ISO/TS 20177:2018
(Main)Vacuum technology — Vacuum gauges — Procedures to measure and report outgassing rates
Vacuum technology — Vacuum gauges — Procedures to measure and report outgassing rates
This document describes procedures to measure outgassing rates from components designed for vacuum chambers and of vacuum chambers as a whole. The outgassing rates are expected to be lower than 10−5 Pa m3 s−1 (10−2 Pa L s−1) at 23 °C and to emerge from devices that are suitable for high or ultra-high vacuum applications. The molecular mass of the outgassing species or vapour is below 300 u. The upper limit 10−5 Pa m3 s−1 of total outgassing rate is specified independent of the size, the total surface area and texture or state of the outgassing material. If a specific outgassing rate (outgassing rate per area) is determined, the area is not a specific surface area including the surface roughness, but the nominal geometrical one. When it is difficult to determine the nominal geometrical surface area of the sample, such as powders, porous materials, very rough surfaces, or complex devices, mass specific outgassing rate (e.g. outgassing rate per gram) is used. For many practical applications, it is sufficient to determine the total outgassing rate. If a measuring instrument, which sensitivity is gas species dependent, is used, the total outgassing rate are given in nitrogen equivalent. In cases, however, where the total outgassing rate is too high, the disturbing gas species is identified, and its outgassing rate is measured in order to improve the sample material. This document covers both cases. Some outgassing molecules can adsorb on a surface with a residence time that is much longer than the total time of measurement. Such molecules cannot be detected by a detecting instrument when there is no direct line of sight. This is considered as a surface effect and surface analytical investigations are more useful than general outgassing rate measurements considered here. Also, molecules that are released from the surface by irradiation of UV light or X-rays, are out of the scope of this document. This document is written to standardize the measurement of outgassing rates in such a way that values obtained at different laboratories and by different methods are comparable. To this end, for any of the described methods, traceability is provided to the System International (SI) for the most important parameters of each method and according to the metrological level. Outgassing rate measurements by mass loss, which were mainly developed for testing of spacecraft and satellite materials, are not gas specific. For acceptable measurement times, mass loss measurements require significantly higher outgassing rates (>10−5 Pa m3 s−1) than typical for high and ultrahigh vacuum components. Also, it is not possible to measure the sample in situ due to the weight of the vacuum chamber, since the balances are not vacuum compatible. For these reasons, mass loss measurements are not considered in this document. It is assumed that the user of this document is familiar with high and ultra-high vacuum technology and the corresponding measuring instrumentation such as ionization gauges and quadrupole mass spectrometers.
Technique du vide — Manomètres à vide — Méthodes de mesurage et de rapport du taux de dégagement de gaz
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© ISO 2018 – All rights reserved
TC /SCISO/TC 112/SC
Date: 2018-06
ISO/TS 20177:2018(E)
ISO/TC /112/SC //WG 2
Secretariat: DIN
Vacuum technology — Vacuum gauges — Procedures to measure and
report outgassing rates
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Document type: Technical Specification
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ISO/TS 20177:2018(E)
COPYRIGHT PROTECTED DOCUMENT
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. + 41 22 749 01 11
Fax + 41 22 749 09 47
copyright@iso.org
www.iso.org
© ISO 2018 – All rights reserved
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ISO/TS 20177:2018(E)
Contents Page
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 3
5 Measurement systems . 4
5.1 General . 4
5.1.1 Overview . 4
5.1.2 Recommendations for systems . 4
5.1.3 Vacuum chambers and pumps . 6
5.1.4 Vacuum gauges . 6
5.1.5 Purity of gases . 6
5.2 Systems applying the throughput method . 7
5.2.1 General . 7
5.2.2 Continuous expansion system as flow comparator . 7
5.2.3 Throughput system with calculated conductance element . 8
5.2.4 Throughput system with measured effective pumping speed . 11
5.2.5 Throughput system with modulated conductance . 13
5.3 Accumulation systems . 14
5.3.1 General . 14
5.3.2 Basic accumulation system . 14
5.3.3 Accumulation system with gas analysis system (extended accumulation system) . 14
6 Measurement procedures . 15
6.1 General . 15
6.2 Recommended sample preparation . 16
6.3 Course and time period of measurement . 17
6.4 Measurement procedures . 17
6.4.1 Procedure with continuous expansion system as flow comparator . 17
6.4.2 Procedure with throughput system with calculated conductance element (pressure
difference system) . 18
6.4.3 Procedure with throughput system with measured effective pumping speed . 20
6.4.4 Procedure with throughput system with modulated conductance . 21
6.4.5 Procedure with accumulation systems . 22
7 Measurement uncertainties . 24
7.1 General . 24
7.2 Continuous expansion system as flow comparator (5.2.2) . 25
7.3 Throughput system with calculated conductance element (5.2.3) . 25
7.4 Throughput system with measured effective pumping speed (5.2.4) . 25
7.5 Throughput system with modulated conductance (5.2.5) . 26
7.6 Basic accumulation system (5.3.2) . 26
7.7 Accumulation system with gas analysis system (5.3.3) . 26
8 Reporting results . 29
