SIST EN ISO 6145-6:2017

Gas analysis - Preparation of calibration gas mixtures using dynamic methods - Part 6: Critical flow orifices (ISO 6145-6:2017)

This document specifies a method for the dynamic preparation of calibration gas mixtures containing
at least two gases (usually one of them is a complementary gas) from pure gases or gas pre-mixtures
using critical flow orifices systems.
The method applies principally to the preparation of mixtures of non-reactive gases that do not react
with any of the materials forming the gas circuit inside the critical flow orifices system or auxiliary
equipment. It has the merit of allowing multi-component mixtures to be prepared as readily as binary
mixtures if an appropriate number of critical flow orifices are used.
By selecting appropriate combinations of critical flow orifices, a dilution ratio of 1 × 104 is achievable.
Although it is more particularly applicable to the preparation of gas mixtures at atmospheric pressure,
the method also offers the possibility of preparing calibration gas mixtures at pressures greater than
atmospheric. The upstream pressure will need to be at least two times higher than downstream
pressure.
The range of flow rates covered by this document extends from 1 ml/min to 10 l/min.

Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-volumetrischen Verfahren - Teil 6: Kritische Düsen (ISO 6145-6:2017)

Analyse des gaz - Préparation des mélanges de gaz pour étalonnage à l'aide de méthodes volumétriques dynamiques - Partie 6: Orifices de débit critiques (ISO 6145-6:2017)

L'ISO 6145-6:2017 décrit une méthode de préparation dynamique de mélanges de gaz d'étalonnage, contenant au moins deux gaz (l'un d'eau étant généralement un gaz de complément), à partir de gaz purs ou de pré-mélanges de gaz à l'aide de systèmes à orifices critiques.
La méthode s'applique principalement à la préparation de mélanges de gaz non réactifs qui ne réagissent avec aucun des matériaux formant le circuit de gaz à l'intérieur du système à orifices critiques ou d'un équipement auxiliaire. Elle a le mérite de permettre la préparation de mélanges à plusieurs constituants aussi facilement que des mélanges à deux constituants si un nombre approprié d'orifices critiques est utilisé.
En choisissant des combinaisons appropriées d'orifices critiques et en utilisant des gaz purs, il est possible d'atteindre un rapport de dilution de 1 x 104.
Bien qu'elle soit plus particulièrement applicable à la préparation de mélanges de gaz à la pression atmosphérique, cette méthode offre également la possibilité de préparer des mélanges de gaz d'étalonnage à des pressions supérieures à la pression atmosphérique. La pression amont devra être au moins deux fois plus élevée que la pression aval.
La gamme de débits couverte par le présent document s'étend de 1 ml/min à 10 l/min.

Analiza plinov - Priprava kalibracijskih plinskih zmesi z uporabo dinamičnih volumetričnih metod - 6. del: Kritične šobe (ISO 6145-6:2017)

Ta dokument določa metodo dinamične priprave kalibracijskih plinskih zmesi, ki vsebujejo najmanj dva plina (običajno je eden komplementarni plin), iz čistih plinov ali vnaprej pripravljenih mešanic plinov, ki uporabljajo sisteme s kritičnimi šobami.
Ta metoda se uporablja predvsem pri pripravi mešanic nereaktivnih plinov, ki ne reagirajo z nobenim materialom, ki tvori plinski krog znotraj sistema kritičnih šob ali pomožne opreme. Njena prednost je v tem, da omogoča pripravo večkomponentnih mešanic na enak način kot pri dvokomponentnih mešanicah, če je uporabljeno ustrezno število kritičnih šob.
Z izbiro ustreznih kombinacij kritičnih šob je mogoče doseči razmerje redčenja 1 × 104. Čeprav je metoda še zlasti primerna za pripravo plinskih zmesi pri atmosferskem tlaku, ponuja tudi možnost priprave kalibracijskih plinskih zmesi pri tlakih, večjih od atmosferskega tlaka. Tlak proti toku mora biti vsaj dvakrat večji od tlaka v smeri toka.
Razpon pretoka, ki je zajet v tem dokumentu, obsega od 1 ml/min do 10 l/min.

