ISO 21360-1:2020
(Main)Vacuum technology — Standard methods for measuring vacuum-pump performance — Part 1: General description
Vacuum technology — Standard methods for measuring vacuum-pump performance — Part 1: General description
This document specifies three methods for measuring the volume flow rate and one method each for measuring the base pressure, the compression ratio, and the critical backing pressure of a vacuum pump. The first method for measuring the volume flow rate (the throughput method) is the basic concept, in which a steady gas flow is injected into the pump while the inlet pressure is measured. In practice, the measurement of gas throughput may be complicated or inexact. For this reason, two other methods are specified which avoid the direct measurement of throughput. The second method for measuring the volume flow rate (the orifice method) is used when there is very small throughput at very small inlet pressures (under a high or ultra-high vacuum). It is based on measuring the ratio of pressures in a two-chamber test dome in which the two chambers are separated by a wall with a circular orifice. The third method for measuring the volume flow rate (the pump-down method) is well suited for automated measurement. It is based on the evacuation of a large vessel. The volume flow rate is calculated from two pressures, before and after a pumping interval, and from the volume of the test dome. Different effects, such as leak and desorption rates, gas cooling by nearly isentropic expansion during the pumping interval, and increasing flow resistance in the connection line between test dome and pump caused by molecular flow at low pressures, influence the results of the pressure measurement and the resulting volume flow rate. The choice of the required measurement methods depends on the properties of the specific kinds of vacuum pump, e.g. the measurement of the critical backing pressure is only necessary for vacuum pumps which need a backing pump. All data that are measured on a vacuum pump, but not specified in this document (e.g. measurement of power consumption), are defined in the specific pump standard.
Technique du vide — Méthodes normalisées pour mesurer les performances des pompes à vide — Partie 1: Description générale
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 21360-1
Second edition
2020-06
Vacuum technology — Standard
methods for measuring vacuum-pump
performance —
Part 1:
General description
Technique du vide — Méthodes normalisées pour mesurer les
performances des pompes à vide —
Partie 1: Description générale
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved
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 Volume flow rate (pumping speed) measurement by the throughput method . 4
5.1.1 General. 4
5.1.2 Test dome for the throughput method . 5
5.1.3 Experimental setup . 6
5.1.4 Determination of the volume flow rate . 7
5.1.5 Measuring procedure . 8
5.1.6 Measuring uncertainties . 8
5.1.7 Evaluation of the measurement . 8
5.2 Volume flow rate (pumping speed) measurement by the orifice method . 9
5.2.1 General. 9
5.2.2 Test dome for the orifice method . 9
5.2.3 Experimental setup .10
5.2.4 Determination of the volume flow rate .11
5.2.5 Measuring procedure for the orifice method .12
5.2.6 Adjustment of the pressure-measuring gauges .12
5.2.7 Measurement of the volume flow rate .12
5.2.8 Measuring uncertainties .12
5.2.9 Evaluation of the measurement .13
5.3 Volume flow rate (pumping speed) measurement by the pump-down method .14
5.3.1 General.14
5.3.2 Test dome for the pump-down method .14
5.3.3 Quick-acting valve .15
5.3.4 Experimental setup .15
5.3.5 Determination of the volume flow rate .16
5.3.6 Measuring procedure .17
5.3.7 Limits of applicability .18
5.3.8 Evaluation of the measurement .18
5.3.9 Measurement uncertainty .18
5.4 Measurement of the base pressure .18
5.4.1 Operating conditions .18
−4
5.4.2 Test procedure for pumps with a base pressure >10 Pa .19
−4
5.4.3 Test procedure for pumps with a base pressure <10 Pa .19
5.4.4 Evaluation of the measurement .19
5.5 Measurement of the compression ratio and the critical backing pressure .19
5.5.1 Experimental setup .20
5.5.2 Determination of the compression ratio and the critical backing pressure .20
5.5.3 Measurement procedure .21
5.5.4 Measurement uncertainty .22
5.5.5 Evaluation of the measurements .22
5.5.6 Specific recommendations for extremely high compression ratio
measurements.22
Annex A (informative) Mean free path of some important gases .24
Annex B (informative) Measuring uncertainties .25
Bibliography .28
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 of 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 www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 112, Vacuum technology.
