ISO/TS 3567:2005

TECHNICAL ISO/TS
SPECIFICATION 3567
First edition
2005-08-01

Vacuum gauges — Calibration by direct
comparison with a reference gauge
Manomètres — Étalonnage par comparaison directe avec un
manomètre de référence




Reference number
ISO/TS 3567:2005(E)
©
ISO 2005

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ISO/TS 3567:2005(E)
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ISO/TS 3567:2005(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms . 3
5 General principle. 3
6 Requirements . 4
7 Calibration . 7
8 Calibration certificate . 10
Annex A (informative) Example of possible calibration system set-up. 11
Annex B (informative) Problems in practice. 12
Bibliography . 15

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ISO/TS 3567:2005(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
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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.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of normative document:
⎯ an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
⎯ an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
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/TS 3567 was prepared by Technical Committee ISO/TC 112, Vacuum technology.
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ISO/TS 3567:2005(E)
Introduction
The purpose of this Technical Specification is to establish the physical, technical and metrological conditions
necessary for adequately disseminating the pressure scale in the vacuum regime by calibration with a
reference gauge. It is assumed that the user will be familiar with the general procedures of vacuum generation
and measurement in the vacuum ranges considered.

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TECHNICAL SPECIFICATION ISO/TS 3567:2005(E)

Vacuum gauges — Calibration by direct comparison with a
reference gauge
1 Scope
This Technical Specification lays down the physical, technical and metrological conditions to be fulfilled when
calibrations of vacuum gauges are performed by direct comparison with a reference gauge. From the
conditions described can be deduced how to design an apparatus that can perform vacuum gauge
calibrations in an adequate manner.
The vacuum gauges to be calibrated can be of any kind. Many types of gauges consist of several parts.
Typically, these are gauge head, cable, operational device and signal read out. This whole set is considered
as the unit that has to be calibrated. Whereas, if only the gauge head (i.e. that part of the vacuum gauge
directly exposed to the vacuum) is calibrated, all set-ups and conditions would have to be recorded such that
the user of the calibrated gauge head would be able to perform the measurements in the same manner as
during the calibration.
The reference gauge is either a calibrated gauge traceable to a vacuum primary or national standard (normal
case), with a calibration certificate according to ISO/IEC 17025, or an absolutely measuring instrument (rare
case), traceable to the SI units and to which a measurement uncertainty can be attributed.
This Technical Specification does not give guidance on how to treat special types of vacuum gauges, be they
reference standards or units under calibration, and it is intended that such guidance be given in other
technical specifications.
The pressure range for calibrations treated in this Technical Specification depends on the realised design of
−6
the calibration apparatus and on the type of reference gauge. The range varies in its limits from 10 Pa to
110 kPa.
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/IEC 17025:2005, General requirements for the competence of testing and calibration laboratories
ISO/IEC Guide to the expression of uncertainty in measurement (GUM), BIPM, IEC, IFCC, ISO, IUPAC,
IUPAP, OIML, 1st edition 1993, corrected and reprinted in 1995
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
primary standard
standard designated or widely acknowledged as having the highest metrological qualities and whose value is
accepted without reference to other standards of the same quantity in the same range
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ISO/TS 3567:2005(E)
3.2
national standard
standard recognized by a national decision to serve, in a country, as a basis for assigning values to other
standards of the quantity concerned
3.3
reference standard
standard generally having the highest metrological quality available at a given location or in a given
organization, from which measurements made there are derived
3.4
vacuum gauge
instrument for measuring gas or vapour pressure that is less than the prevailing atmospheric pressure
NOTE 1 Some types of vacuum gauge commonly in use do not measure a pressure directly, but measure some other
physical quantity which, under specific conditions, is related to pressure.
NOTE 2 For terms and definitions of the various vacuum gauges in use, see ISO 3529-3.
3.5
gauge head
part of a vacuum gauge exposed to the vacuum whose pressure has to be measured
NOTE Gauge heads that include their operational device are usually called transmitters.
3.6
operational device
part of a vacuum gauge that operates the gauge head and/or delivers the signal related to pressure
3.7
unit under calibration
UUC
vacuum gauge to be calibrated
3.8
entrance flange
flange by which the unit under calibration or the reference gauge is connected to the calibration chamber
3.9
calibration chamber
vacuum chamber that serves as a common vacuum medium for the reference gauge and unit under
calibration
3.10
entrance mouth
opening in the calibration chamber which leads to a unit under calibration, reference gauge or any other part
of the calibration system
3.11
calibration gas
gas species or mixture that is used to change the pressure in the calibration chamber
3.12
sorption
taking up of a gas or vapour by a solid or liquid
3.13
desorption
liberation of gases or vapours sorbed by a material
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ISO/TS 3567:2005(E)
3.14
outgassing rate
rate at which molecules and atoms desorb from a material exposed to a vacuum
3.15
total pressure
sum of pressures of all the components of a gaseous mixture
NOTE A vacuum is usually measured as the absolute pressure of gas prevalent in an enclosed chamber, expressed
5 2
in pascals (Pa) or millibars (mbar): 1 mbar = 100 Pa; 1 bar = 0,1 MPa = 10 Pa; 1 MPa = 1 N/mm .
3.16
residual pressure
lowest pressure that can be reached in the calibration chamber, typically after 24 h of pumping
NOTE The residual pressure depends, among others things, on the bake-out condition of the calibration chamber.
3.17
base pressure
pressure in the calibration chamber that exists either before gas is admitted into the calibration chamber for
calibration, or later, after the gas inlet has been valved off for some time
NOTE The base pressure can be higher than the residual pressure, but cannot be lower.
4 Symbols and abbreviated terms
D diameter of cylinder, expressed in millimetres (mm)
e error of reading
P total vacuum pressure, expressed in pascals (Pa) or millibar (mbar)
P base pressure, expressed in pascals (Pa) or millibar (mbar)