General Information

• Status: Published
• Public Enquiry End Date: 02-Jun-2016
• Publication Date: 11-Sep-2017
• 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: 30-Aug-2017
• Due Date: 04-Nov-2017
• Completion Date: 12-Sep-2017
• Ref Project: EN ISO 6145-6:2017 - Gas analysis - Preparation of calibration gas mixtures using dynamic methods - Part 6: Critical flow orifices (ISO 6145-6:2017)

Relations

• Replaces: SIST EN ISO 6145-6:2008 - Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 6: Critical orifices (ISO 6145-6:2003)
• Effective Date: 01-Oct-2017
• Replaces: SIST EN ISO 6145-6:2008 - Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 6: Critical orifices (ISO 6145-6:2003)
• Effective Date: 08-Jun-2022

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 6145-6:2017
01-oktober-2017
1DGRPHãþD
SIST EN ISO 6145-6:2008
$QDOL]DSOLQRY3ULSUDYDNDOLEUDFLMVNLKSOLQVNLK]PHVL]XSRUDERGLQDPLþQLK
YROXPHWULþQLKPHWRGGHO.ULWLþQHãREH,62
Gas analysis - Preparation of calibration gas mixtures using dynamic methods - Part 6:
Critical flow orifices (ISO 6145-6:2017)
Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-
volumetrischen Verfahren - Teil 6: Kritische Düsen (ISO 6145-6:2017)
Analyse des gaz - Préparation des mélanges de gaz pour étalonnage à l'aide de
méthodes volumétriques dynamiques - Partie 6: Orifices de débit critiques (ISO 6145-
6:2017)
Ta slovenski standard je istoveten z: EN ISO 6145-6:2017
ICS:
71.040.40 Kemijska analiza Chemical analysis
SIST EN ISO 6145-6:2017 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 6145-6:2017

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SIST EN ISO 6145-6:2017


EN ISO 6145-6
EUROPEAN STANDARD

NORME EUROPÉENNE

August 2017
EUROPÄISCHE NORM
ICS 71.040.40 Supersedes EN ISO 6145-6:2008
English Version

Gas analysis - Preparation of calibration gas mixtures
using dynamic methods - Part 6: Critical flow orifices (ISO
6145-6:2017)
Analyse des gaz - Préparation des mélanges de gaz Gasanalyse - Herstellung von Kalibriergasgemischen
pour étalonnage à l'aide de méthodes volumétriques mit Hilfe von dynamisch-volumetrischen Verfahren -
dynamiques - Partie 6: Orifices de débit critiques (ISO Teil 6: Kritische Düsen (ISO 6145-6:2017)
6145-6:2017)
This European Standard was approved by CEN on 7 August 2017.

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-CENELEC 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-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.





EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6145-6:2017 E
worldwide for CEN national Members.

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SIST EN ISO 6145-6:2017
EN ISO 6145-6:2017 (E)
Contents Page
European foreword . 3

2

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SIST EN ISO 6145-6:2017
EN ISO 6145-6:2017 (E)
European foreword
This document (EN ISO 6145-6:2017) has been prepared by Technical Committee ISO/TC 158 “Analysis
of gases”.
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 2018, and conflicting national standards
shall be withdrawn at the latest by February 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 6145-6:2008.
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,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 6145-6:2017 has been approved by CEN as EN ISO 6145-6:2017 without any
modification.
3

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SIST EN ISO 6145-6:2017

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SIST EN ISO 6145-6:2017
INTERNATIONAL ISO
STANDARD 6145-6
Third edition
2017-07
Gas analysis — Preparation of
calibration gas mixtures using
dynamic methods —
Part 6:
Critical flow orifices
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage
à l’aide de méthodes volumétriques dynamiques —
Partie 6: Orifices de débit critiques
Reference number
ISO 6145-6:2017(E)
©
ISO 2017

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
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
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2017 – All rights reserved

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 4
5 Principle . 5
6 Calculation of mass flow rate and volume flow rate . 6
6.1 General . 6
6.2 Calculation under ideal conditions . 7
6.2.1 Calculation of mass flow rate . 7
6.2.2 Calculation of volume flow rates . 7
6.3 Calculation of mass flow rate using flow calibration with pure nitrogen . 8
6.4 Flow rate uncertainty calculation . 9
6.4.1 General. 9
6.4.2 Sources of uncertainty . 9
6.4.3 Uncertainty estimation . .10
7 Calculation of amount of substance fraction and volume fraction and associated
uncertainty evaluation .10
7.1 General .10
7.2 Amount of substance fraction calculation and associated uncertainty .10
7.2.1 Case of gases with purity ≥ 99,99 % .10
7.2.2 Case of pre-mixtures . .13
7.3 Remarks about uncertainty for the amount fraction .15
8 Application to the preparation of gas mixtures .15
8.1 Example of a mixing system .15
8.2 Conditions of operation .16
9 Calibration and verification .17
9.1 General .17
9.2 Calibration of the mixing system in the flow rate .17
9.3 Calibration of the mixing system with gas mixtures for a specific gas and concentration .17
9.4 Verification of the mixing system .17
Annex A (informative) Example of calculation of isentropic coefficient, viscosity and critical
flow coefficient .19
Annex B (informative) Calculation of mass and volume flow rates under real conditions .21
Annex C (informative) Example of flow calculation for toroidal critical flow orifices under
ideal and real conditions .23
Annex D (informative) Calculation of mass flow rate using flow calibration with pure
nitrogen: examples .25
Bibliography .27
© ISO 2017 – All rights reserved iii