This second edition cancels and replaces the first edition (ISO 21360-1:2012), of which it constitutes a
minor revision. The changes compared to the previous edition are as follows:
— Note in 3.3 has been deleted;
— K in 3.7 has been corrected;
— 3.9 the definition of the volume has been changed to "is the volume of transported gas";
— Figure 1 has been corrected;
— Figure 2 has been corrected;
— 5.2.7: change to "for at least 60s" instead of " for the following minute".
A list of all parts in the ISO 21360 series can be found on the ISO website.
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.
iv © ISO 2020 – All rights reserved
Introduction
This document is a basic standard for measuring the performance data of vacuum pumps. The methods
specified here are well known from existing national and International Standards. In developing this
document, the aim has been to provide a single document containing the measurements of performance
data of vacuum pumps and to simplify the future development of specific vacuum pump standards.
Specific vacuum pump standards will contain a suitable selection of measurement methods from
this document in order to determine the performance data, limiting values and specific operational
conditions on the basis of the specific properties of the particular kind of pump. Whenever a discrepancy
exists between this document and the specific standard, it is the specific standard which is valid.
INTERNATIONAL STANDARD ISO 21360-1:2020(E)
Vacuum technology — Standard methods for measuring
vacuum-pump performance —
Part 1:
General description
1 Scope
This document specifies three methods for measuring the volume flow rate and one method each for
measuring the base pressure, the compression ratio, and the critical backing pressure of a vacuum pump.
The first method for measuring the volume flow rate (the throughput method) is the basic concept, in
which a steady gas flow is injected into the pump while the inlet pressure is measured. In practice, the
measurement of gas throughput may be complicated or inexact. For this reason, two other methods are
specified which avoid the direct measurement of throughput.
The second method for measuring the volume flow rate (the orifice method) is used when there is
very small throughput at very small inlet pressures (under a high or ultra-high vacuum). It is based on
measuring the ratio of pressures in a two-chamber test dome in which the two chambers are separated
by a wall with a circular orifice.
The third method for measuring the volume flow rate (the pump-down method) is well suited for
automated measurement. It is based on the evacuation of a large vessel. The volume flow rate is
calculated from two pressures, before and after a pumping interval, and from the volume of the test
dome. Different effects, such as leak and desorption rates, gas cooling by nearly isentropic expansion
during the pumping interval, and increasing flow resistance in the connection line between test dome
and pump caused by molecular flow at low pressures, influence the results of the pressure measurement
and the resulting volume flow rate.
The choice of the required measurement methods depends on the properties of the specific kinds of
vacuum pump, e.g. the measurement of the critical backing pressure is only necessary for vacuum
pumps which need a backing pump. All data that are measured on a vacuum pump, but not specified
in this document (e.g. measurement of power consumption), are defined in the specific pump standard.
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 3529-2, Vacuum technology — Vocabulary — Part 2: Vacuum pumps and related terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3529-2 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
volume flow rate
q
V
dV
q = where
V
dt
V is volume;
t is time
[4]
[SOURCE: ISO 80000-4:2006 , 4-30]
EXAMPLE In the context of this document, the volume flow rate is the volume of gas which, under ideal
conditions, flows from the test dome through the pump inlet per time.
Note 1 to entry: For practical reasons, the volume flow rate of a given pump and for a given gas is conventionally
considered to be equal to the quotient of the throughput of this gas and of the equilibrium pressure at a given
location. The volume flow rate is expressed in cubic metres per hour or litres per second.
Note 2 to entry: The term “pumping speed” and symbol “S” are often used instead of “volume flow rate”.
3.2
inlet pressure
p , p , p
1 d e
pressure at the inlet of the pump, measured at a defined location in the test dome
3.3
base pressure
p
b
pressure obtained in the test dome after conditioning the vacuum pump and the test dome
Note 1 to entry: See 5.4.