0
p calibration pressure, expressed in pascals (Pa) or millibar (mbar)

cal
p indicated pressure, expressed in pascals (Pa) or millibar (mbar)

ind
p residual pressure, expressed in pascals (Pa) or millibar (mbar)

res
q outgassing rate, expressed in pascal litres per second (Pa ⋅ L/s), pascal cubic metres per second

out
3
(Pa ⋅ m /s) or millibar litres per second (mbar ⋅ L/s)
3
S effective pumping speed — effective litres per second (L/s) or cubic metres per second (m /s)

eff
volume flow rate into pump
u standard uncertainty
U expanded uncertainty
CF correction factor
UUC unit under calibration
5 General principle
The UUC is connected to the same calibration chamber as the reference gauge.
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ISO/TS 3567:2005(E)
The idea of the calibration of a vacuum gauge (UUC) by comparison with a reference gauge is to expose the
entrance flange of the UUC and entrance flange of the reference gauge to the same density and velocity
distribution of the calibration gas molecules. The same density and velocity distribution of the calibration gas
molecules also means the same pressure at the two locations, but not vice versa. Since there are many types
of vacuum gauge that do not measure pressure — but instead, for example, gas density or the impingement
rate of gas molecules — the above requisite is both necessary and more stringent than only calling for equal
pressures at the two entrance flanges.
The gas density (pressure) in the calibration chamber can be varied and the gauge readings of the UUC can
be compared with the pressures indicated by the reference gauge.
From this general principle are deduced the requirements (Clause 6) for the design of the calibration
apparatus.
6 Requirements
6.1 Design of calibration chamber
The chamber shall be designed to ensure that the distribution of gas in the measuring volume is sufficiently
uniform in space and stable in time.
In addition, the material of the calibration chamber shall be chosen such that the residual pressure, p ,
res
determined by the effective pumping speed, S (effective volume flow rate into pump), and the total
eff
outgassing rate in the calibration chamber, q (absence of leaks), is low enough to perform the calibrations.
out
See Equation (1) and 6.3.
q
out
p = (1)
res
S
eff
In detail, the calibration chamber shall be designed and operated as follows. However, design criteria a) to e)
may be disregarded when the minimum pressures to be realised in the vacuum chamber are larger than
100 Pa and static pressures only (see 7.1) are established.
a) The calibration chamber shall have a volume of at least twenty times the total volume of all the gauges
and associated plumbing connected to it.
b) The shape of the calibration chamber (see Figure 1) shall be cylinder-symmetrical to at least one axis.
A sphere is ideal, but two symmetrical domes, each a part of a sphere and attached to on another, or
cylinders, are equally possible. Where a cylinder is used, its overall length shall be within one and two
times its diameter, and domed ends are recommended.
c) The centre of the cross-sectional area of the pumping outlet and the gas inlet (if applicable) shall lie on
the same cylindrical axis of symmetry of the calibration chamber. The gas inlet may be positioned
between pump outlet and pump system (see 6.3), in which case there is no need to have the gas inlet on
the axis of symmetry.
d) All entrance mouths and their respective flanges to which either the UUCs or the reference gauges are to
be connected shall be on a common equatorial plane, perpendicular to the cylindrical axis of symmetry
chosen for the pumping outlet.
Where a cylinder is used, it is recommended that this equatorial plane separate the cylinder into two
halves of equal length. Where a cylinder with a length of 3/2D in relation to its diameter is used (suitable
for pump speed measurements), the gauges may be placed at 1/3rd of the length (D/2) above the bottom
flange.
e) Temperature differences between arbitrary points across the calibration chamber shall be less than 1 K.
Points closer than 5 cm from a heated vacuum gauge head (e.g. ionization gauge) may be disregarded.
f) The spatial [see e)] mean temperature of the calibration chamber shall be (23 ± 3) °C during calibrations,
while the mean temperature should not change by more than 1 K.
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ISO/TS 3567:2005(E)
If the design criteria a) to e) are not fulfilled, the possible correction owing to unequal molecular density and
velocity distribution at the entrance flanges of the reference gauge and UUC (see 7.3) shall be measured and
the uncertainty of the correction term estimated.