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(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 (see 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 (see 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 voluntary nature of standards, on the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 158, Analysis of gases.
This third edition cancels and replaces the second edition (ISO 6145-6:2003) which has been technically
revised.
A list of all parts in the ISO 6145 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

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SIST EN ISO 6145-6:2017
INTERNATIONAL STANDARD ISO 6145-6:2017(E)
Gas analysis — Preparation of calibration gas mixtures
using dynamic methods —
Part 6:
Critical flow orifices
1 Scope
This document specifies a method for the dynamic preparation of calibration gas mixtures containing
at least two gases (usually one of them is a complementary gas) from pure gases or gas pre-mixtures
using critical flow orifices systems.
The method applies principally to the preparation of mixtures of non-reactive gases that do not react
with any of the materials forming the gas circuit inside the critical flow orifices system or auxiliary
equipment. It has the merit of allowing multi-component mixtures to be prepared as readily as binary
mixtures if an appropriate number of critical flow orifices are used.
4
By selecting appropriate combinations of critical flow orifices, a dilution ratio of 1 × 10 is achievable.
Although it is more particularly applicable to the preparation of gas mixtures at atmospheric pressure,
the method also offers the possibility of preparing calibration gas mixtures at pressures greater than
atmospheric. The upstream pressure will need to be at least two times higher than downstream
pressure.
The range of flow rates covered by this document extends from 1 ml/min to 10 l/min.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 6143, Gas analysis — Comparison methods for determining and checking the composition of calibration
gas mixtures
ISO 7504, Gas analysis — Vocabulary
ISO 9300, Measurement of gas flow by means of critical flow Venturi nozzles
ISO 12963, Gas analysis — Comparison methods for the determination of the composition of gas mixtures
based on one- and two-point calibration
ISO 16664, Gas analysis — Handling of calibration gases and gas mixtures — Guidelines
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9300, ISO 7504 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
© ISO 2017 – All rights reserved 1

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

3.1
critical flow orifice
orifice for which the geometrical configuration and conditions of use are such that the flow rate at the
throat is critical (3.12)
3.2
wall pressure tap
orifice pierced in a pipework wall in such a way that the orifice edge on the inside pipework wall is
flattened off
Note 1 to entry: This pressure tap is set up so that the pressure in the orifice equals the static pressure at this
point of the circuit pipework.
3.3
static pressure
actual pressure of a gas stream, which can be measured by connecting a pressure gauge to a wall
pressure tap
Note 1 to entry: This document uses only absolute pressure values.
3.4
stagnation pressure
pressure that would be found in a gas if the flowing gas stream was isentropically slowed down to zero
velocity
Note 1 to entry: This document uses only absolute pressure values.
3.5
stagnation temperature
temperature that would be found in a gas if the flowing gas stream was isentropically slowed down to
zero velocity
Note 1 to entry: This document only uses absolute temperature values.
3.6
mass flow rate
q
m
mass of gas per unit of time passing through the orifice
3.7
molar flow rate
q
n
amount of substance of gas per unit of time passing through the orifice
3.8
volume flow rate
q
V
volume of gas per unit of time passing through the orifice
3.9
throat Reynolds number
Re
dimensionless parameter calculated from the gas flow rate and dynamic viscosity under critical flow
orifice inlet stagnation conditions
Note 1 to entry: The characteristic dimension is taken as the throat diameter at stagnation conditions. The throat
Reynolds number is given by the formula:
2 © ISO 2017 – All rights reserved