Annex A (informative) Schemes of principles of measurement systems . 32
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ISO/TS 20177:2018(E)
Annex B (informative) Applicability and characteristics of the different measurement
system . 39
Annex C (informative) Traceability of the different measurement systems to the SI . 41
Bibliography . 43
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ISO/TS 20177:2018(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 onof the voluntary nature of standards, 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:
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee TC 112, Vacuum technology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO/TS 20177:2018(E)
Introduction
Outgassing from the inner wall of a vacuum chamber and from components in a vacuum chamber limit
the achievable lowest pressure in a vacuum system and its cleanliness. The lowest achievable pressure is
usually important in research facilities as accelerators, in facilities that need to ensure a clean surface,
e.g. molecular beam epitaxy, or in devices that need to ensure high vacuum without pump for long times,
such as transmitters or X-ray tubes, medical instruments, surface analytical instrumentation or insulation
panels. Cleanliness of a vacuum, i.e. the absence or sufficiently low partial pressure of specific gas species
or vapours, is important in many different industrial applications such as coating, EUV lithography,
catalysis, drying processes in the pharmaceutical or food industry but also in accelerators, fusion
reactors, etc. The measurement of outgassing rates is therefore an important tool of quality assurance in
vacuum technology. This document recommends well-defined procedures with the possibility of getting
traceability of the results of an outgassing rate measurement.
Annex A lists schemes of principles of measurement systems.
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TECHNICAL SPECIFICATION
Vacuum technology — Vacuum gauges — Procedures to measure and
report outgassing rates
1 Scope
This document describes procedures to measure outgassing rates from components designed for vacuum
chambers and of vacuum chambers as a whole. The outgassing rates are expected to be lower than
−5 3 −1 −2 −1
10 Pa m s (10 Pa L s ) at 23 °C and that theto emerge from devices that are suitable for high or
ultra-high vacuum applications. The molecular mass of the outgassing species or vapour is below 300 u.
−5 3 −1
The upper limit 10 Pa m s of total outgassing rate is specified independent of the size, the total surface
area and texture or state of the outgassing material. If a specific outgassing rate (outgassing rate per area)
is determined, the area is not a specific surface area including the surface roughness, but the nominal
geometrical one. When it is difficult to determine the nominal geometrical surface area of the sample,
such as powders, porous materials, very rough surfaces, or complex devices, mass specific outgassing
rate (e.g. outgassing rate per gram) is used.
For many practical applications, it is sufficient to determine the total outgassing rate. If a measuring
instrument, which sensitivity is gas species dependent, is used, the total outgassing rate are given in
nitrogen equivalent. In cases, however, where the total outgassing rate is too high, the disturbing gas
species is identified, and its outgassing rate is measured in order to improve the sample material. This
document covers both cases.
Some outgassing molecules can adsorb on a surface with a residence time that is much longer than the
total time of measurement. Such molecules cannot be detected by a detecting instrument when there is
no direct line of sight. This is considered as a surface effect and surface analytical investigations are more
useful than general outgassing rate measurements considered here. Also, molecules that are released
from the surface by irradiation of UV light or X-rays, are out of the scope of this document.
This document is written to standardize the measurement of outgassing rates in such a way that values
obtained at different laboratories and by different methods are comparable. To this end, for any of the
described methods, traceability is provided to the System International (SI) for the most important
parameters of each method and according to the metrological level.
Outgassing rate measurements by mass loss, which were mainly developed for testing of spacecraft and
satellite materials, are not gas specific. For acceptable measurement times, mass loss measurements
−5 3 −1
require significantly higher outgassing rates (>10 Pa m s ) than typical for high and ultrahigh vacuum
components. Also, it is not possible to measure the sample in situ due to the weight of the vacuum
chamber, since the balances are not vacuum compatible. For these reasons, mass loss measurements are
not considered in this document.
It is assumed that the user of this document is familiar with high and ultra-high vacuum technology and
the corresponding measuring instrumentation such as ionization gauges and quadrupole mass
spectrometers.
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.
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ISO/TS 20177:2018(E)
ISO 3529 (all parts), Vacuum technology — Vocabulary
ISO 14291, Vacuum gauges — Definitions and specifications for quadrupole mass spectrometers
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO/TS 20175, Vacuum technology — Vacuum gauges — Characterization of quadrupole mass
spectrometers for partial pressure measurement
ISO 27894, Vacuum technology — Vacuum gauges — Specifications for hot cathode ionization gauges
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3529-1, ISO 3529-2, ISO 3529-
3, ISO 14291, ISO 27894 and the following apply.
