ISO 21360-5:2023
(Main)Vacuum technology — Standard methods for measuring vacuum-pump performance — Part 5: Non-evaporable getter (NEG) vacuum pumps
Vacuum technology — Standard methods for measuring vacuum-pump performance — Part 5: Non-evaporable getter (NEG) vacuum pumps
This document specifies methods for the measurement of pumping characteristics of non-evaporable getters (NEGs). It is applicable to all sizes and all types of NEGs, including those: — with the shape of pill, disk, ring, strip, module, cartridge; — with pump structures; — and NEG coatings on inner surface of pipes and vacuum chamber. A significant difference of pumping characteristics of the NEG and other vacuum pumps is that the pumping speed of the NEG depends on the sorption quantity. Furthermore, especially in the case of NEG coating, the sticking probability rather than the pumping speed is often the index of the pumping performance. Therefore, this document specifies the methods for measuring the pumping speed, the sorption quantity, and the sticking probability of NEGs. WARNING It is assumed that the user is familiar with the handling of combustible gases and poisonous ones and with ultra-high vacuum technology.
Technique du vide — Méthodes normalisées pour mesurer les performances des pompes à vide — Partie 5: Pompes à piégeur non évaporable (NEG)
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 21360-5
First edition
2023-11
Vacuum technology — Standard
methods for measuring vacuum-pump
performance —
Part 5:
Non-evaporable getter (NEG) vacuum
pumps
Technique du vide — Méthodes normalisées pour mesurer les
performances des pompes à vide —
Partie 5: Pompes à piégeur non évaporable (NEG)
Reference number
© ISO 2023
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms.3
5 Test methods . 4
5.1 General . 4
5.1.1 Test gases . 4
5.1.2 Vacuum chamber . 4
5.1.3 Orifice . 4
5.1.4 Vacuum pumping system for rough pumping . 4
5.1.5 Vacuum gauges . 5
5.1.6 Temperature . 5
5.1.7 Activation method of NEG . 5
5.1.8 Procedure of sample installation and activation . 5
5.2 Throughput method for small NEG samples . 6
5.2.1 Experimental setup . 6
5.2.2 Sample. 7
5.2.3 Determination of getter pumping speed, S, and sorption quantity, C . 7
q
5.2.4 Determination of sticking probability, α . 10
5.2.5 Measurement procedure . 10
5.2.6 Measurement uncertainty . 11
5.3 Throughput method with test dome . 11
5.3.1 Experimental setup . 11
5.3.2 Sample. 13
5.3.3 Determination of getter pumping speed S and sorption quantity, C .13
q
5.3.4 Determination of sticking probability, α .13
5.3.5 Measurement procedure . 13
5.3.6 Measurement uncertainty . 14
5.4 Transmission method for NEG coatings . 14
5.4.1 Experimental setup . 14
5.4.2 Sample. 15
5.4.3 Determination of average getter pumping speed per unit area, S , and
A
sorption quantity C . 15
q
5.4.4 Determination of sticking probability, α . 16
5.4.5 Measurement procedure . 16
5.4.6 Measurement uncertainty . 17
5.5 Combination of transmission method and throughput method with test dome . 17
6 Reporting .18
6.1 General . 18
6.2 Small size of NEGs with the structure of pill, disk, ring, strip, module and cartridge . 18
6.3 NEG pumps . 19
6.4 NEG coatings . . 19
Annex A (informative) Calculation method of the molecular conductance of the orifice .21
Annex B (informative) Example of diagrams for pumping characteristics of NEG .22
Annex C (informative) Typical value of initial sticking probability α of NEG at room
temperature .23
Bibliography .25
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 112, Vacuum technology.
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Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
This document specifies methods for measuring the performance data of non-evaporable getters (NEGs)
with the shape of pill, disk, ring, strip, module, cartridge, pump structures and coatings. This document
complements ISO 21360-1, which provides a general description of the measurement of performance
data of vacuum pumps.
