EN ISO 6145-1:2008

SLOVENSKI STANDARD
01-oktober-2008
$QDOL]DSOLQRY3ULSUDYDNDOLEUDFLMVNHSOLQVNH]PHVL]XSRUDERGLQDPLþQLK
YROXPHWULþQLKPHWRGGHO.DOLEUDFLMVNHPHWRGH,62
Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods
- Part 1: Methods of calibration (ISO 6145-1:2003)
Gasanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-
volumetrischen Verfahren - Teil 1: Kalibrierverfahren (ISO 6145-1:2003)
Analyse des gaz - Préparation des mélanges de gaz pour étalonnage à l'aide de
méthodes volumétriques dynamiques - Partie 1: Méthodes d'étalonnage (ISO 6145-
1:2003)
Ta slovenski standard je istoveten z: EN ISO 6145-1:2008
ICS:
71.040.40 Kemijska analiza Chemical analysis
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 6145-1
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2008
ICS 71.040.40
English Version
Gas analysis - Preparation of calibration gas mixtures using
dynamic volumetric methods - Part 1: Methods of calibration
(ISO 6145-1:2003)
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 1:
dynamiques - Partie 1: Méthodes d'étalonnage (ISO 6145- Kalibrierverfahren (ISO 6145-1:2003)
1:2003)
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 6145-1:2008: E
worldwide for CEN national Members.

Contents Page
Foreword.3

Foreword
The text of ISO 6145-1:2003 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 6145-1:2008 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 6145-1:2003 has been approved by CEN as a EN ISO 6145-1:2008 without any modification.

INTERNATIONAL ISO
STANDARD 6145-1
Second edition
2003-11-15
Gas analysis — Preparation of calibration
gas mixtures using dynamic volumetric
methods —
Part 1:
Methods of calibration
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage
à l'aide de méthodes volumétriques dynamiques —
Partie 1: Méthodes d'étalonnage

Reference number
ISO 6145-1:2003(E)
©
ISO 2003
ISO 6145-1:2003(E)
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ii © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Calibration methods . 2
4.1 General. 2
4.2 Description of primary or potentially primary measuring devices . 4
4.3 Measurements on the final mixture. 12
5 Techniques for preparation of gas mixtures calibrated by the methods described in
Clause 4. 13
5.1 General. 13
[3]
5.2 Volumetric pumps (see ISO 6145-2 ) . 15
[4]
5.3 Continuous injection (see ISO 6145-4 ). 15
[5]
5.4 Capillary (see ISO 6145-5 ). 15
[6]
5.5 Critical orifices (see ISO 6145-6 ). 16
[7]
5.6 Thermal mass flow controllers (see ISO 6145-7 ). 16
[8]
5.7 Diffusion (see ISO 6145-8 ) . 16
[9]
5.8 Saturation (see ISO 6145-9 ). 17
[10]
5.9 Permeation (see ISO 6145-10 ). 17
Annex A (normative) Volume measurement by weighing the water content. 19
Annex B (informative) Description of secondary devices which need calibration against primary
devices . 23
Bibliography . 32

ISO 6145-1:2003(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6145-1 was prepared by Technical Committee ISO/TC 158, Analysis of gases.
This second edition cancels and replaces the first edition (ISO 6145-1:1986), in which the estimated
uncertainties in the calibration methods and techniques have now been combined in a square-root sum-of-
squares manner to form the relative combined standard uncertainty. In comparison with the previous edition
the periodic injection has been deleted (limited application).
ISO 6145 consists of the following parts, under the general title Gas analysis — Preparation of calibration gas
mixtures using dynamic volumetric methods:
 Part 1: Methods of calibration
— Part 2: Volumetric pumps
— Part 4: Continuous injection methods
— Part 5: Capillary calibration devices
— Part 6: Critical orifices
— Part 7: Thermal mass-flow controllers
— Part 9: Saturation method
— Part 10: Permeation method
Diffusion will be the subject of a future Part 8 to ISO 6145. Part 3 to ISO 6145, entitled Periodic injections into
a flowing gas, has been withdrawn.
iv © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
Introduction
This part of ISO 6145 is one of a series of standards which describes the various dynamic volumetric methods
used for the preparation of calibration gas mixtures.
In dynamic volumetric methods a gas, A, is introduced at volume or mass flow rate q into a constant flow rate
A
q of a complementary gas B. Gas A can be either a pure calibration component, i, or a mixture of i in A.
B
The volume fraction, ϕ of i in the final calibration gas mixture is given in the following equation:
i,M