3.4
maximum working pressure
p
1max
highest pressure on the inlet side that the vacuum pump and the driving device can withstand for a
prolonged period of operation time without being damaged
3.5
backing pressure
p
pressure at the outlet of a vacuum pump
3.6
critical backing pressure
p
c
maximum backing pressure for which the conditions are defined in the instruction manual or in a
specific standard for the particular vacuum pump
3.7
compression ratio
K
compression ratio without gas load, wherein p is the base pressure of the backing pump and p is the
b3 b1
base pressure of the test pump
pp−
33b
K =
pp−
11b
2 © ISO 2020 – All rights reserved
3.8
test dome
special vacuum vessel with precisely defined size, diameter and connection flanges on specified
locations, used for standard performance data measurements on vacuum pumps
3.9
throughput
Q
amount of gas flowing through a duct, expressed by the formula:
pV
Q==pq
1 V
t
where
p is the (high) vacuum pressure on the inlet;
q is the volume flow rate of the test pump;
V
t is time;
V is the volume of transported gas
3.10
standard gas flow rate
q
Vstd
volume flow rate at standard reference conditions, i.e. 0 °C and 101 325 Pa
4 Symbols and abbreviated terms
Symbol Designation Unit
a inner diameter of the connection pipe between test pump m
and quick-acting valve (items 3 and 5 in Figure 6)
A cross-section of the connection pipe between test pump and m
quick-acting valve (items 3 and 5 in Figure 6)
3 3
C conductance m /s (= 10 l/s)
d diameter of orifice m
D inner diameter of test dome m
D diameter of the inlet pipe m
i
D nominal diameter of test dome m
N
K compression ratio of vacuum pump with zero throughput —
l length of the connection pipe between test pump and m
quick-acting valve (items 3 and 5 in Figure 6)
mean free path m
l
M molar mass of gas kg/mol
p standard atmospheric pressure — 101 325 Pa Pa
p (high) vacuum pressure on inlet Pa (or mbar)
p maximum working pressure on inlet Pa (or mbar)
1max
p vacuum pressure in backing line Pa (or mbar)
p , p ,p pressures in the test dome for the pump-down method, Pa (or mbar)
t t t
1 2 3
measured before and after time intervals Δt , Δt , Δt
1 2 3
p , p , p base pressures Pa (or mbar)
b1 b2 b3
p critical backing pressure Pa (or mbar)
c
p , p pressures in the test dome for the orifice method Pa (or mbar)
d e
Q gas throughput of vacuum pump Pa·l/s (or mbar·l/s)
Q test gas load Pa·l/s (or mbar·l/s)
r
q volume flow rate of test pump l/s (or m /h)
V
q volume flow rate of backing pump l/s (or m /h)
VBP
q volume flow rate at standard reference conditions for gases, sccm (or cm /min)
Vsccm
i.e. 0 °C and 101 325 Pa
q volume flow rate at standard reference conditions for gases, l/s (or m /h)
Vstd
i.e. 0 °C and 101 325 Pa
Q maximum gas throughput of vacuum pump which the pump Pa⋅l/s (or mbar·l/s)
max
can withstand without damage
R ideal gas constant 8,314 J/(mol·K)
T thermodynamic temperature K
T 273,15 K (defined as 0 °C) K
T temperature of the test dome K
D
T temperature of the flow meter K
f
u measurement uncertainty —
V volume of the test dome l, m
V volume of connection pipe between test pump and quick-act- l, m
i
ing valve (items 3 and 5 in Figure 6)
δ thickness of the orifice wall at the orifice diameter m
5 Test methods
5.1 Volume flow rate (pumping speed) measurement by the throughput method
5.1.1 General
The throughput method is the one most used for vacuum pumps and is applicable to all pressure ranges
and pump sizes where flow meters for gas throughput measurements are available with sufficient
4 © ISO 2020 – All rights reserved
accuracy. The gas flow measuring ranges shall be chosen by multiplying the expected volume flow rate
by the maximum and minimum working pressure of the test pump.
All measuring devices shall be calibrated either:
a) in a traceable way to a vacuum primary or to a national standard, or
b) by means of instruments of absolute measure which are traceable to the SI units and to which
measurement uncertainties can be attributed.