ISO and IEC maintain terminological databases for use in standardization in the following address:
— IEC Electropedia: available at — ISO Inline browsing platform: available at
https://www.iso.org/obp
— IEC Electropedia: available at http://ww.electropedia.org/
3.1
outgassing rate
rate of molecules that leave a surface in vacuum, in units of throughput, measured at 23 °C or calculated
for a gas temperature T = 23 °C
Note 1 to entry: For scientific investigations, it is distinguished between desorption and outgassing. The first is a
surface effect, the second a bulk (diffusion) effect. For many measurement procedures, however, it is not possible
to distinguish between the two effects. To measure the true outgassing rate, all molecules from a surface are
removed by a bake-out or a similar procedure. A bake-out, however, in particular in industrial applications, is not
possible or not desirable. For this reason, in this document, the term "outgassing rate" includes both desorption and
outgassing rate.
Note 2 to entry: The temperature of the sample or the measurement chamber may be different from 23 °C, but the
measured outgassing value in units of throughput shall be calculated as if released gas would have temperature
23 °C. In this way, it is possible, to state a measured throughput (at 23 °C) for a sample at 150 °C.
Note 3 to entry: In vacuum applications, the unit of throughput is commonly used for outgassing rates, but for known
outgassing gas species it is also possible to use the unit of mass flow, e.g. g/s.
Note 4 to entry: In some cases, when outgassing gas molecules of a certain species easily adsorb on the surrounding
surfaces of the measurement system, it is distinguished between intrinsic and measured outgassing rate. The
intrinsic outgassing rate is the outgassing rate that leave the sample surface. It is higher than the measured one,
when no equilibrium of ad- and desorption on the surfaces of the walls has been reached. In this document, it is
assumed that equilibrium is established, but this may not be true for absorbing gases like water vapour.
3.2
geometrical surface area
area of a surface determined from its geometrical dimensions, not including surface roughness
3.3
mass range of outgassing
mass range of QMS from 1 to the highest m/z of all measured or expected outgassing gas species
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ISO/TS 20177:2018(E)
Note 1 to entry: For the scope of this document, the mass range of outgassing is 300.
3.4
nitrogen equivalent outgassing rate
outgassing rate (3.1) when all gases released from the sample are assumed to be nitrogen molecules
Note 1 to entry: To be consistent, all quantities involved in a physical equation (e.g conductance and pumping speed)
need to be expressed for nitrogen, if pressure is measured in nitrogen equivalent. Otherwise, the same nitrogen
reading of a vacuum gauge could lead to different quantities dependent on gas species (see the following example).
EXAMPLE If the throughput of hydrogen is measured by p C, where pressure p is measured in nitrogen
N2 N2
equivalent and C is the effective conductance (pumping speed), C shall be the value calculated or determined for
nitrogen, and not hydrogen.
3.5
bake-out time
time for which a sample or chamber is maintained at a specified (bake-out) temperature
Note 1 to entry: Bake-out time does not include the warm-up and cool-down phase.
4 Symbols and abbreviated terms
Symbol Designation Unit
combined calibration factor for sensitivity
3 −1 −1
ψ Pa m s A
and effective pumping speed
effective conductance or pumping speed of
3 −1 −1
C m s or L s
eff,i
gas species, i
D diameter of cylindrical chamber m
f fragmentation factor 1
i gas species
I ion current at partial pressure, p A
I ion current at residual pressure, p A
0 0
l length of cylindrical chamber m
m mass kg
−1
M molecular weight kg mol
MDPPp minimum detectable partial pressure Pa
MDPP
p pressure or partial pressure Pa
residual pressure or residual partial
p Pa
0
pressure
p pressure in nitrogen equivalent Pa
N2
3 −1 −1
qpV throughput Pa m s or Pa L s
3 −1 −1
qout outgassing rate Pa m s or Pa L s
3 −1 −1
q outgassing rate in nitrogen equivalent Pa m s or Pa L s
out,N2
relative sensitivity for a specified gas
r species “x” divided by sensitivity SN2 for 1
x
nitrogen
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ISO/TS 20177:2018(E)
−1 −1
R universal gas constant J mol K
S sensitivity (coefficient) A/Pa
S sensitivity for nitrogen A/Pa
N2
t time s
T temperature K
3
V volume m or L (Liter)
z charge state after ionization 1
CDG capacitance diaphragm gauge
QMS quadrupole mass spectrometer
SEM secondary electron multiplier
SI System International
SRG spinning rotor gauge
5 Measurement systems
5.1 General
5.1.1 Overview
This document intentionally leaves a choice of measurement systems for outgassing measurements. The
reason is that, at the present stage, there are no scientific reasons to prefer one method over the other,
as long as traceability to the SI is ensured in some way. In the past, lack of traceability was a great deficit
of systems and procedures. All measurement systems listed in this document ensure some kind of
traceability to the SI. The higher the metrological level, the more direct the traceability is, and the lower
the uncertainties of the traceable quantities are, which are significant. The traceable quantities of each
kind of system are given in Annex C.
In the future, when this document is established and comparisons between different systems have been
carried out, it may turn out that some systems should be preferred against others, because the kind of
traceability is insufficient or data are less reliable for other reasons. This will lead to new
recommendations.