q
A
ϕϕ=

ii,M ,A
qq+
AB
where ϕ is the volume or mass fraction of component, i, in the pre-mixed gas A, and is already known from
i,A
its method of preparation. It is assumed that in this equation, ϕ , the concentration of component, i, in gas B,
i,B
is zero.
The introduction of gas A can be continuous (e.g. permeation tube) or pseudo-continuous (e.g. volumetric
pump). A mixing chamber should be inserted in the system before the analyser and is particularly essential in
the case of pseudo-continuous introduction. The flow rate of component A is measured either directly in terms
of volume or mass, or indirectly by measuring the variation of a physical property.
The dynamic volumetric preparation techniques produce a continuous flow rate of calibration gas mixtures into
the analyser but do not generally allow the build-up of a reserve by storage under pressure.
The main techniques used for the preparation of the mixtures are:
a) volumetric pumps;
b) continuous injection;
c) capillary;
d) critical orifices;
e) thermal mass-flow controllers;
f) diffusion;
g) saturation;
h) permeation;
i) electrochemical generation.
In all cases, and most particularly if very dilute mixtures are concerned, the materials used for the apparatus
are chosen as a function of their resistance to corrosion and low absorption capacity (usually glass, PTFE or
stainless steel). It should, however, be pointed out that the phenomena are less important for dynamic
volumetric methods than for static methods.
Numerous variants or combinations of the main techniques can be considered and mixtures of several
constituents can also be prepared by successive operations.
ISO 6145-1:2003(E)
Some of these techniques allow calculation of the final concentration of the gas mixture from basic physical
information (e.g. mass rates of diffusion, flow through capillaries). However, since all techniques are dynamic
and rely on stable flow rates, this part of ISO 6145 emphasizes calibration of the techniques by measurement
of the individual flow rates or their ratios, or by determination of the composition of the final mixture.
The uncertainty of the composition of the calibration gas mixture is best determined by comparison with a gas
mixture traceable to international standards. Certain of the techniques which may be used to prepare a range
of calibration gas mixtures may require several such traceable gas mixtures to verify their performance over
that range. The dynamic volumetric technique used has a level of uncertainty associated with it. Information
on the final mixture composition depends both on the calibration method and on the preparation technique.

vi © ISO 2003 — All rights reserved

INTERNATIONAL STANDARD ISO 6145-1:2003(E)