The methods described here are well known from existing national and international standards. This
document aims to show a collection of suitable methods for the measurement of performance data of
NEGs. The method specified in this document takes precedence over the volume flow rate (pumping
speed) measurement given in ISO 21360-1:2020, 5.1, 5.2 and 5.3.
v
INTERNATIONAL STANDARD ISO 21360-5:2023(E)
Vacuum technology — Standard methods for measuring
vacuum-pump performance —
Part 5:
Non-evaporable getter (NEG) vacuum pumps
1 Scope
This document specifies methods for the measurement of pumping characteristics of non-evaporable
getters (NEGs). It is applicable to all sizes and all types of NEGs, including those:
— with the shape of pill, disk, ring, strip, module, cartridge;
— with pump structures;
— and NEG coatings on inner surface of pipes and vacuum chamber.
A significant difference of pumping characteristics of the NEG and other vacuum pumps is that the
pumping speed of the NEG depends on the sorption quantity. Furthermore, especially in the case of
NEG coating, the sticking probability rather than the pumping speed is often the index of the pumping
performance. Therefore, this document specifies the methods for measuring the pumping speed, the
sorption quantity, and the sticking probability of NEGs.
WARNING — It is assumed that the user is familiar with the handling of combustible gases and
poisonous ones and with ultra-high vacuum technology.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
non-evaporable getter
NEG
getter material to sorb gases in vacuum chambers without evaporation
Note 1 to entry: Sorbing gases mean the process of removing gases from vacuum chambers by adsorption or
absorption phenomena. The adsorption is a kind of sorption in which the gas is retained at the surface of the
getter material. Most of gas molecules are chemisorbed at the surface of the getter material. The absorption
is also a kind of sorption in which the gas molecules diffuse into the bulk of the getter material. The term of
“sorption”, “adsorption”, “chemisorption” and “absorption” are defined in ISO 3529 1:2019, 3.4.1, 3.4.2, 3.4.4 and
3.4.6, respectively.
Note 2 to entry: NEGs have a variety of forms, such as pellets (pills), bars, chips, powders, sheets, strips, washers,
wires, module and cartridge.
3.2
non-evaporable getter vacuum pump
NEG vacuum pump
entrapment vacuum pump with a reactive porous alloy or powder mixtures getter material
Note 1 to entry: NEG vacuum pumps are typically mounted on a vacuum flange. The internal heaters and the
controller for the activation may be included.
[SOURCE: ISO 3529-2:2020, 3.1.36]
3.3
non-evaporable getter coating
NEG coating
thin films made from non-evaporable getter, which is coated on inner surface of pipes and vacuum
chamber
3.4
surface getter
getter where only the surface shows pumping action
Note 1 to entry: The pumping speed and sorption capacity are essentially proportional to the surface area.
Note 2 to entry: For example, Zr-Fe-V alloy acts as a surface getter for CO at room temperature.
3.5
volume getter
getter where the pumping speed and/or sorption capacity depends on the volume
Note 1 to entry: The dependence of the pumping speed and sorption capacity of the volume getters on the
temperature and operation pressure is more significant compared with the surface getter (3.4).
Note 2 to entry: For example, Zr-Fe-V alloy acts as a volume getter for H at room temperature. Zr-Fe-V alloy also
acts as a volume getter for CO at high temperature.
3.6
activation
conditioning by thermal treatment of a getter to develop its gettering characteristics
Note 1 to entry: Hydrogen reversibly acts with non-evaporable getters (NEGs) and therefore allows to be released
by activation.
Note 2 to entry: Other active gases such as CO, CO , N , and O are chemisorbed irreversibly with NEGs. The
2 2 2
activation promotes the diffusion of these gas atoms into the bulk.
3.7
getter pumping speed
S
volume of gas sorbed per unit time
Note 1 to entry: The pumping speed has the same meaning of the volume flow rate.
Note 2 to entry: Getter pumping speed depends on gas species and the amount of gas being sorbed.
