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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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
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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
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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 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
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).
2 © ISO 2018 – All rights reserved

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
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
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.
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.
4 © ISO 2018 – All rights reserved

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
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,i eff,I
The known throughput is generated by a flowmeter or a gas flow out of a reservoir volume V (see
R
Figure A.1) filled with gas of pressure p through a known conductance C , as shown in Formula (1):
R,i R,i
q = p C (1)
pV,i R,i R,i
The flow injection can also be provided by a flow divider method where the pressure ratio across a flow
restricting element is independent of pressure in molecular flow regime.
NOTE Generating the known flow traceable to the SI is the part of the system which requires a great deal of
experience in vacuum metrology and a comprehensive equipment.
6 © ISO 2018 – All rights reserved

This flow q generates a partial pressure p for species i in the continuous expansion chamber by
pV,i i
Formula (2):
q = p C (2)
pV,i i eff,i
where C is the effective conductance of species i to the pump system.
eff,i
In the same manner, an outgassing sample with rate q = q will generate the same partial pressure,
out,i pV,i
p , in the measurement chamber and can be recorded by a QMS. By this recording, the continuous
i
expansion system acts as a flow comparator between the unknown q and the known q . If the
out,i pV,i
′
rates are not the same, q will generate the partial pressure p according to Formula (3):
out,i
i
′
qp= C (3)
out,ii eff,i
Finally, q is determined by Formula (4):
out,i
′
p
i
q = q (4)
out,i pV ,i
p
i
The partial pressures shall be measured by a QMS and the total pressure additionally by a total pressure
vacuum gauge. The outgassing can be analysed in terms of gas species as shown in Formula (4). Since
the signal of a QMS is normally a current I , for the purpose of this document, it makes sense to define a
i
calibration factor ψ as given in Formula (5):
i
q C
pV ,i eff ,i
ψ == (5)
i
()II− Sp()
ii,0 i
where
S(p ) is the sensitivity as defined in ISO 14291;
i
I is the QMS signal at p ;
i i
I is residual pressure conditions.
i,0
With this definition, the outgassing rate is calculated from the QMS signal as Formula (6):
′
()II−
ii,0
′
qI==ψ ()−Iq (6)
out,ii ii,,0 pV i
()II−
ii,0
′ ′
where I as the QMS signal at p . If only a total pressure vacuum gauge is used, the total outgassing
i,0 i
rate in nitrogen equivalent can be determined by using Formula (7):
′
p
N2
q = q (7)
out,N2 pV ,N2
p
N2
The position of the calibration gas inlets shall be at such positions that equal flows from them and the
outgassing sample generate the same signal on the QMS. This can be accomplished by a cylindrical
chamber with the conductance at one endface and all gas inlets and the inlet from the outgassing
sample located on the same equatorial plane perpendicular to the cylindrical axis.
For the purpose of a primary outgassing rate measurement system, it shall be possible to establish
at least three calculable partial pressures at the same time in the chamber. The three gases shall
correspond to the three major outgassing species (if applicable) from the outgassing sample. The QMS
is calibrated in situ for these one to three major gas species and for other outgassing species of interest
as described by Formula (5).
The measurement chamber should have a volume of at least four times the total volume of all the gauges
and associated pipe work connecting the chamber and the gauges (e.g. elbows shall be considered as
part of the gauge volume). Another chamber, separated by a valve from the measurement chamber
and to host the sample, shall be available. The injected q shall not change by more than 1 % within
pV,i
30 min. The system shall be bakeable.
A load-lock for the sample introduction shall be available, when small outgassing rates shall be
measured, but the sample stay in non-baked-out condition. In all other cases, a load-lock is strongly
recommended.
5.2.3 Throughput system with calculated conductance element
5.2.3.1 General
This method is considered as of medium or high metrological level, depending on effort (see below).
The system can be also named pressure difference system both for one (5.2.3.2) and two path design
(5.2.3.3).
5.2.3.2 One path design
The idea of the throughput system with calculated conductance element is to measure the pressure
difference Δp across an orifice, short duct (length lower or equal to the diameter) or other element of
known conductance, C, located between two chambers. The upstream pressure p shall be measured,
the downstream pressure p can be either measured or known from an estimation. In particular, if
the pumping speed behind the duct is much higher than the conductance of the duct (>20 times), the
downstream pressure can be neglected. In this case, the downstream chamber is optional.