Gas analysis — Preparation of calibration gas mixtures using
dynamic volumetric methods —
Part 1:
Methods of calibration
1 Scope
This part of ISO 6145 specifies the calibration methods involved in the preparation of gas mixtures by dynamic
volumetric techniques. It also gives a brief presentation of a non-exhaustive list of examples of dynamic
volumetric techniques which are described in more detail in other parts of ISO 6145.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6142, Gas analysis — Preparation of calibration gas mixtures — Gravimetric method
ISO 6143, Gas analysis — Comparison methods for determining and checking the composition of calibration
gas mixtures
ISO 7504, Gas analysis — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7504 and the following apply.
3.1
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
NOTE 1 Values of the individual statistical uncertainties found in some methods and techniques in this part of
ISO 6145 are combined with the values of systematic uncertainties that also occur in a square-root sum-of-squares
manner to provide a relative combined uncertainty, or in some cases as a relative expanded uncertainty by application of
the coverage factor “2”.
NOTE 2 In keeping with Reference [1] of the Bibliography, the uncertainty of the composition of a mixture is expressed
as a relative expanded uncertainty.
ISO 6145-1:2003(E)
4 Calibration methods
4.1 General
4.1.1 The uncertainty in the composition i,M of a component i of a calibration mixture M depends at any
time on
a) the uncertainty of the calibration method,
b) the frequency with which it is applied,
c) the stability of the control devices involved in the dynamic preparation technique.
To assess the uncertainty of the whole procedure, possible instantaneous variations and drift of the principle
parameters of the technique during the calibration procedure shall be considered.
According to the preparation technique for the gas mixtures used, calibration can be carried out by one of the
following methods:
 measurement of flow rate (mass or volume);
 comparison method;
 tracer method;
 direct chemical analysis.
Table 1 shows the applicability of each calibration method to the different preparation techniques.
Table 1 — Calibration methods applicable to the preparation techniques
Calibration methods
Preparation techniques
Comparison with Flow rate
a
Tracer Direct analysis
a a
ISO 6143  measurement
Volumetric pumps + — +
Continuous injection + — +
Capillary + + +
May be applicable;
Critical orifice + + +
depends on nature
Thermal mass flow controllers + + +
of components
Diffusion + — —
Saturation + — —
Permeation + — —
a
The pluses refer to the measurement of a volume flow. In principle, flow rate measurement can also be performed for continuous
injection methods, diffusion methods and permeation methods. Here, mass flows are measured rather than volume flows. For diffusion
and permeation tubes the mass flow may be measured continuously using a suspension balance.