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

4 q
()
m
Re =
πη d
0 N
3.10
isentropic coefficient
γ
ratio of the relative variation in pressure to the corresponding relative variation in density under
elementary reversible adiabatic (isentropic) transformation conditions
Note 1 to entry: In real gases, the forces exerted between molecules as well as the volume occupied by the
molecules have a significant effect on the gas behaviour. In an ideal gas, intermolecular forces and the volume
occupied by the molecules can be considered as negligible.
3.11
discharge coefficient
c
dimensionless ratio of the actual flow rate to the ideal flow rate of a non-viscous gas that would be
obtained with one-dimensional isentropic flow for the same upstream stagnation conditions
Note 1 to entry: This coefficient corrects for viscous and flow field curvature effects. For each type of critical
flow orifice design and installation conditions specified in this document, this coefficient is solely a function of
the throat Reynolds number.
3.12
critical flow rate
maximum flow rate through a given orifice under the given upstream conditions
Note 1 to entry: At critical flow, the throat velocity is equal to the local value of the speed of sound (acoustic
velocity), the velocity at which small pressure disturbances propagate.
3.13
critical flow function
C
*
dimensionless function which characterizes the thermodynamic flow properties of an isentropic one-
dimensional flow between the inlet and the throat of a orifice
Note 1 to entry: It is a function of the nature of the gas and of the stagnation pressure (3.4) and stagnation
temperature (3.5).
3.14
critical flow coefficient of a real gas
C
R
alternative form of the critical flow function, more convenient for gas mixtures
Note 1 to entry: This coefficient can be deduced from critical flow function via the formula:
CC= z
*
R 0
3.15
critical pressure ratio
*
r
ratio of the static pressure at the critical flow orifice throat to the stagnation pressure for which the gas
mass flow rate through the critical flow orifice is maximal
Note 1 to entry: This ratio is calculated according to the formula given in Clause 5.
© ISO 2017 – All rights reserved 3

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

γ
 
p
 γ −1
2
out
r* =   =
 
 
p γ +1
 
 in 
crit
3.16
compressibility factor
Z
0
correction factor numerically expressing the fact that the behaviour of a real gas deviates from the
ideal gas law at stagnation pressure and temperature
Note 1 to entry: It is defined by the following formula:
pM
0
Z =
0
ρ RT
00
where R, the universal gas constant, equals 8, 314 4621 J/(mol·K)
4 Symbols
Symbol Definition SI unit
2
A critical flow orifice throat area m
a,b coefficients of the discharge coefficient equation —
C discharge coefficient calculated for the critical flow orifice —
C critical flow function for the gas under ideal conditions dependent on p and T —
*i
C sensitivity coefficient —
i
C critical flow coefficient for the gas under real conditions —
R
dependent on p and T
C molar specific heat capacity of the gas at constant pressure J/(mol·K)
p
C molar specific heat capacity of the gas at constant volume J/(mol·K)
V
d critical flow orifice throat diameter M
N
d conduit diameter upstream of the critical flow orifice M
t
M molar mass of the gas kg/mol
n coefficient n of the critical flow orifice discharge coefficient equation —
p absolute static pressure measured upstream of the critical flow orifice Pa
in
p absolute pressure under normal conditions (101,325 kPa) Pa
n
p absolute static pressure measured downstream of the critical flow orifice
out
p absolute stagnation pressure dependent on p , T and q
0 in in m
mass flow rate kg/s
q
m
q molar flow rate mol/s
n
3
volume flow rate m /s
q
V
*
r critical pressure ratio —
R universal gas constant J/(mol.K)
Re critical flow orifices throat Reynolds number —
T temperature measured upstream of the critical flow orifice K
in
T temperature under normal conditions (273,15 K) K
n
T absolute stagnation temperature dependent on P , T and q K
0 in in m
v speed of gas through the critical flow orifices m/s
g
v velocity of sound at the throat m/s
s
Z compressibility factor under normal conditions (T , P ) —
n n n
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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

Symbol Definition SI unit
Z compressibility factor at P , T —
0 0 0
γ isentropic coefficient dependent on P and T —
η dynamic viscosity calculated for the gas at P and T Pa.s
0 0 0
3
ρ gas density upstream of the critical flow orifice kg/m
in
3
ρ gas density at the critical flow orifices throat kg/m
n
3
ρ gas density at stagnation condition kg/m
0
5 Principle
When passed through a critical orifice at increasing upstream pressure p , the volume flow rate of gas
in
passing through the orifice will increase. When the ratio of the gas pressure downstream p and the
out
gas pressure upstream p of the orifice has reached the critical value, the volume flow rate of the gas
in
becomes independent with respect to p and is proportional to p .
out in
An example of a critical flow orifice is illustrated in Figure 1.
Key
1 inlet
2 critical flow orifice throat diameter (d )
N
3 direction of flow
4 conduit diameter upstream of the critical flow orifice (d )
t
5 outlet
NOTE 1 The temperature T and the pressure p are measured in point 4 and the pressure p in point 5.
in in out
NOTE 2 The temperature T and the pressure p are calculated in point 2 (see 5.2).
0 0
Figure 1 — Example of a critical flow orifice
For a given gas at constant temperature, the critical pressure ratio, (r*), is:
γ
 