3.8
initial pumping speed of NEG
instantaneous pumping speed 3 min after the start of the test at the chosen pressure and temperature
Note 1 to entry: This time delay is necessary to allow initial transient effects, until the pressure equilibrium has
become negligible.
3.9
intrinsic sticking probability
sticking coefficient
ratio of the number of sorbed gas molecules to that of impinging ones at a unit area per unit time, where
the surface is assumed to be flat.
3.10
sticking probability
α
ratio of the number of sorbed gas molecules to that of impinging ones at a unit apparent area per unit
time
Note 1 to entry: Sticking probability depends on gas species, surface chemical composition, surface roughness
and coverage.
Note 2 to entry: Sticking probability is typically measured as pumping characteristics of NEGs.
3.11
sorption quantity
C
q
quantity of gas sorbed by the getter
3.12
sorption capacity
C
C
quantity of gas sorbed by the getter until the getter pumping speed decrease to 10 % of the initial
pumping speed
4 Symbols and abbreviated terms
Symbol Designation Unit
A apparent surface area of getter material m
C conductance of orifice m /s
C sorption quantity Pa m
q
C sorption capacity Pa m
C
F correction factor of vacuum gauge 1, where F
1 1
= 1/K
F correction factor of vacuum gauge 2, where F
2 2
= 1/K
K sensitivity of vacuum gauge 1
K sensitivity of vacuum gauge 2
p pressure reading of vacuum gauge 1, which is Pa
R1
located at the upstream side of orifice
p pressure reading of vacuum gauge 2, which is Pa
R2
located at the downstream side of orifice
p base pressure of vacuum gauge 1 Pa
B1
p base pressure of vacuum gauge 2 Pa
B2
Q gas flow rate Pa m /s
pv
Q molar flow rate mol/s
mol
R ideal gas constant 8,134 J/(mol K)
S getter pumping speed m /s
T temperature K
α sticking probability
α initial sticking probability
5 Test methods
5.1 General
5.1.1 Test gases
H and CO shall be used to test for NEG. CO can be replaced by N or CO from a safety perspective when
2 2 2
an agreement is made between customer and testing laboratory. In addition, other gases such as O can
be required depending on the application. The purity of the test gas in the gas cylinder shall be higher
than 99,99 % for H and 99,95 % for CO, respectively. It is also recommended to measure the purity of
the test gas by using quadrupole mass spectrometer (QMS) in the vacuum chamber because the test gas
can be polluted during the transportation from the gas cylinder to the vacuum chamber.
5.1.2 Vacuum chamber
The vacuum chamber shall consist of all-metal vacuum components with a baking system. When valves
with elastomer sealing parts are used, they shall be bakeable and fabricated for the usage of UHV
condition. The cleanliness shall be appropriate to obtain sufficiently low base pressure in the range of
ultrahigh vacuum or extreme-high vacuum (XHV), depending on the application. The apparatus shall
-6
be capable of reaching a base pressure of less than 1×10 Pa without NEG sample installation or with
uncoated tube. In addition, it is recommended to measure the residual gas by QMS to make sure that
both the air leak and the outgassing of H O, CO, CO , and hydrocarbons are sufficiently small.
2 2
Note that H should be the dominating gas species at the base pressure.
5.1.3 Orifice
An orifice is used to determine the gas flow rate for the throughput method as shown in 5.2 and 5.3.
The molecular conductance of the orifice shall be calculated from the molecular mass, temperature of
the gas, and the diameter and the thickness of the orifice. The calculation method is shown in Annex A.
The conductance of the orifice C is carefully selected from four points of view:
— the diameter of the orifice is smaller than the mean free path of the test gas;
— C is sufficiently smaller than the system conductance which is obtained by combining conductances
of the pipe and vacuum chamber. The ratio of system conductance to C shall be larger than 100;
— C shall be selected so that the pressure ratio of the upstream pressure p of the orifice to the
0 1
downstream pressure p , p /p , during the test is larger than their error measured by the vacuum
2 1 2
gauges specified in 5.1.5. In this document, the p /p value during test is recommended to be larger
1 2
than 2;
— C shall be selected so that the pressure during the test is in the linear response range of the vacuum
gauge specified in 5.1.5. When C is too small, the upstream pressure p may be higher than the
0 1
linear response range of the Bayert-Alpert gauge (BAG) (key reference 3 in Figure 1) because the
downstream pressure p is set to satisfy the requirements in 5.2.5.