5.1.2 Recommendations for systems
The criteria for which system is best to use are as follows:
a) need to measure time dependence;
b) need to identify outgassing species;
c) outgassing of vapour (e.g. water vapour) in a significant magnitude;
d) expected outgassing rate;
e) needed accuracy or uncertainty of measurement;
f) effort, budget and experience.
Annex B gives an overview on which system is suitable or not in specific cases.
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ISO/TS 20177:2018(E)
If the time dependence of outgassing shall be measured, one of the throughput method systems described
in 5.2.2 to 5.2.4 should be applied. The throughput system described in 5.2.5, however, is not useful for
measuring time dependence.
Accumulation systems are not suitable for gases or vapours (e.g. water vapour) that adsorb on the inner
walls, since this will lead to an underestimation of the outgassing rates.
If a significant amount of vapour, in particular water vapour, is outgassing from the sample, one of the
throughput methods described in 5.2 should be applied.
If a UHV-compatible sample after a bake-out has to be measured, one of the accumulation methods
described in 5.3 should be applied. This method is usually more accurate than the throughput method
−12 3 −1
and at the same time associated with less effort. For very low outgassing rates (<10 Pa m s ), the
accumulation method is also applicable, but will need long measurement times. If this shall be avoided,
the throughput method systems described in 5.2.2, 5.2.3 or 5.2.5 should be applied.
If the outgassing species have to be identified, the system described in 5.3.2 cannot be applied. This
system is, however, well-suited for measuring total outgassing rates.
−9 3 −1
All systems can be used for outgassing rates >10 Pa m s , however, the effort for the systems described
in 5.2.3 with two path design and 5.2.5 is not adapted to these rates. For lower outgassing rates
−9 3 −1
<10 Pa m s , the two designs described in 5.2.3 and 5.2.5 or the accumulation systems described in 5.3
should be used.
A great deal of experience is required when systems described in 5.2.2, 5.2.3, 5.2.5 and 5.3.3 are
established and operated.
The budgetary investments as well the effort to establish and validate the systems roughly rises in the
following order:
— basic accumulation system (5.3.2);
— throughput system with measured effective pumping speed (5.2.4);
— throughput system with calculated conductance element, two path design is more expensive (5.2.3);
— accumulation system with gas analysis system (extended accumulation system) (5.3.3);
— throughput system with modulated conductance (5.2.5);
— continuous expansion system as flow comparator (5.2.2).
The lowest uncertainties in terms of nitrogen equivalent of total outgassing rate can be achieved with the
basic accumulation system. The measurement uncertainties typically rise in the following order:
— basic accumulation system (5.3.2);
— continuous expansion system as flow comparator (5.2.2);
— throughput system with modulated conductance (5.2.5);
— throughput system with calculated conductance element; two path design is more accurate at lower
rates (5.2.3);
— accumulation system with gas analysis system (extended accumulation system) (5.3.3);
— throughput system with measured effective pumping speed (5.2.4).
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ISO/TS 20177:2018(E)
The so-called mass change method, where the change of mass of an outgassing sample or the mass of
detected outgassing species from a sample is measured, was introduced for testing of spacecraft and
[3] −5 3 −1
satellite materials . It cannot be recommended for outgassing rate measurements <10 Pa m s which
are within the scope of this document, because the method is not sensitive enough for high and ultrahigh
vacuum applications. Measurement times of more than 1 000 h are needed for typical outgassing rates of
−7 3 −1
10 Pa m s .
5.1.3 Vacuum chambers and pumps
The vacuum chambers should be prepared such that an increase of the signal (p-p or I-I ) due to the
0 0
lowest sample outgassing rate during the measurement time is at least the same as the background signal
(p or I ). As a minimum, the system shall at least contain a measurement chamber with a pump system
0 0
and appropriate vacuum gauges. In addition, a separate sample chamber, a load-lock system and a pump
chamber may be used. If a separate sample chamber is used, it should have a separate pump system with
a valve in between. The conductance from the sample chamber to the measurement chamber should be
at least 2 L/s for nitrogen.
NOTE 1 A separate sample chamber to load the sample allows to keep the measurement chamber under
background conditions all the time and also independent bake-outs of sample and measurement chamber.
NOTE 2 A much smaller conductance than 2 L/s does not affect measurement results, but can lead to long
relaxation times and inhibit the measurement of time dependent outgassing rates.
All systems for outgassing rate measurements should be all metal systems. Large measurement chambers
that can host large samples, however, will need large doors which require elastomer sealing. Systems
with elastomer sealing will also be suitable, if the expected outgassing rates from the sample are relatively
−8 3 −1
high, typically >10 Pa m s . Double sealing techniques can be applied, if the permeation through
elastomer seals is too high. Also, valve seat sealings which are not exposed to the air from outside, may
consist of elastomers.