In the case of calibrated measuring instruments, there should exist a calibration certificate in
[3]
accordance with ISO/IEC 17025 .
5.1.2 Test dome for the throughput method
For these measurements, use a test dome as shown in Figure 1 with the same nominal diameter, D ,
N
as that of the pump inlet. The face of the dome opposite the inlet flange may be flat, conical or slightly
curved, with the same average height above the flange as the flat face. Three flanges are preferable for
pressure measurement at a height of D/2 above the bottom flange if more than one pressure gauge is
used. The diameter of these flanges should be greater than or equal to the flanges of the gauges used,
and their mounting dimensions shall be noted. No measuring port shall be located in the angle range
±45° next to a gas inlet port. The connection pipes between flange and dome shall not protrude beyond
the dome wall on the inside, with the exception of the gas inlet pipe.
If necessary for the test pump, the test dome shall be fitted with a device for bake-out that ensures
uniform heating of the dome to achieve the base pressure.
The volume of the test dome may depend on the pump type. Refer to the specific pump standard for
details.
For pumps with an inlet flange diameter of less than D = 100 mm, the diameter of the dome shall
N
correspond to D = 100 mm. The transition to the pump inlet flange shall be made through a 45° conical
N
adaptor, as shown in Figure 1.
Key
1 gas inlet pipe and temperature measuring point for T
D
2 vacuum gauge and mass spectrometer connections
D diameter of the inlet pipe
i
D inner diameter of test dome, in metres
NOTE D should be big enough to allow homogenous gas conditions at the pump flange. A diameter of 0,1D is
i
generally appropriate.
Figure 1 — Test dome for the throughput method
5.1.3 Experimental setup
See Figure 2.
The test dome shall be clean and dry. The cleanness of the pump, seals and other components shall
be appropriate for the expected base pressure. All components are mounted together under clean
conditions in accordance with Figure 2. Because of the narrow measuring range, flow meters with
different ranges may be switched in series. If flow is restricted by a small flow meter, they may be used
in parallel with a manifold, adding a valve between every flow meter and the manifold. Instead of the
flow meter and the gas inlet valve, mass flow controllers with programmable throughputs may be used.
They shall be combined in parallel on a manifold.
The leak-tightness of large mass flow controllers is not sufficient in many cases. In such cases, it is
advisable that valves be used between the flow controller and the manifold.
Ionization gauges and mass spectrometers shall be installed in such a way that there is no direct
geometrical path between them.
CAUTION — Observe the safety instructions of the vacuum pump manufacturer.
6 © ISO 2020 – All rights reserved
Key
1 test dome 4 gas inlet valve 7 heating jacket (optional)
2 backing pump 5 flow meters to measure Q 8 vacuum gauge to measure p
3 test pump 6 vacuum gauge to measure p 9 temperature measuring point for T
1 D
NOTE Items 2 and 8 are only used in connection with high-vacuum test pumps.
Figure 2 — Arrangement for measuring volume flow rate (pumping speed) with throughput
method
5.1.4 Determination of the volume flow rate
The method adopted for the measurement of the volume flow rate, q , is the throughput method for
V
which the gas throughput, Q, is measured outside the dome. If the pressure, p , in the test dome,
measured by a vacuum gauge at the specified height above the bottom flange (see Figure 1), is held
constant, the volume flow rate, q , is obtained by the relationship
V
Q
q = (1)
V
pp−
1b
where p is the base pressure in the test dome (see 5.4).
b
An analogue equation is valid for the volume flow rate of the backing pump, q .
VBP
Q
q = (2)
VBP
pp−
33b
The gas throughput can be measured volumetrically (gas burettes, gas counters) by means of viscous
flow effects (rotameter, capillaries) or, in most cases, by means of thermoelectric mass flow meters (see
Reference [6] pp. 109–113).
Because of the dependence of the temperature on the gas volume, for all volumetric measurements,
corrections by a factor of T /T are necessary if the temperature, T , of the flow meter and T of the test
D f f D
dome are different.
