(A)
m
(4)
xA() =
qM/
m,c g c g
The results may be expressed in any appropriate units.
It should be noted that Equations (1), (3) and (4) are related by constants, which have a negligible uncertainty
−5
associated with them (the typical relative uncertainty in molar mass is ± 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 in the following sub-clauses.
7.2.2 Impurities
As discussed above, before using a permeation apparatus for the preparation of gas mixtures, any impurities in the
calibration component and carrier gases shall be identified and quantified, or upper limits placed on their relative
concentrations. Appropriate analytical techniques (e.g. GC or FTIR) shall be used for this purpose. Frequent (e.g.
weekly) checks on the permeation rate at a fixed temperature are a good means of verifying that a single
component is permeating.
7.2.3 Substances undergoing polymerization or combining together
Some substances (e.g. vinyl chloride) may undergo polymerization or may combine together. This will have the
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