4.1.2 In general, the principles of the methods fall into two categories, as follows.
 Principles in which the flow rates of component gases are measured either by volume or by mass and in
which the concentration in the final mixture is calculated from the flow rate. Different techniques may be
used for the individual components of a mixture and these may be calibrated by different methods. The
principle of measurements of individual flow rates, however, remains.
 Principles which operate directly on the final mixtures.
2 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
Since different principles are involved, they are given separately under each individual method.
Since the calibration methods rely upon different principles and the equipment used for the realization of the
gas flow rates is different, different units can be used to express the contents.
For calibrations using the comparison method, the content is expressed as a mole fraction or mole
concentration because most of the calibration gas mixtures used for the comparison, if possible, are described
in this way.
Using techniques based on volume flow rate leads in the first instance to volume fractions. Recalculation of
these data to mole fractions is possible but leads to an increase in the uncertainty because of the uncertainty
of the density and molar-volume data. In this case, the expression in volume fractions is preferred.
Calibration by the gravimetric method gives mass fractions for the contents of components in gas mixtures.
These can be recalculated to mole fractions by dividing by the respective atomic or molar masses. Expression
in mole fraction is therefore preferred.
Under some circumstances, the total flow rate cannot be taken as the sum of two individual flow rates q and
A
q which have been measured separately. These problems can be caused by deviations from the ideal gas
B
laws or by changes in conditions such as backpressure or viscosity resulting from the blending of the two flow
rates. Deviations from ideal behaviour can be predicted with reasonable accuracy and other uncertainties can
be minimized by careful attention to apparatus design.
4.1.3 Flow rate measurement is normally carried out using one of the following:
a) primary devices, based on absolute principles, for example:
 gravimetric method;
b) methods which may be considered as potentially primary when the volume of the device is determined by
weighing the relevant volume of water, or another suitable liquid of higher density:
 mercury-sealed piston,
 bell-prover;
c) many other devices available for the measurement of volume flow, some of which are listed below
(calibration of these devices is carried out by using one of the above primary or potentially primary
methods):
 soap-film meter,
 wet-gas meter,
 thermal mass flow sensor,
 variable area flow meter.
The soap-film and mercury-sealed piston flow meters share a common principle, i.e. that of timing the travel of
a soap bubble or piston between carefully defined points either electronically or by observation, for example
by means of a cathetometer. The volume between these points can be determined by filling with water, which
is subsequently weighed (see Annex A).
The wet-gas meter is an integrating device which indicates the total volume of gas that has been passed
through it (the dry-gas meter, familiar from the domestic environment, has a similar integrating property but
has not been included because it is less accurate). The variable area flow meter is a continuously indicating
device. The thermal mass flow sensor measures mass flow rate as a function of heat flux.
NOTE These devices are fully described in Annex B.
ISO 6145-1:2003(E)
4.1.4 Calibration of these flow-rate measuring devices is carried out using one of the primary or potentially
primary methods:
a) gravimetric method;
b) mercury-sealed piston;
c) bell prover.
The gravimetric method measures the mass of gas, which has flowed at a constant rate for a defined time
through the device to be calibrated. The mercury-sealed piston drives a defined volume of gas over a
measured time period into the device to be calibrated. The bell prover is a device for creating a constant and
defined flow rate of gas, acting as a mechanically driven gasholder.
The bell prover and the gravimetric method may be used directly, where appropriate, to calibrate the various
preparation techniques, but the information is more commonly transferred via one of the flow-rate measuring
devices.
4.2 Description of primary or potentially primary measuring devices
4.2.1 Gravimetric method
4.2.1.1 Principle
Gas from a cylinder flows at a constant rate through the device to be calibrated. This is continued for a
sufficiently long period for the loss of mass from the cylinder to be accurately measured. The procedure
provides data in terms of mass flow, which can then be converted to molar flow rate or, with assessed
uncertainty, to a volume flow rate.
The gas cylinder and flow-rate measuring device are set up as shown in Figure 1. The cylinder (1) is fitted with
a pressure regulator (2) on the outlet of which a precision needle valve (3) and shut-off valve (4) lead to the
device to calibrated (5). The dead volume between the needle valve outlet and the shutoff valve is minimized
by using the smallest size of tubing and fittings commensurate with the desired gas flow rate. The temperature
and pressure of the gas are measured at the inlet to the device to be calibrated.
The cylinder valve is opened, the pressure regulator is set to a value of, e.g. 100 kPa (1 bar) gauge, and the
needle valve is adjusted to the desired flow rate. When conditions are seen to be steady, the shut-off valve is
closed and the pipe-work is disconnected at the outlet of this valve. The cylinder, regulator, needle valve and
shut-off valve are weighed as a single unit. The pipe-work is reconnected and the shut-off valve is opened to
re-start the flow at the same rate. After the gas has flowed for a period long enough for the mass used to be
measured accurately, the shut-off valve is closed and the cylinder, regulator, needle valve and shut-off valve
weighed as before. During this period, the gas flow is accurately measured by first calculating the volume of
gas from the change in mass, then the flow rate from the volume and the time.
4 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
Key
1 cylinder
2 pressure regulator
3 needle valve
4 shut-off valve
5 device to be calibrated
a
To vent.
Figure 1 — Gravimetric method
ISO 6145-1:2003(E)
4.2.1.2 Uncertainty of measurement
4.2.1.2.1 Uncertainty of weighing
Gravimetric preparation of mixtures is described in ISO 6142. Using the procedures given in ISO 6142, it can
−4
be assumed that the mass of gas used in a test can be weighed to a relative standard uncertainty of 2 × 10
(i.e. 20 g of gas taken from a 10 kg cylinder whose mass before and after the test can be measured with an
−3
−4
uncertainty of 2 mg, giving a relative standard uncertainty of 22/20 ×10 , i.e. 1,4 × 10 ).
4.2.1.2.2 Uncertainty with unstable flows
This uncertainty can be neglected provided the cylinder and its flow-rate control devices are both pressurized