NOTE Thermoelectric mass flow meters do not measure the throughput, but the volume flow rate, q , at
Vstd
standard reference conditions for gases (i.e. p = 101 325 Pa and T = 273,15 K, see 3.10). To obtain the throughput,
0 0
q is multiplied by the factor T p /T . Consequently, q is given by:
Vstd D 0 0 V
qp T
Vstd0 D
q = (3)
V
Tp()−p
01 b
The unit “sccm” (standard cubic centimetre per minute) is frequently used for q . If so, one obtains q ,
Vstd V
−3
in litres per second, by inserting [q = (q /sccm) ⋅ 10 l/60 s], [p = 101 325 Pa] and [T = 273,15 K]
Vstd Vsccm 0 0
in Formula (4), as follows:
31−−3
qT/mcm in ××10 lP101325 a×
()
Vsccm D
q = l//s (4)
V
60sK××273,(15 pp− )
1b
5.1.5 Measuring procedure
The arrangement of the measuring equipment with the test dome from Figure 1 is given in Figure 2.
At the start, when the gas inlet valve is closed, the base pressure shall prevail in the test dome (see
5.4). Then gas is admitted to the test dome through the adjustable valve. Measurements are made with
increasing pressure from a threshold value, allowing the correct use of the flow meter. During this
period of time, the ambient temperature shall be constant within ±2 °C.
When the required pressure, p , is obtained, within a variation of 3 %/min, measure the pressures, p
1 1
and p , the ambient temperature and the test dome temperature, T , as well as the admitted throughput,
3 D
Q. If the throughput remains steady to within ±3 %, the measurement at this point may be regarded as
valid. If the throughput is unsteady due to a transient condition, wait until it stabilizes. If the throughput
measurement lasts for more than 60 s, the pressure, p , in the dome shall be noted at least every minute. In
this case, the pressure is the average of the measured values. If during a measurement, the pressure or the
throughput varies by more than ±3 %, the measurement shall be repeated until the readings are stable.
Measurements shall be made at a minimum of three points per pressure decade of p . If the throughput
is increased to the maximum allowed value, Q , the maximum inlet pressure is obtained whose values
max
may be limited by the manufacturer.
NOTE Volume flow rate measurements can be made with different gases. When the gas is changed, all pipes
connected to the gas inlet valve are purged with the new gas before the beginning of the new measurement.
5.1.6 Measuring uncertainties
The gas flow should be measured with a standard uncertainty of ±2,5 % and the pressure with a
standard uncertainty of less than ±3 %. For the exact calculation, see Annex B. The total uncertainty of
the volume flow rate shall be <10 %.
5.1.7 Evaluation of the measurement
Plot on a semi-logarithmic graph (similar to Figure 5) the volume flow rate, q , of the test pump,
V
calculated by means of Formula (1), with respect to the inlet pressure, and plot on the same graph the
volume flow rate, q , of the backing pump (if used), calculated from Q and p , with respect to p , so
VBP std 3 3
as to show the size of the backing pump. The range of abscissa shall cover the whole range of pressures
p and p . The base pressures of the vacuum pump, p , and of the backing pump, p , shall be indicated.
1 3 b1 b3
The test report shall include as a minimum:
a) type, serial number, measuring uncertainty and operational conditions of all vacuum gauges and
flow meters used;
b) type and serial number of the test pump;
c) rotational frequency ("speed") and/or other operating conditions of the test pump;
d) fluids and their vapour pressures at 20 °C used in the test pump;
e) D (nominal diameter of the test dome and flange type);
N
8 © ISO 2020 – All rights reserved
f) type and volume flow rate of the backing pump (if used);
g) type of seals used upstream from the inlet flange of the test pump;
h) type of baffles and traps employed during the test, as well as their temperatures;
i) cooling water temperatures and water flow rate;
j) ambient and test dome temperatures;
k) baking time and temperatures.
5.2 Volume flow rate (pumping speed) measurement by the orifice method
5.2.1 General
The orifice method is applicable to high-vacuum pumps. Molecular flow conditions shall be present
in the test dome. This method is recommended for low gas throughputs where no suitable gas flow
meters are available. The orifice diameter in the test dome shall be adapted to the expected volume
flow rate of the test pump in order to avoid excessively high pressures which would result in laminar
flow conditions through the orifice.