All systems shall be equipped with high vacuum pumps that are without the risk of changing pumping
speed and emitting previously pumped gases. To this end, turbomolecular pumps and cryopumps are
recommended. Diffusion pumps are also acceptable. Ion getter pumps, sublimation pumps or passive
getter pumps, however, are only acceptable, if it can be ensured at all times that the change of pumping
speed or ejection of previously pumped gases does not influence the results. Dry pump systems should
be used when low hydrocarbon outgassing rates are expected or when the sample requires an oil-free
environment.
NOTE 3 Also cryopumps can change pumping speed to some extent, depending on the gas amount already
pumped.
5.1.4 Vacuum gauges
Vacuum gauges used for the measurement of outgassing rate or to calibrate the sensitivity of the QMS
shall have a certificate according to ISO/IEC 17025.
QMS are needed to identify species and measure their outgassing rates. The QMS shall be calibrated
according to ISO/TS 20175, either in a separate system or in situ in the outgassing measurement system.
If calibration was performed in a separate system, an in situ calibration check has to follow.
If a procedure requires in situ calibration of vacuum gauges, then the gauges to be compared should have
symmetrical positions to optional gas inlets and pump outlet and both ionization gauges and QMS should
have no line of sight to any other gauge.
5.1.5 Purity of gases
For the purpose of this document, a pure gas specifies a purity of >99,9 %.
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ISO/TS 20177:2018(E)
5.2 Systems applying the throughput method
5.2.1 General
These methods are based on measuring a steady flow through the measuring system.
5.2.2 Cont
...
TECHNICAL ISO/TS
SPECIFICATION 20177
First edition
2018-06
Vacuum technology — Vacuum gauges
— Procedures to measure and report
outgassing rates
Technique du vide — Manomètres à vide — Méthodes de mesurage et
de rapport du taux de dégagement de gaz
Reference number
ISO/TS 20177:2018(E)
©
ISO 2018
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ISO/TS 20177:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved
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ISO/TS 20177:2018(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 3
5 Measurement systems . 4
5.1 General . 4
5.1.1 Overview . 4
5.1.2 Recommendations for systems .4
5.1.3 Vacuum chambers and pumps .5
5.1.4 Vacuum gauges .6
5.1.5 Purity of gases .6
5.2 Systems applying the throughput method .6
5.2.1 General. 6
5.2.2 Continuous expansion system as flow comparator .6
5.2.3 Throughput system with calculated conductance element .8
5.2.4 Throughput system with measured effective pumping speed . 10
5.2.5 Throughput system with modulated conductance . 12
5.3 Accumulation systems . 13
5.3.1 General.13
5.3.2 Basic accumulation system . 13
5.3.3 Accumulation system with gas analysis system (extended
accumulation system) . 13
6 Measurement procedures .14
6.1 General .14
6.2 Recommended sample preparation . 15
6.3 Course and time period of measurement . 16
6.4 Measurement procedures . 16
6.4.1 Procedure with continuous expansion system as flow comparator . 16
6.4.2 Procedure with throughput system with calculated conductance element
(pressure difference system) . 17
6.4.3 Procedure with throughput system with measured effective pumping speed . 18
6.4.4 Procedure with throughput system with modulated conductance . 19
6.4.5 Procedure with accumulation systems . 20
7 Measurement uncertainties .23
7.1 General .23
7.2 Continuous expansion system as flow comparator (5.2.2) .23
7.3 Throughput system with calculated conductance element (5.2.3) .23
7.4 Throughput system with measured effective pumping speed (5.2.4) .24
7.5 Throughput system with modulated conductance (5.2.5) .24
7.6 Basic accumulation system (5.3.2) .24
7.7 Accumulation system with gas analysis system (5.3.3) .25
8 Reporting results .28
Annex A (informative) Schemes of principles of measurement systems .30
Annex B (informative) Applicability and characteristics of the different measurement system .36
Annex C (informative) Traceability of the different measurement systems to the SI .37
Bibliography .39
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ISO/TS 20177:2018(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
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ISO/TS 20177:2018(E)
Introduction
Outgassing from the inner wall of a vacuum chamber and from components in a vacuum chamber
limit the achievable lowest pressure in a vacuum system and its cleanliness. The lowest achievable
pressure is usually important in research facilities as accelerators, in facilities that need to ensure
a clean surface, e.g. molecular beam epitaxy, or in devices that need to ensure high vacuum without
pump for long times, such as transmitters or X-ray tubes, medical instruments, surface analytical
instrumentation or insulation panels. Cleanliness of a vacuum, i.e. the absence or sufficiently low partial
pressure of specific gas species or vapours, is important in many different industrial applications such
as coating, EUV lithography, catalysis, drying processes in the pharmaceutical or food industry but also
in accelerators, fusion reactors, etc. The measurement of outgassing rates is therefore an important
tool of quality assurance in vacuum technology. This document recommends well-defined procedures
with the possibility of getting traceability of the results of an outgassing rate measurement.
Annex A lists schemes of principles of measurement systems.