5.1.4 Vacuum pumping system for rough pumping
A turbomolecular vacuum pump (TMP) shall be used to obtain the sufficiently low base pressure
and to evacuate outgases during degassing and/or activation of test chamber and NEGs under test. In
addition, an ion pump may be useful to obtain lower base pressure before measurements. A dry pump
is recommended to be used as a roughing vacuum pump to avoid oil pollution, but it should be carefully
chosen because gases released from fluorine elements such as F and Cl can also pollute the surface of
the NEG.
Installing the valve between TMP and the vacuum chamber (for example, key reference 8 in Figure 1) is
strongly recommended so as to keep the inside of vacuum chamber clean in order not to be contaminated
by oil vapor and dust. In addition, it is useful to adjust the pressure p or to keep the inside of vacuum
chamber under vacuum while the system is not in operation.
5.1.5 Vacuum gauges
Bayert-Alpert vacuum gauges (BAGs), extractor gauges or ion analysing gauges shall be used to
measure the getter pumping speed and sorption capacity of NEG. BAGs shall be calibrated in a traceable
way to an applicable SI unit. In addition, using a quadrupole mass spectrometer (QMS) is strongly
recommended to not only to measure the performance of NEGs but also for other purposes such as leak
testing, checking a purity of test gas, evaluation of outgassing during activation. A spinning rotor gauge
or an ionization gauge according to ISO/TS 6737 instead of BAGs can be used, but a Magnetron gauge is
not recommended because the high pumping effect can cause overestimation of pumping performance
of NEG.
There are two methods to calibrate the BAGs. One is that the BAGs are calibrated in a laboratory meeting
the requirements of ISO/IEC 17025 or a national metrology institute. The other is that the BAGs are
calibrated from the direct comparison with a reference gauge in situ.
A spinning rotor gauge (SRG) or a high accuracy capacitance diaphragm gauge (CDG) with a full scale
of 133 Pa or lower shall be used as the reference gauge for in situ calibration. The position where the
reference gauge is attached shall be the upstream side of gas inlet against to TMP. The calibration gas
shall be the same as the one to be tested because BAG has gas species dependence. The nonlinearity
of the sensitivity of BAGs is recommended to be evaluated in advance although BAGs has liner
characteristics in principle. The QMS is similarly calibrated from the direct comparison with SRG, CDG,
and/or BAG. For information on the calibration method of QMS, refer to ISO/TS 20175.
5.1.6 Temperature
The measurements shall be taken at an ambient temperature of (23 ± 7) °C and the temperature shall
not change by more than 2K (peak-to-peak) during the measurement. The temperature of the vacuum
chamber shall be recorded.
5.1.7 Activation method of NEG
NEGs shall be activated according to the method specified by manufacturer if available. Various
methods are used to heat NEGs for activation such as induction heating, joule (resistance) heating,
radiant heating, conductance heating and electron bombardment. The non-uniformity of temperature
of NEG during heating shall be minimized. The temperature during the activation and activation time
shall be measured and recorded.
5.1.8 Procedure of sample installation and activation
The procedures of sample installation and activation shall be followed to the operation manual provided
by the manufacture if available. The general procedure is given in below.
a) A NEG sample to be tested is installed by using clean tools to the test chamber/dome.
b) The whole vacuum system is evacuated by the vacuum pumping system (see 5.1.4).
c) Bake-out the whole vacuum system including the test chamber/dome (e.g. 150 °C – 300 °C). The
bake-out time is from several hours to several days depending on the condition of the vacuum
system.
d) After cooling down the vacuum system, the NEG sample is heated up to the specified temperature
and time to activate. This activation should be initiated under high vacuum conditions of
-4
approximately 1×10 Pa or lower.
e) After activation, cooling the sample down to the operating temperature.
f) Reactivation of NEG shall be performed before each test.