Whenever size and shape of the sample will allow, the upper measurement chamber should have
a cylindrical volume of diameter, D, and length, l, and its size of at least four times the total volume
of all the gauges and associated pipe work connecting the chamber and the gauges (e.g. elbows shall
be considered as part of the gauge volume). The ratio l/D shall be ≥1,5 and ≤3. The system should be
bakeable. The flanges for vacuum measurement in the upstream vessel should be located at a height of
D/2 above the orifice plane, where D is the diameter of the cylinder. The lower measurement chamber
(pump chamber) may have a cylindrical volume of the same diameter as the upper one. For reason of
symmetry, the flanges for vacuum measurement in the downstream vessel should be located at a height
of D/2 below the orifice plane.
NOTE 1 The cylindrical design of the chambers with the positioning of the gauge inlet ports at D/2 allows
the use of the same domes as described in ISO 21360-1:2012, 5.2, if l/D is as required by ISO 21360-1. The
downstream vessel, however, can be put upside down. An additional flange at half of the length of the cylinder
and a sample holder inside can be required.
If the downstream pressure p has to be measured, a bypass with two valves (all metal valves are
recommended) and the measuring instrument should be installed (QMS and/or total pressure gauge)
in between. With the corresponding valve open, either the upstream or downstream pressure can be
measured with the same instrument.
The upstream pressure p shall be measured by a calibrated total pressure gauge and optionally in
addition by a calibrated QMS. The calibration certificates shall state the sensitivities for nitrogen as a
minimum.
The sample may be placed in the upstream pressure vessel. A separate vessel, however, has the
advantage because the background from the outgassing of the vessels can be measured just before and
after the measurement of the sample. The additional background from the sample chamber shall be
determined in a separate experiment. If a separate vessel is used, its connecting flange should be located
at l/2 where l is the maximum length of the cylinder. In addition, any holder of a sample should be at this
8 © ISO 2018 – All rights reserved

same height close to l/2. If the sample is placed inside the upstream vessel, it should be positioned at a
height of at least D/2 above the orifice plane. A height of l/2 is recommended.
NOTE 2 A lower position of the sample holder can reduce the calculated conductance. This can result to a
significant part of the molecules leaving the sample to not reach the QMS or gauges.
See Figure 1.
Key
1 upper measurement chamber
2 lower measurement chamber (pump chamber)
3 recommend sample position
4, 5, 6 vacuum gauge or QMS
7 gas inlet to separate sample chamber (not shown)
8 vacuum pump system
Figure 1 — Measurement chamber configuration for throughput method with calculated
conductance
A separate chamber also allows the outgassing flow being measured to be injected either in the
upstream or downstream vessel. This modification is known as two path system (see 5.3.3). The
system is of greater value when calibrated standard leaks, at least for nitrogen, are attached to the
upstream vessel. Gas injection lines with variable known flow rates may also be installed. This enables
the user to check the stability of the total vacuum gauge and the optional QMS and check or measure
the conductance in situ. In this case, the system is of high metrological level and very similar to the
system described in 5.2.2.
A load-lock for the sample introduction shall be available, when small outgassing rates shall be
measured, but the sample stay in non-baked out condition. In other cases, a load-lock is recommended.
If no micro-technical parts for QMS or conductance are used, the pressure shall be <0,01 Pa at all times
and in all parts of the system during the measurement. The value of C of the conductance element
shall be known from measuring the geometry (this is the preferred option) or be measured in situ by a
−5 −2
standard leak for nitrogen of throughput q 10 Pa L/s to 10 Pa L/s which is attached to the chamber.
pV
It shall have a valve and this should be positioned as close as possible to the place of the outgassing
sample. In this case, C is calculated for nitrogen using Formula (8):
C = q /(p − p) (8)
N2 pV 1 2
where p and p are measured as nitrogen equivalent. The corresponding pressures need to be
1 2
corrected for the background signal from the measurement system and the gauge itself.
Since the flow is of molecular type, the effective conductance C for another gas species i with molecular
i
mass M can be calculated from the one for nitrogen by Formula (9):
i
CC= (9)
i N2
M
i
NOTE 3 This formula is true even in the case when the net flow through the conductance element is influenced
by the pumping speed behind the conductance element. This effect is considered by the measurement of the
pressure difference across the conductance element.