5.2.2 Test dome for the orifice method
The test dome shall be cylindrical and of the shape shown in Figure 3. A wall with a (changeable) circular
orifice divides the dome into two chambers. A device for bake-out that ensures uniform heating of the
dome is needed.
The diameter of the thin-wall orifice plate (δ/d < 0,1) shall be chosen according to the expected flow
rate and shall be such that the ratio of the pressures p and p is between 3 and 30. Care shall be taken
d e
to ensure that in the orifice the mean free path, l , of the gas particles is not smaller than twice the
orifice diameter, 2d.
For specific values of l , see Annex A.
For pumps with an inlet flange diameter greater than or equal to D = 100 mm, the nominal diameter,
N
D , of the dome shall be equal to the actual diameter of the inlet flange.
N
For pumps with an inlet flange diameter of less than D = 100 mm, the diameter of the dome shall
N
correspond to D = 100 mm. In this case, the transition to the pump inlet flange shall be made through
N
a 45° taper fitting in accordance with Figure 1.
Key
1 gas inlet
2 gas inlet and temperature measuring point for T
D
3 vacuum gauge and mass spectrometer connections
D inner diameter of test dome, in metres
δ thickness of the orifice wall at the orifice diameter, in metres
p , p pressures in the test dome for the orifice method, in pascals (or millibars)
d e
Figure 3 — Test dome for the orifice method
5.2.3 Experimental setup
See Figure 4.
The test dome shall be clean and dry. For all connections on the high-vacuum side, bakeable knife-edge
flanges are recommended.
CAUTION — Do not touch inner surfaces with your hands. Use gloves during mounting.
10 © ISO 2020 – All rights reserved
Key
1 test dome 6 vacuum gauge to measure p
2 backing pump 7 vacuum gauge to measure p
d
3 test pump 8 vacuum gauge to measure p
e
4 gas inlet valve 9 heating jacket
5 gas inlet valve 10 temperature measuring point for T
D
Figure 4 — Arrangement for measuring volume flow rate (pumping speed) with orifice method
5.2.4 Determination of the volume flow rate
A thin circular orifice plate divides the test dome into two volumes (see Figure 3). The volume flow rate
is given by
pp−
dbd
qC= −1 (5)
V
pp−
ebe
where C is the calculated conductance, taking into account the orifice size and the gas properties.
The base pressures, p and p , in the upper and lower chamber of the test dome are measured after
bd be
baking (see 5.4) and before admission of the gas. The conductance of the orifice with diameter, d, and
thickness, δ, can be calculated using Formula (6):
πRT
D
C= d (6)
32Md1+ δ/
()
The term 1/[1 + (δ/d)] is a correction factor (only valid for δ<< d) that can be defined as the average
transition probability through the orifice.
Take care that the formula is used with consistent units. Inserting the values
R = 8,314 J/(mol⋅K)
−3
M = 28,97 ⋅ 10 kg/mol
air
T = 293 K (20 °C)
D
gives, in cubic metres per second,
91d
C = (7)
air
1+ δ/d
()
or, in litres per second,
91000d
C = (8)
air
1+()δ/d
where δ and d are measured in metres.
5.2.5 Measuring procedure for the orifice method
The arrangement of the measuring equipment is given in Figure 4. At the start, after baking with all
inlet valves closed, the base pressures, p and p , shall prevail in the test dome (see 5.4).
bd be
5.2.6 Adjustment of the pressure-measuring gauges
After reaching and recording the base pressures, p and p , in the test dome, the test gas is admitted to
bd be
valve (Figure 4, label 4) to check the sensitivity of the gauges (Figure 4, label 7) and (Figure 4, label 8).
Because the gas flows directly to the pump inlet, the actual pressures, p − p and p − p , are equal at
d bd e be
a constant gas flow through the valve.
CAUTION — Use only dry gases (99,9 % by mass) for the measurements in order to avoid
adsorption and desorption processes.