with gas to the same degree for both weighings. However, when the gas is shut off before weighing, the pipe-
work between the needle valve and the shutoff valve becomes pressurized to the value set on the regulator,
and this will cause a surge when the gas flow rate restarts. The uncertainty caused by this surge is the
amount of gas required to pressurize the volume between the needle valve and the shut-off valve relative to
the amount of gas having flowed. If 2 ml of dead-space is pressurized to 1 bar gauge in a test in which 20 g of
−5
methane flows, the standard uncertainty is 7 × 10 .
To reduce pressure surge effects which can cause oscillations of flow, stabilize the gas flow before taking any
readings. This avoids any uncertainty.
4.2.1.2.3 Uncertainty on conversion of mass to volume
The temperature, pressure, compression (Z) factor and molar mass of the gas, all affect the uncertainty on
conversion of mass to volume. Measurement of temperature with an uncertainty of 0,05 °C and pressure to
−4 −4
10 Pa (0,1 mbar) represents relative standard uncertainties of 1,7 × 10 and 10 , respectively. Compression
−4
factors are commonly quoted to four decimal places, which implies an uncertainty of 10 , and molar masses
are known with sufficient accuracy not to contribute significantly. The relative standard uncertainty is therefore
−4
not greater than 2,2 × 10 .
4.2.1.2.4 Uncertainty due to flow rate variation
If the device to be calibrated measures either instantaneous flow rates or volumes which are small by
comparison with the volume taken from the cylinder, then variations in flow rate are a contribution to the
uncertainty.
A high quality pressure regulator and needle valve should ensure a flow rate constancy of 0,2 % relative, apart
from the initial flow surge (see 4.2.1.2.2), but should be checked for each installation. This level of flow-rate
−3
control represents a relative standard uncertainty of 2 × 10 .
4.2.1.2.5 Uncertainty of time measurement
The time during which the gas flows from the cylinder can be measured by an electronic timer with a relative
−4
standard uncertainty of 2 × 10 .
NOTE The uncertainty of the time measurement generally depends on the discharge time. The timer can be very
accurate, but if "hand" clocking is used to start and stop the timer the uncertainty in the time measurement is in the order
of ± 0,2 s, requiring a 1 000 s discharge time to reach the stated relative uncertainty.
4.2.1.2.6 Relative combined standard uncertainty
The combination of the standard uncertainties described in 4.2.1.2.1 to 4.2.1.2.5 is as follows:
−4
 weighing 2 × 10
−5
 flow transients 7 × 10
6 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
−4
 mass to volume 2,2 × 10
−3
 flow rate variation 2 × 10
−4
 timing 2 × 10
−3
 relative combined standard uncertainty 2,0 × 10
4.2.2 Mercury-sealed piston flow meter
4.2.2.1 Principle
A glass measuring tube (see Figure 2) of known diameter and uniformity is set vertically in an insulated box
fitted with temperature control. The temperature is maintained constant to within ± 0,02 °C.
The measuring tube is divided into a number of sections by photoelectric cells serving as sensors, and the
actual volume between two adjacent photoelectric cells is determined by filling with water and weighing (see
Annex A). Greater accuracy is achieved in the calibration if a liquid of higher density is used.
A constant flow moves a frictionless piston with a constant speed upwards. The displaced volume can be
estimated from the dimensions of the tube or measured with reference to the water calibration.
The piston, made of plastics (e.g. PVC) or glass contains a horizontal, circular groove, filled with mercury. The
purity of the mercury is such as to ensure that the piston does not stick in operation. The use of triple distilled
mercury is recommended.
The piston is allowed to attain a constant speed before time measurement is started at Sensor 1.
Depending on the flow rate and the tube size, time measurement is stopped when the piston passes Sensor 2
or Sensor 3. Sensors may be of the reflection type because of the high reflectance of the mercury ring.
Because of a high back-pressure caused by the weight of the piston, the measured pressure difference is
approximately from 0,1 kPa (1 mbar) up to 1 kPa (10 mbar).
The measuring sequence starts by closing Side A of the 3-way valve (see Figure 2). As soon as the piston
passes Sensor 1, time measurement starts; it stops after the piston passes the next sensor. The three-way
valve resets its position and the piston falls down on the spring. The flow meter is then ready to restart.
4.2.2.2 Uncertainty of measurement
4.2.2.2.1 Influence of temperature variation
−6 −1
The measuring tube is made of borosilicate glass having a coefficient of linear expansion of 3,3 × 10 K .
The result is that, taking into account the control of temperature to ± 0,02 °C, there are relative standard
−7 −5
uncertainties in the volume of the tube of approximately 2 × 10 and in the volume of gas of 7 × 10 .
NOTE The user should be aware that there can be a temperature gradient if flow sensors are heated to operate (e.g.
MFCs) in the upstream system. The expansion effects on glass can be neglected.
4.2.2.2.2 Correction for pressure differences and piston pressure
Correction for pressure differences of the flow device between calibration (p ) and use (p ) is made using
cal use
the factor (p + p ) / p , in which the piston pressure, p , generally takes values between 0,1 kPa
cal piston use piston
and 1 kPa.
Assuming the absolute pressure to be measurable with a relative uncertainty of ± 0,1 %, and the piston
pressure to be measurable with an uncertainty of less than ± 10 Pa, then the relative uncertainty of the
−3
pressure correction is 1,4 × 10 .
ISO 6145-1:2003(E)
Key
1 photoelectric cell Sensor 2 (first volume)
2 photoelectric cell Sensor 3 (second volume)
3 piston
4 photoelectric cell Sensor 1 (start counting)
5 pressure sensor
6 spring
7 3-way valve (Sides A, B, C)
a
Flow in.
b
To vent.
Figure 2 — Mercury-sealed piston flow meter
4.2.2.2.3 Diffusion across the piston
The construction of the mercury-sealed piston does not provide for the possibility of keeping the same
composition of the gas on both sides. Although diffusion along the mercury seal is still possible, the effect is
considered negligible in general practice.
4.2.2.2.4 Relative combined standard uncertainty
The combination of the standard uncertainties described in parts 4.2.2.2.1. to 4.2.2.2.3 is as follows:
−5
 temperature 7 × 10
−3
 pressure 1,4 × 10
 diffusion across piston 0
−3
 relative combined standard uncertainty 1,4 × 10
8 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
4.2.3 Bell prover
4.2.3.1 General
A gas flow measurement shall be provided by displacing a defined volume of gas at constant flow from the
holder of a bell prover within a measured time period.
4.2.3.2 Principle
A schematic diagram of a bell prover is given in Figure 3 and shows the bell (1) in a stationary tank (2) filled
with the sealing liquid (3). The measuring scale (4) is used to take readings of the position of the bell, which is
supported on a chain passing over rollers (5) and balanced by the counterweight (6).

