ISO/TS 20177:2018

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 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

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
Foreword
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iv © ISO 2018 – All rights reserved

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.
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 lo
...