It should be avoided that the pressure during activation becomes too high to avoid the pollution of NEGs.
NEG coatings should be especially taken care of the pollution during activation because their maximum
sorbed amount is smaller than other getters. Also, a safety guide provided by the manufacturer should
be kept for the handling of NEGs. In general, heating NEGs at low vacuum condition should be avoided.
BA gauges and QMS should be degassed between bake-out (procedure step c) and NEG activation
(procedure step d) if necessary.
NOTE In the case of a CO test after a H test, the influence of H gas exposure on the pumping performance of
2 2
CO is small. However, the opposite is not the case.
5.2 Throughput method for small NEG samples
5.2.1 Experimental setup
Figure 1 shows a schematic diagram of the measurement system of the throughput method for small
1) [14]
NEG samples. This method is based on ASTM F798-97 . The system consists of two chambers, a
gas manifold and a test chamber. The gas manifold includes a variable leak valve to introduce test
gases, a BA gauge (BAG-1), and a turbomolecular vacuum pump. An isolation valve may be located
between the manifold and the turbomolecular vacuum pump. An ion pump can also be added between
the turbomolecular vacuum pump and the isolation valve to obtain lower base pressure. The test
chamber shall include a test sample (NEGs or NEG pumps) including a heater to activate the sample,
a thermometer to measure the sample temperature, and another BA gauge (BAG-2). The test chamber
and the gas manifold are connected by the orifice with known conductance and a bypass valve. The
conductance of the orifice C is carefully selected to satisfy the requirement of the 5.1.3. Equipping a
sample chamber with the test chamber can be useful as shown in Figure 1 b) to increase the efficiency
of testing, but special care shall be taken so that the conductance between the test chamber and the
sample chamber is sufficiently large.
The controller of BA gauges shall be set to be the constant emission current against the changing in the
pressure. The lower emission current such as from 0,01 mA to 0,1 mA is recommended to decrease the
influence of outgassing and pumping effect of the BA gauge.
a) Method without sample chamber
1) Withdrawn.
b) Method with sample chamber
Key
1 manifold
2 test chamber
3 BA gauge (BAG-1)
4 BA gauge (BAG-2)
5 orifice, C
6 bypass valve
7 variable leak valve
8 valve
9 gas cylinder
10 sample (NEG)
11 thermometer
12 sample chamber
13 gate valve
14 turbomolecular vacuum pump
15 roughing vacuum pump
Figure 1 — Throughput method for small NEG sample
5.2.2 Sample
Relatively small size of NEGs with the type of pill, disk, ring, strip, module, cartridge, NEG coating
disk shall be tested by using either system shown in Figure 1a) or Figure 1b). NEG pumps with a small
diameter of connection flange can also be tested by this system.
5.2.3 Determination of getter pumping speed, S, and sorption quantity, C
q
The flow rate, Q, passing through the orifice equals to that pumped by NEGs or NEG pumps with the
pumping speed of S at the quasi-equilibrium condition. Then, the following relation is established as
shown in Formula (1).
QC=−()pp =⋅Sp (1)
01 22
where C is the conductance of orifice and should be noted that C depends on gas species and
0 0
temperature. This document strongly recommends selecting parameters such as the measurement
pressure and the inner diameter of orifice so that the molecular flow condition is realized for gases
passing through the orifice. The data analysis becomes much simple because the conductance C
becomes constant and is obtained analytically.
The pressure p and p , which are the pressures in the gas manifold and test chamber, respectively, are
1 2
obtained by Formulae (2) and (3):
pp−
RB11
p = =−Fp p , (2)
()
1 11RB1
K
pp−
RB22
p = =−Fp()p , (3)
2 21RB2
K
where
p is the pressure reading of BAG-1;
R1
p is the base pressure of BAG-1;
B1
K is the sensitivity of BAG-1;
F is the correction factor of BAG-1;
p is the pressure reading of BAG-2;
R2
p is the base pressure of BAG-2;
B2
K is the sensitivity of BAG-2;
F is the correction factor of BAG-2;
The sensitivity K is equal to the inverse of the correction factor F.