The total outgassing rate in nitrogen equivalent is given by Formula (10):
q = C (p − p) (10)
out,N2 N2 1 2
the outgassing rate of a particular species i by Formula (11):
q = C (p − p) (11)
out,i i 1,i 2,i
5.2.3.3 Two path design
[5][6]
In the two path system , a symmetrical lower measurement chamber should be used. The outgassing
flow can be directed to the upstream vessel via an all metal valve "A" (see Figure A.3) so that a pressure
p is generated, and to the downstream vessel via an all metal valve "B" (see Figure A.3) so that a
1A
pressure p is generated in the upstream vessel. Inlet 12 shall be positioned as close as possible to the
1B
conductance element. The downstream pressures will be equal in the two cases so that they cancel out.
For this reason, it is sufficient to have a QMS and calibrated vacuum gauge installed on the upstream
("upper") measurement chamber only. It is, however, also recommended to install a total pressure
vacuum gauge in the downstream chamber for monitoring purposes.
The total outgassing rate from the sample or test chamber in nitrogen equivalent is given by
Formula (12):
q = C (p − p) (12)
out,N2 N2 1A 1B
the outgassing rate of a particular species i by Formula (13):
q = C (p − p) (13)
out,i i 1A,i 1B,i
The two path system is of high metrological level and best suited for very low outgassing rates, since it
cancels out the inherent residual pressure indication of the gauge due to outgassing and x-rays.
5.2.4 Throughput system with measured effective pumping speed
This method is considered as of low and medium metrological level, depending on the shape of the
chamber and on metrological effort.
The throughput measuring system is the most flexible and lowest demanding system. The experimental
equipment is kept to a minimum, but traceability is still assured. Its key elements are:
— the flow through the system is of molecular type;
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— the effective pumping speed C is known for nitrogen by a calibrated nitrogen standard leak;
eff
— calibrated gauges (high and ultrahigh vacuum gauge and optionally QMS) are used to measure total
pressure p in nitrogen equivalent or partial pressure, p .
i
The system shall be equipped with a total pressure high and ultrahigh vacuum gauge, calibrated for
nitrogen, as a minimum and a QMS optionally in addition. The QMS shall be calibrated in situ for pure
nitrogen by the total pressure gauge and a standard leak for nitrogen of throughput q between
pV
−5 −2
10 Pa L/s and 10 Pa L/s.
NOTE The external calibration of the ion gauge and the in situ calibration of the QMS is not only important to
obtain traceability but also to obtain information about the linearity of the gauge and the QMS.
If no micro-technical parts for QMS or conductance are used, the pressure shall be <0,01 Pa at all times
and in all parts of the system during the measurement.
Preferably, the outgassing sample is located in a separate chamber (sample chamber) connected to
the measurement chamber via a valve. If the outgassing sample is placed directly in the chamber, the
outgassing background of the measurement is normally more difficult and less reliable to measure.
This method can also be applied when the outgassing of a vacuum chamber has to be measured. In this
case, the outgassing chamber is both sample and measuring system.
A load-lock for the sample introduction shall be available, when small outgassing rates shall be
measured, but the sample stay in non-baked out condition. In other cases, a load-lock is recommended.
The effective pumping speed C of the system for nitrogen is determined by a standard leak for
eff
−5 −2
nitrogen of throughput q between 10 Pa L/s and 10 Pa L/s which is attached to the chamber.
pV
It shall be equipped with a valve and positioned as close as possible to the outgassing sample. If the
chamber itself is the outgassing source that has to be measured, the standard leak shall be positioned
close to the middle of the longest direct path of a particle in the chamber toward the vacuum pump. The
throughput shall be such that the pressure signal, p, is approximately 100 times larger than residual
pressure p . C is calculated using Formula (14):
0 eff
C = q /(p −p) (14)
eff pV 0
where
p is the pressure indicated by the calibrated high vacuum gauge when the flow from the standard
leak is present;
p is the pressure indicated by the calibrated high vacuum gauge when it is valved off.
In contrast to the throughput system with calculated conductance element, in general, the known
effective pumping speed C for nitrogen cannot be extrapolated to other gas species, since there is
eff
a significant influence of the pump on the effective pumping speed for the different species. For t
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