Key
1 bell 4 measuring scale
2 stationary tank 5 rollers
3 sealing liquid 6 counterweight
Figure 3 — Schematic diagram of a bell prover
ISO 6145-1:2003(E)
The principle of operation is as follows.
a) The bell is raised and filled with air.
b) A definite volume of air is displaced from the prover by lowering the bell (1) into the stationary tank (2)
while maintaining constant pressure in the conduits. The time interval over which the air is displaced is
measured by a timer. The air flow rate is calculated using the measured values of volume and time
interval.
4.2.3.3 Uncertainty of measurement
4.2.3.3.1 Uncertainty on prover capacity
The volume of the bell prover is determined at various points over the usable range and the uncertainty on
each volume determination is determined at less than 0,5 cm . A best-fit line is drawn through the volume
determinations to provide a calibration graph having a relative standard uncertainty of ± 0,05 %. The volume
discharged from the bell prover is the difference in volume between the start and finish point, giving an
uncertainty of 2 times the relative standard uncertainty in calibration, i.e. ± 0,07 %.
4.2.3.3.2 Uncertainties in the use of the measuring scale
The position of the bell prover is determined using a measuring scale which may be read to better than
0,2 mm. Assuming a change in position of 1 m, the relative standard uncertainty would be ±
−4
( 2 × 0,03/ 3 ) / 1 000 = 0,16 mm in 1 m, or 1,6 × 10 .
4.2.3.3.3 Uncertainty on displacement time interval
The time interval may be electronically measured to better than ± 0,001 s. Assuming a discharge time of 40 s,
−5
the relative uncertainty is ±×( 2 0,001 / 3 ) / 40= 2× 10 .
4.2.3.3.4 Uncertainty on the gas-distributing device
Random variations in the speed of operation of the solenoid valve which starts and stops the gas discharge
should not exceed ± 0,03 s. On a discharge time of 40 s, the relative uncertainty is ± ( 2×=0,03 / 3 ) / 40
−4
6 × 10 .
4.2.3.3.5 Combined uncertainty due to the recalculation of flow rates to reference conditions
These should normally be avoided by carrying out calibrations under the required conditions.
The combination of the standard uncertainties described in 4.2.3.3.1 to 4.2.3.3.4 is as follows:
−4
 capacity 7 × 10
−4
 measuring rule 1,6 × 10
−5
 timing 2 × 10
−4
 distribution 6 × 10
 recalculation 0
−3
 relative combined standard uncertainty 0,9 × 10
This total is the combined uncertainty on the mean flow rate and instability of the flow rate has not been taken
into consideration.
10 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
4.2.4 Measurement of time
Timing is necessary for some of the flow-rate measuring devices. Photoelectric cells fitted to the soap-film flow
meter and mercury-sealed piston flow meter define the upper and lower measuring points between which the
film or piston moves. Similarly a photoelectric cell can register the movement of the pointer of a wet-gas meter
past a particular point on its scale. The shut-off valve for the gravimetric calibration can be linked to a timer. In
all cases, the timer should be an accurate electronic device capable of measuring the time intervals with a
−4
relative standard uncertainty of no greater than 2 × 10 .
4.2.5 Correction for pressure differences
With the exception of the mercury-sealed piston meter (see 4.2.2.2), a correction for pressure differences
between calibration and use of the flow device needs to be applied using a factor p / p . Assuming the
cal use
absolute pressure to be measurable with a relative uncertainty of ± 0,1 %, then the relative uncertainty in the
−3
correction is 1,4 × 10 .
4.2.6 Correction for temperature differences
Correction for temperature differences of the flow device between calibration (T ) and use (T ) is made
cal use
using a factor T / T , where T is the absolute temperature of the flowing gas expressed in kelvins.
use cal
Assuming the temperature measurement to have a relative uncertainty of ± 0,1 %, then the relative
−3
uncertainty in the correction is 1,4 × 10 .
4.2.7 Uncertainty calculation
The relative combined standard uncertainties of the primary calibration methods (see 4.2.1.2.6, 4.2.2.2.4 and
4.2.3.3.5) are given in the first column of Table 2. When this method has been used to calibrate one of the
secondary methods (see Annex B), the contribution has been added under calibration. Individual standard
uncertainties for the measurement and time contributions for each secondary method are included. These
have been combined in a square root sum of squares method to provide a combined uncertainty u for each