Following the above relations, the getter pumping speed, S, and the sorption quantity, C , are obtained
q
by Formulae (4) and (5):
Cp −p
()
01 2
S = , (4)
p
tt
CQ==dt Cp()−pdt, (5)
qpV 01 2
∫∫
0 0
where
Q is the gas flow rate
pV
t is the elapsed time
When the sample chamber is employed (see Figure 1b), the pressure in the test chamber may be
[15]
different with that in the sample chamber. Then, the Formula (4) shall be corrected as follows .
Cp()−p
01 2
S = , (6)
C
p −−()pp
2 12
C
p
where C is the conductance of the pipe and the valve (key reference 13 in Figure 1 b)) between the
p
test chamber and the sample chamber. Monte Carlo simulation is also useful to correct the pressure
[16-18]
distribution in the test chamber and sample chamber .
NOTE 1 Commercially available BA gauges are calibrated for N in general. Relative sensitivities of H , K /
2 2 H2
K , and CO, K /K , for N are about 0,40 and 1,03, respectively, although those depend on the type and the
N2 CO N2 2
[19]
operational parameter of BA gauge .
NOTE 2 When NEG pumps are tested by this method, the measurement result depends on the connection
method to the test chamber. For example, the pumping speed when the NEG pump is inserted into the test
chamber [see Figure 2a)] becomes larger than that when NEG pump is connected to the test chamber by using
a nipple [see Figure 2b)]. Since the latter case is close to the method shown in 5.3, refer to the requirements
specified in 5.3.
a) NEG pump is inserted into the test chamber
b) NEG pump is connected by using a nipple
Key
1 manifold
2 test chamber
3 NEG pump
4 nipple
Figure 2 — The connection method of NEG pump to the test chamber.
5.2.4 Determination of sticking probability, α
When the pressure distribution in the test chamber is homogeneous, in other words, the gas molecules
in the test chamber hit on the NEG surface with the same probability independent on the position,
the sticking probability, α, on the NEG surface is obtained by Formula (7). The sticking probability, α,
corresponds to the intrinsic sticking probability or sticking coefficient, α , when the NEG surface is flat.
4S
α = (7)
Av
where
S is the getter pumping speed S obtained by Formulae (4) or (6) (m /s);
A is the apparent surface area of NEG (m );
v
is the arithmetic mean velocity of gas molecule for the test gas (= 8RT/πM ) (m/s);
R is ideal gas constant (=8,134 J/(mol K)) ;
T is the temperature (K) ;
M is the mass of gas molecule (kg) ;
NOTE 1 The assumption that the pressure distribution in the test chamber is negligible is satisfied only when
the size of NEG sample is sufficiently small compared with that of the test chamber. If not, for example, when the
NEG sample is located in the nipple as shown in Figure 2b), the α estimated by Formula (7) gives an incorrect
value.
NOTE 2 The accuracy of α can be improved by calculating the pressure distribution in the test chamber by
[16-18]
Monte Carlo simulation .
NOTE 3 The typical values of initial sticking probability measured by the method of 5.2 are listed in Table C.1
of Annex C.
5.2.5 Measurement procedure
-6
a) Base pressures of both the test chamber and gas manifold shall be less than 1×10 Pa.
b) The bypass valve is closed. No significant increase of the pressure p shall be observed.
c) The test gas is introduced into the gas manifold and adjusted by leak valve so that the pressure of
p becomes constant. The pressure ratio of p /p shall be kept higher than 2.
2 1 2
d) Record the pressure p , p , and temperature of NEG at least with a suitable time interval. The several
1 2
ten seconds of time interval should be suitable at the beginning of the measurement because of
rapid change in getter pumping speed. It can be extended as the sorption quantity of NEG increases
because the change becomes slow.