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TECHNICAL SPECIFICATION ISO/TS 20177:2018(E)
Vacuum technology — Vacuum gauges — Procedures to
measure and report outgassing rates
1 Scope
This document describes procedures to measure outgassing rates from components designed for
vacuum chambers and of vacuum chambers as a whole. The outgassing rates are expected to be lower
−5 3 −1 −2 −1
than 10 Pa m s (10 Pa L s ) at 23 °C and to emerge from devices that are suitable for high or
ultra-high vacuum applications. The molecular mass of the outgassing species or vapour is below 300 u.
−5 3 −1
The upper limit 10 Pa m s of total outgassing rate is specified independent of the size, the total
surface area and texture or state of the outgassing material. If a specific outgassing rate (outgassing
rate per area) is determined, the area is not a specific surface area including the surface roughness, but
the nominal geometrical one. When it is difficult to determine the nominal geometrical surface area of
the sample, such as powders, porous materials, very rough surfaces, or complex devices, mass specific
outgassing rate (e.g. outgassing rate per gram) is used.
For many practical applications, it is sufficient to determine the total outgassing rate. If a measuring
instrument, which sensitivity is gas species dependent, is used, the total outgassing rate are given in
nitrogen equivalent. In cases, however, where the total outgassing rate is too high, the disturbing gas
species is identified, and its outgassing rate is measured in order to improve the sample material. This
document covers both cases.
Some outgassing molecules can adsorb on a surface with a residence time that is much longer than the
total time of measurement. Such molecules cannot be detected by a detecting instrument when there
is no direct line of sight. This is considered as a surface effect and surface analytical investigations
are more useful than general outgassing rate measurements considered here. Also, molecules that are
released from the surface by irradiation of UV light or X-rays, are out of the scope of this document.
This document is written to standardize the measurement of outgassing rates in such a way that values
obtained at different laboratories and by different methods are comparable. To this end, for any of the
described methods, traceability is provided to the System International (SI) for the most important
parameters of each method and according to the metrological level.
Outgassing rate measurements by mass loss, which were mainly developed for testing of spacecraft and
satellite materials, are not gas specific. For acceptable measurement times, mass loss measurements
−5 3 −1
require significantly higher outgassing rates (>10 Pa m s ) than typical for high and ultrahigh
vacuum components. Also, it is not possible to measure the sample in situ due to the weight of the vacuum
chamber, since the balances are not vacuum compatible. For these reasons, mass loss measurements
are not considered in this document.
It is assumed that the user of this document is familiar with high and ultra-high vacuum technology
and the corresponding measuring instrumentation such as ionization gauges and quadrupole mass
spectrometers.
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 3529 (all parts), Vacuum technology — Vocabulary
ISO 14291, Vacuum gauges — Definitions and specifications for quadrupole mass spectrometers
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ISO/TS 20177:2018(E)
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO/TS 20175, Vacuum technology — Vacuum gauges — Characterization of quadrupole mass
spectrometers for partial pressure measurement
ISO 27894, Vacuum technology — Vacuum gauges — Specifications for hot cathode ionization gauges
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3529-1, ISO 3529-2, ISO 3529-3,
ISO 14291, ISO 27894 and the following apply.
ISO and IEC maintain terminological databases for use in standardization in the following address:
— ISO Inline browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //ww .electropedia .org/
3.1
outgassing rate
rate of molecules that leave a surface in vacuum, in units of throughput, measured at 23 °C or calculated
for a gas temperature T = 23 °C
Note 1 to entry: For scientific investigations, it is distinguished between desorption and outgassing. The first is a
surface effect, the second a bulk (diffusion) effect. For many measurement procedures, however, it is not possible
to distinguish between the two effects. To measure the true outgassing rate, all molecules from a surface are
removed by a bake-out or a similar procedure. A bake-out, however, in particular in industrial applications, is not
possible or not desirable. For this reason, in this document, the term "outgassing rate" includes both desorption
and outgassing rate.
Note 2 to entry: The temperature of the sample or the measurement chamber may be different from 23 °C, but the
measured outgassing value in units of throughput shall be calculated as if released gas would have temperature
23 °C. In this way, it is possible, to state a measured throughput (at 23 °C) for a sample at 150 °C.
Note 3 to entry: In vacuum applications, the unit of throughput is commonly used for outgassing rates, but for
known outgassing gas species it is also possible to use the unit of mass flow, e.g. g/s.
Note 4 to entry: In some cases, when outgassing gas molecules of a certain species easily adsorb on the surrounding
surfaces of the measurement system, it is distinguished between intrinsic and measured outgassing rate. The
intrinsic outgassing rate is the outgassing rate that leave the sample surface. It is higher than the measured one,
when no equilibrium of ad- and desorption on the surfaces of the walls has been reached. In this document, it is
assumed that equilibrium is established, but this may not be true for absorbing gases like water vapour.