Figure 1 — Examples of possible calibration chamber shapes
6.2 Plumbing of gauges to calibration chamber
6.2.1 In order to minimize unbalanced molecular (pressure) distribution from sorption, gauge pumping and
outgassing etc., the tubing connecting the calibration chamber and the gauges shall be as short as possible
and shall have a diameter of at least the open area of the entrance flange of the gauge.
6.2.2 Care shall be taken to ensure that the simultaneous operation of the reference gauges and UUC does
not result in any significant mutual influence of their respective readings in steady operation. An influence on
the order of the uncertainty of the base pressure is acceptable.
NOTE The mutual influence can be checked by observing the reading of a gauge when switching another gauge off
and on.
6.2.3 No significant ambient air flow cooling or heating of the UUC or reference gauge shall be present.
A protective cover could be necessary.
6.3 Vacuum and gas inlet system
6.3.1 The base pressure, p , in the calibration chamber shall be less than one tenth of the lowest pressure,
0
p , realised for a calibration as determined by the reference gauge. The vacuum pump and its tubing to the
cal
calibration chamber shall be sized accordingly.
Lowest uncertainties due to the base pressure effect can be achieved if the value of the base pressure is
below the resolution limit of the UUC and/or reference gauge. It is strongly recommended that a base
pressure lower than the resolution limit of the UUC and/or reference gauge be established, if this resolution
limit is equal to or higher than 1 mPa.
NOTE Where a low residual pressure and base pressure in the calibration chamber is required, it could be necessary
to provide heating of the chamber to accelerate the removal of sorbed gases or vapours from the chamber walls.
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ISO/TS 3567:2005(E)
6.3.2 A throughput pumping system that discharges the pumped gas continuously into the atmosphere is
recommended. If no throughput pump is used, it shall be ensured that the effective pumping speed remains
stable throughout the calibration procedure.
6.3.3 Any significant backstreaming of oil into the vacuum chamber shall be excluded.
6.3.4 The base pressure and residual pressure should be monitored using an extra gauge.
6.3.5 The gas inlet may be provided either by admitting gas into the tubing between the calibration chamber
and pump system or separately on the axis of symmetry of the calibration chamber. If the latter option is
chosen, the inlet shall be designed such that each gas molecule coming from the gas inlet has to make at
least one hit with a wall of the calibration chamber or a baffle before it can enter the entrance mouth of the
UUC or reference gauge.
NOTE A valve reducing the effective pumping speed could help to reduce gas consumption. A corresponding rise of
residual pressure has to be considered as a trade-off.
6.4 Calibration gas
For the calibration gas, nitrogen 99,9 % pure or better is recommended. Other gases of the same purity, even
well defined gas mixtures, may also be used for calibration. At pressures below 100 Pa the gases shall not
stick significantly to the surface (sorption). Vapours shall not condense at the conditions in the calibration
chamber.
If the gas purity is relevant for the uncertainty budget, the possibility has to be considered that the reservoir
gas purity might not be present in the calibration chamber, due to desorbing gases between the gas reservoir
and (including) the calibration chamber.
6.5 Thermometers and ambient conditions
Thermometers with an overall expanded uncertainty (k=2) of less than to or equal 0,5 K shall be used. The
temp
...