-4 -3 -
In the absence of specified request, p should be set between 1×10 Pa and 1×10 Pa, preferably 4×10
Pa for applications in medium and high vacuum conditions. This test pressure is more than 100 times
higher than the base pressure before the gas admitting at procedure step b) and is sufficiently low to
realize the molecular flow condition in typical. In addition, it is within the linear response range of BA
gauges and close to the calibration pressure range in typical.
For UHV applications the lower base and test pressures at p may be required. For example, setting the
-8 -10
test pressure from 10 Pa to 10 Pa is required for high performance surface analysis devices and
electron microscopy systems.
The dependence of S, C , and C on NEG temperature may be measured. The S, C , and C of CO, for
q c q c
[7,8,12,13]
example, depend on the NEG temperature in general .
When the volume getter is dominant, for example H pumping, note that the pumping speed may
depend on the test pressure because it is competing reaction between gas injection onto the surface
and diffusion into the bulk.
5.2.6 Measurement uncertainty
The measurement uncertainties of S and C are estimated from the uncertainty to measure p and p by
q 1 2
BA gauges, the uncertainty of the orifice conductance, and the uncertainty due to the pressure
distribution in the test chamber. In the case of α, the uncertainties of the apparent surface area of NEG,
A, and the arithmetic mean velocity of gas molecule for the test gas, v , are also added. A total uncertainty
of several tens of percentage is sufficient for general purposes.
When the testing devise has a sample chamber as shown in Figure 1b), it is recommended that the
sensitivity of two BA gauges are cross-checked by admitting the test gas after opening the bypass valve
(key reference 6 in Figure 1) and closing the gate valve (key reference 13 in Figure 1).
EXAMPLE References [15] and [18] include examples of the evaluation of measurement uncertainty.
5.3 Throughput method with test dome
5.3.1 Experimental setup
Three types of test domes are shown in Figure 3. The test dome shown in Figure 3a) is allowed when a
suitable gas flow measuring instrument is available. Leak elements such as small orifice, capillary, and
[20-25]
sintered filter are used .
The test domes of Figure 3b) and Figure 3c) are used for the orifice flow method. The test dome
[26-30]
Figure 3c) is the so-called Fischer- Mommsen dome. The diameter and the thickness of the orifice
shall be measured for the calculation of the conductance in advance. The calculation method of the
molecular conductance is shown in Annex A. The conductance of the orifice C is carefully selected so as
to satisfy the requirements of the 5.1.3.
a) Test dome
b) Test dome
c) Fischer – Mommsen dome
Key
1 gas inlet
2 connections for BA gauge, mass spectrometer, and high vacuum pumping system
3 connections for BA gauge, mass spectrometer
4 gas inlet for vacuum gauge calibration and temperature measuring point, T
D
5 orifice
D inner diameter of test dome
a) SOURCE: Reproduced with permission from ISO 21360-1:2020, Figures 1.
b) SOURCE: Reproduced with permission from ISO 21360-1:2020, Figures 3.
c) SOURCE: Based on references [20-24].
Figure 3 — Three types of test domes for throughput method
NEG pumps or NEG coated chambers under test shall be located at the bottom of each test dome. Another
high pumping system for rough pumping (5.1.4) shall be located at the connection flange labelled as key
reference 2 in Figure 3 via an isolation valve.
The main problem in using the test dome is its pressure distribution. In the case of the test dome shown
in Figure 3 a), the gauge position (key reference 2 in Figure 3) has been determined so that the pressure
at the gauge position is comparable with that at the inlet of the NEG pumps or NEG coated chambers
under test. In the case of the test dome shown in Figures 3b) and 3 c), the gauge position of downstream
side of the orifice (key reference 3 in Figure 3) has been similarly determined. On the other hand, the
gauge position of upstream side of the orifice (key reference 2 in Figure 3) has been selected so that the
pressure at the gauge position is comparable with that at the inlet of the orifice. A different shape of test
dome can be used when the pressure measured by vacuum gauges is compensated by considering the
pressure distribution in the test dome.