3.2
geometrical surface area
area of a surface determined from its geometrical dimensions, not including surface roughness
3.3
mass range of outgassing
mass range of QMS from 1 to the highest m/z of all measured or expected outgassing gas species
Note 1 to entry: For the scope of this document, the mass range of outgassing is 300.
3.4
nitrogen equivalent outgassing rate
outgassing rate (3.1) when all gases released from the sample are assumed to be nitrogen molecules
Note 1 to entry: To be consistent, all quantities involved in a physical equation (e.g conductance and pumping
speed) need to be expressed for nitrogen, if pressure is measured in nitrogen equivalent. Otherwise, the same
nitrogen reading of a vacuum gauge could lead to different quantities dependent on gas species (see the following
example).
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ISO/TS 20177:2018(E)
EXAMPLE If the throughput of hydrogen is measured by p C, where pressure p is measured in nitrogen
N2 N2
equivalent and C is the effective conductance (pumping speed), C shall be the value calculated or determined for
nitrogen, and not hydrogen.
3.5
bake-out time
time for which a sample or chamber is maintained at a specified (bake-out) temperature
Note 1 to entry: Bake-out time does not include the warm-up and cool-down phase.
4 Symbols and abbreviated terms
Symbol Designation Unit
combined calibration factor for sensitivity
3 −1 −1
ψ Pa m s A
and effective pumping speed
effective conductance or pumping speed
3 −1 −1
C m s or L s
eff,i
of gas species, i
D diameter of cylindrical chamber m
f fragmentation factor 1
i gas species
I ion current at partial pressure, p A
I ion current at residual pressure, p A
0 0
l length of cylindrical chamber m
m mass kg
−1
M molecular weight kg mol
p minimum detectable partial pressure Pa
MDPP
p pressure or partial pressure Pa
residual pressure or residual partial
p Pa
0
pressure
p pressure in nitrogen equivalent Pa
N2
3 −1 −1
q throughput Pa m s or Pa L s
pV
3 −1 −1
q outgassing rate Pa m s or Pa L s
out
3 −1 −1
q outgassing rate in nitrogen equivalent Pa m s or Pa L s
out,N2
relative sensitivity for a specified gas
r species “x” divided by sensitivity SN2 for 1
x
nitrogen
−1 −1
R universal gas constant J mol K
S sensitivity (coefficient) A/Pa
S sensitivity for nitrogen A/Pa
N2
t time s
T temperature K
3
V volume m or L (Liter)
z charge state after ionization 1
CDG capacitance diaphragm gauge
QMS quadrupole mass spectrometer
SEM secondary electron multiplier
SI System International
SRG spinning rotor gauge
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ISO/TS 20177:2018(E)
5 Measurement systems
5.1 General
5.1.1 Overview
This document intentionally leaves a choice of measurement systems for outgassing measurements.
The reason is that, at the present stage, there are no scientific reasons to prefer one method over the
other, as long as traceability to the SI is ensured in some way. In the past, lack of traceability was a great
deficit of systems and procedures. All measurement systems listed in this document ensure some kind
of traceability to the SI. The higher the metrological level, the more direct the traceability is, and the
lower the uncertainties of the traceable quantities are, which are significant. The traceable quantities
of each kind of system are given in Annex C.
In the future, when this document is established and comparisons between different systems have
been carried out, it may turn out that some systems should be preferred against others, because the
kind of traceability is insufficient or data are less reliable for other reasons. This will lead to new
recommendations.
5.1.2 Recommendations for systems
The criteria for which system is best to use are as follows:
a) need to measure time dependence;
b) need to identify outgassing species;
c) outgassing of vapour (e.g. water vapour) in a significant magnitude;
d) expected outgassing rate;
e) needed accuracy or uncertainty of measurement;
f) effort, budget and experience.
Annex B gives an overview on which system is suitable or not in specific cases.
If the time dependence of outgassing shall be measured, one of the throughput method systems
described in 5.2.2 to 5.2.4 should be applied. The throughput system described in 5.2.5, however, is not
useful for measuring time dependence.
Accumulation systems are not suitable for gases or vapours (e.g. water vapour) that adsorb on the inner
walls, since this will lead to an underestimation of the outgassing rates.
If a significant amount of vapour, in particular water vapour, is outgassing from the sample, one of the
throughput methods described in 5.2 should be applied.
If a UHV-compatible sample after a bake-out has to be measured, one of the accumulation methods
described in 5.3 should be applied. This method is usually more accurate than the throughput method
−12 3 −1
and at the same time associated with less effort. For very low outgassing rates (<10 Pa m s ), the
accumulation method is also applicable, but will need long measurement times. If this shall be avoided,
the throughput method systems described in 5.2.2, 5.2.3 or 5.2.5 should be applied.
If the outgassing species have to be identified, the system described in 5.3.2 cannot be applied. This
system is, however, well-suited for measuring total outgassing rates.
−9 3 −1
All systems can be used for outgassing rates >10 Pa m s , however, the effort for the systems
described in 5.2.3 with two path design and 5.2.5 is not adapted to these rates. For lower outgassing
−9 3 −1
rates <10 Pa m s , the two designs described in 5.2.3 and 5.2.5 or the accumulation systems
described in 5.3 should be used.