c
method.
The uncertainty contributions depend upon the characteristics of the calibration method and the flow-rate
control device. Thus, if a soap-film meter is calibrated by weighing its water content, there are three sources of
uncertainty, since the time taken by the soap-film between the graduation marks has to be measured. If,
however, the measurement gives a continuous indication (variable area flow meter or thermal mass flow
sensor), then once the calibration method flow rate has been established, there is no further need for time
measurement and hence no time measurement uncertainty.
The relative combined standard uncertainties listed in Table 2 relate only to the calibration methods described
in 4.2 and, when used, the flow-rate measuring devices described in Annex B. When a mixture is prepared
using one of the techniques described in subsequent parts of ISO 6145 (see the Bibliography), the relative
standard uncertainties associated with the technique should also be taken into account.
ISO 6145-1:2003(E)
Table 2 — Estimated uncertainties of flow-rate measuring methods (see 4.1.3)
a
Secondary flow-rate measuring device
Primary
Source of
calibration
uncertainty Soap film Wet-gas Variable area Thermal mass
method
flow meter meter flow meter flow sensor
−3 −3 −3 −3
Calibration 2,0 × 10 2,0 × 10 2,0 × 10 2,0 × 10
−4 −4 −2 −4
Gravimetric Measurement 3,3 × 10 5,1 × 10 2,3 × 10 1,0 × 10
−3
−4 −4
u u 2,0 × 10
Time 2,0 × 10 2,0 × 10 0 0
rel
−3 −3 −2 −3
u 2,0 × 10 2,1 × 10 2,3 × 10 2,0 × 10
c
−3 −3 −3 −3
Calibration 1,4 × 10 1,4 × 10 1,4 × 10 1,4 × 10
Mercury-sealed
−4 −4 −2 −4
Measurement 3,3 × 10 5,1 × 10 2,3 × 10 1,0 × 10
piston flow meter
−4 −4 −4 −4
Time 2,0 × 10 2,0 × 10 2,0 × 10 2,0 × 10
−3
u u 1,4 × 10
rel
−3 −3 −2 −3
u 1,5 × 10 1,5 × 10 2,3 × 10 1,4 × 10
c
−3 −3 −3 −3
Calibration 0,9 × 10 0,9 × 10 0,9 × 10 0,9 × 10