p
 γ −1
2
out
r* =   = (1)
 
 
p γ +1
 
in
 
crit
Different methods for calculating the isentropic coefficient γ are described in A.1.
For monatomic, diatomic and triatomic gases, this critical pressure ratio will be around 0,5, but it is
dependent on pressure and temperature conditions as shown in Table 1.
© ISO 2017 – All rights reserved 5

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

Table 1 — Influence of the pressure and type of gas on the critical pressure ratio
Temperature Pressure Pressure Critical pres-
Gas
T p p sure ratio
in in out
γ = C /C
p V
°C bar bar r*
20,0 2,1 1,0 1,669 7 0,49
Argon 20,0 10,3 5,0 1,681 7 0,49
20,0 20,7 10 1,696 9 0,49
20,0 1,9 1,0 1,401 4 0,53
Nitrogen 20,0 9,5 5,0 1,408 6 0,53
20,0 19,0 10 1,417 7 0,53
20,0 1,8 1,0 1,296 7 0,55
Carbon dioxide 20,0 9,2 5,0 1,322 2 0,54
20,0 18,7 10 1,358 9 0,54
NOTE The γ values were calculated for p = p using the data from the NIST REFPROP V 9.0 database.
out
To prepare calibration gas mixtures, the gas blender mixes the complementary gas flowing at a known
rate out of one or several critical flow orifice(s) and the gas to be diluted flowing out of one or several
critical flow orifice(s). The resulting mixture is generally homogenized in a mixing chamber.
This method is not absolute, as each critical flow orifice system should be calibrated for each gas
used, to obtain optimal accuracy with minimal uncertainty (traceable flow calibration or analytical
comparison). This is because the formula for the volume flow rate of a gas includes its molar mass. If a
different calibration gas is used, a correction factor shall be applied and allowance shall be only made
for its associated uncertainty.
The temperatures of all critical flow orifices shall be the same in order to avoid any effects on the
flow rates.
To obtain a flow rate below 10 l/min, the throat diameter should be less than 0,2 mm.
6 Calculation of mass flow rate and volume flow rate
6.1 General
Mass and volume flow rates in critical flow orifices are directly proportional to stagnation pressure
upstream of critical flow orifices and inversely proportional to the square root of absolute stagnation
temperature.
The flow rates through a critical flow orifice running at sonic conditions (below critical pressure ratio)
can be calculated under ideal and real conditions using ISO 9300. The flow rate in real conditions is
given in Annex B and examples of the calculation of the flow rates under ideal and real conditions in
Annex C.
To calculate mass flow rate under real conditions, additional parameters shall be taken into account
(viscosity, surface roughness, flow field curvature, installation conditions). Due to the complexity for
calculating the flow rate in real conditions, this document presents the flow rate calculation in ideal
conditions only.
The calculation using ideal and real conditions can result in flow rates which could be different. This
could influence the accuracy of the generated gas mixtures. By calibrating the device with each used
gas, this effect will become negligible.
The conversion of mass flow rate to volume flow rate is presented in 6.2.2.
6 © ISO 2017 – All rights reserved

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SIST EN ISO 6145-6:2017
ISO 6145-6:2017(E)

6.2 Calculation under ideal conditions
6.2.1 Calculation of mass flow rate
The mass flow rate of a gas through a critical flow orifice under ideal conditions is given by Formula (2):
AC p
*
i 0
q = (2)
m
TR
0
M
where
is the mass flow rate;
q
m
A is the critical orifice throat area;
is the critical flow function for the gas, calculated as follows:
C
*
i
γ +1
 γ −1
2
C = γ (3)
*  
i
γ +1
 
R is the universal gas constant;
M is the molar mass of the gas.
NOTE 1 The critical flow orifice throat area is dependent on the critical flow orifice thermal expansion
coefficient. It is consequently advisable to use the critical flow orifice within the manufacturer’s stated
temperature range.
NOTE 2 At under a millimetre, getting a dimensional measurement of the throat diameter becomes particularly
difficult. This makes it difficult to obtain an accurate throat area figure.
γ is the isentropic coefficient which can be calculated using different methods described in Annex A:
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