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ISO/TS 20177:2018(E)
A great deal of experience is required when systems described in 5.2.2, 5.2.3, 5.2.5 and 5.3.3 are
established and operated.
The budgetary investments as well the effort to establish and validate the systems roughly rises in the
following order:
— basic accumulation system (5.3.2);
— throughput system with measured effective pumping speed (5.2.4);
— throughput system with calculated conductance element, two path design is more expensive (5.2.3);
— accumulation system with gas analysis system (extended accumulation system) (5.3.3);
— throughput system with modulated conductance (5.2.5);
— continuous expansion system as flow comparator (5.2.2).
The lowest uncertainties in terms of nitrogen equivalent of total outgassing rate can be achieved with
the basic accumulation system. The measurement uncertainties typically rise in the following order:
— basic accumulation system (5.3.2);
— continuous expansion system as flow comparator (5.2.2);
— throughput system with modulated conductance (5.2.5);
— throughput system with calculated conductance element; two path design is more accurate at lower
rates (5.2.3);
— accumulation system with gas analysis system (extended accumulation system) (5.3.3);
— throughput system with measured effective pumping speed (5.2.4).
The so-called mass change method, where the change of mass of an outgassing sample or the mass
of detected outgassing species from a sample is measured, was introduced for testing of spacecraft
[3] −5 3 −1
and satellite materials . It cannot be recommended for outgassing rate measurements <10 Pa m s
which are within the scope of this document, because the method is not sensitive enough for high
and ultrahigh vacuum applications. Measurement times of more than 1 000 h are needed for typical
−7 3 −1
outgassing rates of 10 Pa m s .
5.1.3 Vacuum chambers and pumps
The vacuum chambers should be prepared such that an increase of the signal (p-p or I-I ) due to the
0 0
lowest sample outgassing rate during the measurement time is at least the same as the background
signal (p or I ). As a minimum, the system shall at least contain a measurement chamber with a pump
0 0
system and appropriate vacuum gauges. In addition, a separate sample chamber, a load-lock system
and a pump chamber may be used. If a separate sample chamber is used, it should have a separate
pump system with a valve in between. The conductance from the sample chamber to the measurement
chamber should be at least 2 L/s for nitrogen.
NOTE 1 A separate sample chamber to load the sample allows to keep the measurement chamber under
background conditions all the time and also independent bake-outs of sample and measurement chamber.
NOTE 2 A much smaller conductance than 2 L/s does not affect measurement results, but can lead to long
relaxation times and inhibit the measurement of time dependent outgassing rates.
All systems for outgassing rate measurements should be all metal systems. Large measurement
chambers that can host large samples, however, will need large doors which require elastomer sealing.
Systems with elastomer sealing will also be suitable, if the expected outgassing rates from the sample
−8 3 −1
are relatively high, typically >10 Pa m s . Double sealing techniques can be applied, if the permeation
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ISO/TS 20177:2018(E)
through elastomer seals is too high. Also, valve seat sealings which are not exposed to the air from
outside, may consist of elastomers.
All systems shall be equipped with high vacuum pumps that are without the risk of changing pumping
speed and emitting previously pumped gases. To this end, turbomolecular pumps and cryopumps are
recommended. Diffusion pumps are also acceptable. Ion getter pumps, sublimation pumps or passive
getter pumps, however, are only acceptable, if it can be ensured at all times that the change of pumping
speed or ejection of previously pumped gases does not influence the results. Dry pump systems should
be used when low hydrocarbon outgassing rates are expected or when the sample requires an oil-free
environment.
NOTE 3 Also cryopumps can change pumping speed to some extent, depending on the gas amount already
pumped.
5.1.4 Vacuum gauges
Vacuum gauges used for the measurement of outgassing rate or to calibrate the sensitivity of the QMS
shall have a certificate according to ISO/IEC 17025.
QMS are needed to identify species and measure their outgassing rates. The QMS shall be calibrated
according to ISO/TS 20175, either in a separate system or in situ in the outgassing measurement system.
If calibration was performed in a separate system, an in situ calibration check has to follow.
If a procedure requires in situ calibration of vacuum gauges, then the gauges to be compared should
have symmetrical positions to optional gas inlets and pump outlet and both ionization gauges and QMS
should have no line of sight to any other gauge.
5.1.5 Purity of gases
For the purpose of this document, a pure gas specifies a purity of >99,9 %.
5.2 Systems applying the throughput method
5.2.1 General
These methods are based on measuring a steady flow through the measuring system.
5.2.2 Continuous expansion system as flow comparator
This method is considered as of high metrological level.
The continuous expansion systems are based on the principle that well-known pressures or partial
pressures can be established in a vacuum chamber by the injection of gas of known flow rate (here:
[3]
throughput q ) and pumped out of the vacuum chamber via a duct of known conductance C .
pV,
...
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