−4 −4 −2 −4
Bell prover
Measurement 3,3 × 10 5,1 × 10 2,3 × 10 1,0 × 10
−3 −4 −4 −4 −4
u u 0,9 × 10 Time 2,0 × 10 2,0 × 10 2,0 × 10 2,0 × 10
rel
−3 −3 −2 −3
u 1,0 × 10 1,1 × 10 2,3 × 10 0,9 × 10
c
−5
Calibration 7,7 × 10 — — —
Weighing of volume
−4
Measurement 3,3 × 10 — — —
of water
−4
Time 2,0 × 10 — — —
−5
u u 7,7 × 10
rel
−4
u 3,9 × 10 — — —
c
NOTE The combined uncertainties, u , for the secondary flow-rate measuring device are those which are the best obtainable
c
under controlled conditions.
a
See Annex B.
4.3 Measurements on the final mixture
4.3.1 General
This approach eliminates non-additivity uncertainties, e.g. volume changes on mixing of components.
Calibration of the concentration in the final mixture is carried out as described in 4.3.2 to 4.3.3.
4.3.2 Comparison method
Where possible, the content of the prepared gas mixture shall be verified by comparison with a standard
prepared or certified by an accredited national or international body. The results provided by this verification
shall confirm traceability to that national body within the analytical limits of the comparison method. Use the
comparison method described in ISO 6143.
NOTE This verification also yields information about the accuracy of the technique used to prepare the calibration
gas mixture.
12 © ISO 2003 — All rights reserved

ISO 6145-1:2003(E)
4.3.3 Measurements on the final mixture
4.3.3.1 General
The measurement on the final mixture shall be performed by one of the two following methods:
a) direct chemical analysis; or
b) tracer methods using comparison or direct chemical analysis.
4.3.3.2 Direct chemical analysis
In some cases, an analytical method exists that should be used to determine the amount of component i in the
final mixture without reference to other calibration gas mixtures. The amount of i is determined as the mass or
number of moles. The volume of the mixture used in the analytical procedure shall be measured.
4.3.3.3 Tracer methods
The method relies on previous introduction into gas, A, through another preparation method, of a tracer gas, T.
The gas, A, then contains:
a) a known volume fraction ϕ of tracer gas T;
T,A
b) a known volume fraction ϕ of component i.
i,A
The measurement, either by direct chemical analysis or by a comparison method, of the volume fraction ϕ
TM
of T in the final mixture M, enab
...