5.3.2 Sample
Relatively large size of NEGs such as strip, module, cartridge, NEG pumps, and NEG coated tubes or
chambers shall be tested by using this method.
5.3.3 Determination of getter pumping speed S and sorption quantity, C
q
The method to determine the pumping speed S is specified in ISO 21360-1:2020, 5.1 and 5.2. When the
dome shown in Figure 3a) is used, the getter pumping speed S is obtained by the flow rate Q, which is
determined by the flow meter, divided by the pressure p in the test dome (S = Q/p). This p is the value
after subtracting the base pressure. When the dome shown in Figure 3b) or the Fischer-Mommsen
dome shown in Figure 3c) is used, the pumping speed S is obtained by Formula (4) by using p and p to
1 2
the upstream pressure for the orifice and the downstream one, respectively. The sorption quantity C is
q
also obtained by the Formula (5).
5.3.4 Determination of sticking probability, α
It is impossible to determine the α by using Formula (7) except for strip, module, and NEG coating
chambers with the small length to diameter ratio, because the pressure distribution in the NEG pump
or NEG coating chamber under test is significant. However, the α can be obtained by comparing the
calculation result by a Monte Carlo simulation.
NOTE 1 The typical values of initial sticking probability measured by the method of 5.3 is listed in Table C.1 of
Annex C.
5.3.5 Measurement procedure
-6
a) It is confirmed that the base pressure of the test dome is less than 1×10 Pa.
b) In the case of test dome Figure 3a), the isolation valve, which is located at key reference 2 in Figure 3
between the test dome and the high vacuum pumping system, is closed. No significant increase of
the pressure p is confirmed. In the case of test dome Figure 3b) or c), it is not necessary to close the
valve located at the connection to TMP (key reference 2) but it can be throttled.
c) Test gas is introduced into the test dome through the gas inlet (key reference 1 in Figure 3). The test
pressure p in the test dome Figure 3a) should be set more than twice as large as the base pressure
before the gas admitting. The pressures p and p in the dome Figure 3b) and Figure 3c) is similar.
1 2
d) Record the flow rate Q and the pressure p for the test dome Figure 3a) or the pressures p and p
1 2
for the test dome Figure 3b) or Figure 3c), and temperature of NEG at least with a suitable time
interval.
NOTE 1 The test pressure p is similarly chosen in 5.2.5.
NOTE 2 The dependence of S and the sorption capacity C on NEG temperature can be measured. The S and C
C
of CO, for example, depend on the NEG temperature in general.
NOTE 3 The pumping speed can depend on the gas flow rate when volume getter acts, for example H pumping,
because it is a competing reaction between gas injection onto the surface and diffusion into the bulk.
5.3.6 Measurement uncertainty
When the dome shown in Figure 3a) is used, the measurement uncertainties of S and C are estimated
q
from the uncertainty to measure the flow rate Q and p, and that due to the pressure distribution in the
test chamber. When the dome shown in Figure 3b) or Figure 3 c) is used, the measurement uncertainty
is estimated from the uncertainty to measure p and p by BA gauges, the orifice conductance, and the
1 2
pressure distribution in the test chamber. In the case of α, the uncertainties of the apparent surface
area of NEG, A, and the arithmetic mean velocity of gas molecule for the test gas, v , are also added. The
total uncertainty of several tens of percentage is sufficient for general purposes.
When the domes shown in Figure 3b) 3c) are used, it is recommended that the sensitivities of two BA
gauges are cross-checked by admitting the test gas from the gas inlet (labelled as key reference 4) in
Figure 3.
5.4 Transmission method for NEG coatings
5.4.1 Experimental setup
Schematic diagrams of the measurement system of transmission method for NEG coating are shown
[31,32]
in Figure 4. In the case of the closed design illustrated in Figure 4a), the test gas is admitted at
right-hand end of the coated pipe by the variable leak valve. Although leak elements such as small
orifice, capillary, and sintered filter may be also used instead of the variable leak valves, special care is
necessary to select their conductance
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