Take at least three measurements per decade of p with increasing pressures, beginning from a
e
threshold value of twice that of the base pressure, p .
be
Calculate the ratio (p − p )/(p − p ) for every couple of pressure values which should be equal to 1. If
d bd e be
there are deviations from 1, the sensitivity of one gauge shall be corrected by the mean deviation factor
for each decade.
After this adjustment, the test dome is pumped down to almost the base pressure and the measurement
of the volume flow rate can start.
5.2.7 Measurement of the volume flow rate
The gas is admitted to the test dome through the adjustable valve (Figure 4, label 5). Take measurements
with increasing pressures, starting from a threshold value of twice that of the base pressure, p . When
be
the required pressure, p , is obtained and remains stable for at least 60s to within ±3 %, this point may
be
be regarded as valid. If pressure is unsteady due to a transient condition, wait until it stabilizes.
−3
Take measurements at a minimum of three points per pressure decade up to p = 1 ⋅ 10 Pa or to a
e
pressure at which the mean free path (see Reference [7] p. 43) of the gas molecules in the upper part of
the test dome becomes less than 2d, where d is the diameter of the orifice (see Annex A). The pressures
p , p and p are recorded at each measurement.
d e 3
Calculate the volume flow rate, q , with Formula (5).
V
NOTE Volume flow rate measurements can be made with different gases. When the gas is changed, all pipes
connected to the gas inlet valve are purged with the new gas before the beginning of the new measurement.
5.2.8 Measuring uncertainties
The pressure ratios should be measured with an uncertainty of ≤3 % and the orifice diameter with an
uncertainty of 0,5 %. If the pressure in the upper chamber rises to a value where the mean free path
approaches double the orifice diameter, the conductance grows by 3 % of the molecular flow value (see
12 © ISO 2020 – All rights reserved
Reference [7] pp. 147–150). For the exact calculation, see Annex B. The total uncertainty of the volume
flow rate shall be <10 %.
5.2.9 Evaluation of the measurement
Plot on a semi-logarithmic graph (see Figure 5) the volume flow rate, q , of the test pump, calculated
V
by means of Formula (5), with respect to the inlet pressure, and plot on the same graph the volume
flow rate, q , of the backing pump (if used), calculated from Q = p q = C(p − p ) and p , with respect
VBP e V d e 3
to p , so as to show the size of the backing pump. The range of abscissa shall cover the whole range of
pressures p and p . The base pressures of the vacuum pump, p , and of the backing pump, p , shall be
e 3 be b3
indicated.
The test report shall include as a minimum:
a) type, serial number, measuring uncertainty and operational conditions of all gauges used;
b) type and serial number of the test pump;
c) rotational frequency ("speed") and/or other operating conditions of the test pump;
d) fluids and their vapour pressures at 20 °C used in the test pump;
e) D (nominal diameter of the test dome and flange type);
N
f) type and volume flow rate of the backing pump (if used);
g) type of seals used upstream from the inlet flange of the test pump;
h) type of baffles and traps employed during the test, as well as their temperatures;
i) cooling water temperatures and water flow rate;
j) ambient and test dome temperatures;
k) baking time and temperatures.
Key
X inlet pressure, in pascals q volume flow rate of test pump
V
Y volume flow rate, in litres per second q volume flow rate of backing pump
VBP
Figure 5 — Example of volume flow rate (pumping speed) curve
5.3 Volume flow rate (pumping speed) measurement by the pump-down method
5.3.1 General
The pump-down method should be used for small pumps. The pumping speed is obtained by evacuating
a test dome with the test pump. This method requires a measurement of pressure versus time as well as
a knowledge of the volume of the test dome. The advantages of the method are that no gas flow needs to
be measured and that automation of the procedure is easy.
However, continuous evacuation has certain disadvantages, such as those cited in the following.
— The pressure measurement can be disturbed by the response times of pressure gauges and of the
data accumulation system.
— Evacuation of a vessel corresponds to an expansion of the gas out of the vessel, resulting in a cooling
of the gas. As a result, the observed pressure drop originates from both the removal of gas by the
pump and the cooling of the gas in the vessel. The cooling effect changes in the course of pumping
down since the speed of heat transfer between gas and vessel walls depends on pressure. At
atmospheric pressure, the gas expansion is close to isentropic (resulting in substantial cooling), but
in the fine vacuum regime, it is close to isothermal (resulting in a quick warming up to the ambient
temperature of the cooled gas).
These problems are avoided by intermittent pumping, such that the vessel is evacuated in repeated
pump cycles, Δt , with intermediate waiting times, Δt . At the beginning of a cycle, the vessel is valved
1 2
off and the pressure is recorded as initial pressure. The vessel is pumped for a certain time interval,
Δt , until the pressure has decreased by several per cent. The pumping process is then interrupted, and
the second pressure value recorded after the time interval, Δt , which allows for thermal equalization.
The equalization is achieved when the pressure assumes a stationary value. The pump cycle is then
repeated in the same way.
Using this method for pumps with high back-streaming from the exhaust to the inlet side can increase
the volume flow rate of these pumps for light gases by a purge gas effect. During the waiting time for
thermal equalization, the pump reaches the base pressure with a residual gas composition similar to air
at the exhaust of the pump. At the beginning of the new pump interval, this residual gas accelerates the
pumping of a light gas like hydrogen. Consequently, the pump-down method cannot be recommended
in such cases.
All measuring devices shall be calibrated either:
a) in a traceable way to a vacuum primary or to a national standard; or
b) by means of instruments of absolute measure which are traceable to the SI units and to which
measurement uncertainties can be attributed.
In the case of calibrated measuring instruments, there should exist a calibration certificate in
[3]
accordance with ISO/IEC 17025 .
5.3.2 Test dome for the pump-down method
For the measurement of the volume flow rate with the pump-down method, a test dome with a volume
not smaller than the expected volume flow rate multiplied by 120 s shall be used. The dimensions of the
dome in the three directions in space shall not differ by more than a factor of 10. All internal surfaces
of the test dome and the connection line to the pump shall be clean and dry. The test dome shall have
one suction port with a nominal diameter greater than or equal to the inlet flange of the test pump,
additional ports for a gas inlet valve and one or more ports for vacuum gauges. The ports for the vacuum
gauges shall not be close to the pumping port (see Figure 6).
14 © ISO 2020 – All rights reserved
Key
1 test dome
2 gas inlet valve
3 test pump
vacuum gauge to measure p , p , p
t t t
1 2 3
5 quick-acting valve
V volume of test dome
Vi volume of connection pipe between test pump and quick-acting valve
Figure 6 — Arrangement for measuring volume flow rate (pumping speed) with pump-down
method
5.3.3 Quick-acting valve
The quick-acting valve should have an opening or closing time of <0,5 s. The measurement interval,
Δt , shall be large compared to this time (i.e. Δt > 8 s) in order to minimize its contribution to the
1 1
measurement uncertainty of the pumping speed. For an exact measurement of Δt , the actual opening
time of the quick-acting valve shall be measured with sufficient accuracy and included in the calculation.
This opening time may deviate from the valve drive time depending on the type of valve.
Because the conductance of the valve reduces the measured volume flow rate of the test pump, a
straight valve with a high cross-section should be chosen.
5.3.4 Experimental setup
The cleanness of the vacuum pump, seals and other components shall be appropriate for the expected
base pressure. All components are mounted together under clean conditions in accordance with
Figure 6. The vacuum pump shall be connected via the quick-acting valve to the test dome using a
short connection pipe with sufficient cross-section (see 5.3.7). The valve shall be mounted closely to
the pump inlet flange in order to minimize the volume, V , of that part of the connection line. The pipe
i
between the valve and the test dome can be expanded to a large cross-section. The nominal diameter of
the connecting elements should be greater than or equal to the inlet port of the pump. The volume, V , of
i
the connection pipe between the quick-acting valve and the entry of the pumping system of the vacuum
pump shall be smaller than 1 % of the volume, V, of the test vacuum vessel, i.e. V < 0,01V.
i
The pressure measur
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