ISO 21360:2007

INTERNATIONAL ISO
STANDARD 21360
First edition
2007-06-01

Vacuum technology — Standard methods
for measuring vacuum-pump
performance — General description
Technique du vide — Méthodes normalisées pour mesurer les
performances des pompes à vide — Description générale




Reference number
ISO 21360:2007(E)
©
ISO 2007

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ISO 21360:2007(E)
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ISO 21360:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 Symbols and abbreviated terms . 3
5 Test Methods. 4
5.1 Volume flow rate (pumping speed) measurement with the throughput method . 4
5.2 Volume flow rate (pumping speed) measurement with the orifice method. 8
5.3 Volume flow rate (pumping speed) measurement with the pump-down method . 13
5.4 Measurement of the base pressure . 18
5.5 Measurement of the compression ratio and the critical backing pressure. 19
Annex A (informative) Mean free path of some important gases. 23
Annex B (informative) Measuring uncertainties. 24
Bibliography . 27

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ISO 21360:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 21360 was prepared by Technical Committee ISO/TC 112, Vacuum technology.
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ISO 21360:2007(E)
Introduction
This International Standard is a basic standard for measuring the performance data of vacuum pumps. The
methods described here are well known from existing national and international standards. In developing this
International Standard, the aim was to provide one 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 basic
standard 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 the
basic standard and the specific standard, it is the specific standard which is valid.

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INTERNATIONAL STANDARD ISO 21360:2007(E)

Vacuum technology — Standard methods for measuring
vacuum-pump performance — General description
1 Scope
This International Standard is a basic standard which defines three different 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 described 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 occurring at very small inlet pressures (in the high and ultra-high vacuum). It is based on
measuring the ratio of pressures in a two-chamber test dome where 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 is measured on a vacuum pump but not described in this International Standard
(e.g. measurement of power consumption) is defined in the specific pump standard.
2 Normative references
The following referenced documents are indispensable for the application 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 3529-1, Vacuum technology — Vocabulary — Part 1: General terms
ISO 3529-2, Vacuum technology — Vocabulary — Part 2: Vacuum pumps and related terms
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ISO 21360:2007(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3529-2 and the following apply.
3.1
volume flow rate
q
V
volume of gas which, under ideal conditions, flows from the test dome through the pump inlet per unit time
NOTE 1 For practical reasons, however, 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
3
units adopted for the volume flow rate are cubic metres per hour (m /h) or litres per second (l/s).
NOTE 2 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
See 5.4.
NOTE The ultimate pressure is the value which the pressure in the test dome approaches asymptotically. It is the
lowest pressure obtainable with the pump, but there is no practical method of measurement or specification.
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
3
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
0
ratio of the backing pressure, p , to the inlet pressure, p , of the vacuum pump without throughput, expressed
3 1
by the following equation:
p
3
K =
0
p
1
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ISO 21360:2007(E)
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 following equation:
pV
1
Qp==q
1 V
t
where
p is the (high) vacuum pressure on the inlet;
1
q is the volume flow rate of the test pump;
V
t is time;
V is the volume of the test dome.
3.10
standard gas flow rate
q
Vstd
volume flow rate at standard reference conditions for gases in accordance with ISO 3529-1
4 Symbols and abbreviated terms
Symbol Designation Unit
a
inner diameter of the connection pipe between test pump and m
quick-acting valve (items 3 and 5 in Figure 6)
2
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 nominal diameter of test dome m

N
K compression ratio of vacuum pump with zero throughput —

0
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

L thickness of the orifice wall at the orifice diameter m
M molar mass of gas kg/mol

p 101 325 Pa in accordance with ISO 3529-1 Pa
0
p
(high) vacuum pressure on inlet Pa (or mbar)
1
p maximum working pressure on inlet Pa (or mbar)
1max
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ISO 21360:2007(E)
p vacuum pressure in backing line Pa (or mbar)
3
p , p , p pressures in the test dome for the pump-down method, Pa (or mbar)
t1 t2 t3
measured before and after time intervals ∆t , ∆t , ∆t
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
3
q volume flow rate of test pump
l/s (or m /h)
V
3
q volume flow rate of backing pump
l/s (or m /h)
VB
3
q volume flow rate at standard reference conditions for gases in
sccm (or cm /min)
Vsccm
accordance with ISO 3529-1
3
q volume flow rate at standard reference conditions for gases in
l/s (or m /h)
Vstd
accordance with ISO 3529-1
Q maximum gas throughput of vacuum pump which the pump can Pa·l/s (or mbar·l/s)
max
withstand without damage
R ideal gas constant 8,314 J/(mol·K)
T
thermodynamic temperature K
T 273,15 K in accordance with ISO 3529-1 K
0
T temperature of the test dome K
D
T temperature of the flow meter K
f
u measurement uncertainty —
3
V
volume of the test dome
l, m
3
V volume of connection pipe between test pump and quick-acting
l, m
i
valve (items 3 and 5 in Figure 6)
5 Test Methods
5.1 Volume flow rate (pumping speed) measurement with the throughput method
5.1.1 General
The throughput method is the one most used for vacuum pumps and is applicable for all pressure ranges and
pump sizes where flow meters for the gas throughput measurements are available with sufficient accuracy.
The gas flow measuring ranges shall be chosen by multiplying the expected volume flow rate with 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 accordance with
ISO/IEC 17025.
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ISO 21360:2007(E)
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 , as that
N
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 for pressure measurement at the
height of D/2 above the bottom flange are preferable if more than one pressure gauge is used. The diameter
of these flanges should be equal to or larger than 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 correspond
N
to D = 100 mm. The transition to the pump inlet flange shall be made through a 45° conical adaptor, as
N
shown in Figure 1.

Key
1 gas inlet pipe and temperature measuring point for T
D
2 vacuum gauge and mass spectrometer connections
D inner diameter of test dome, in metres
Figure 1 — Test dome for the throughout method
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ISO 21360:2007(E)
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. In the case of a flow restriction by a small flow meter, they may be used in parallel with a
manifold and 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.
NOTE 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 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 manufacturer of the vacuum pump.

Key
1 test dome
2 backing pump
3 test pump
4 gas inlet valve
5 flow meters to measure Q
6 vacuum gauge to measure p
1
7 heating jacket (optional)
8 vacuum gauge to measure p
3
9 temperature measuring point for T
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
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ISO 21360:2007(E)
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 which the
V
gas throughput, Q, is measured outside the dome. If the pressure, p , in the test dome (measured by a
1
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
p − p
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 .
VB
Q
q = (2)
VB
p − p
3b3
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 [1], Ch.19, pp. 109-113).
Because of the dependence of the temperature on the gas volume, in all cases of volumetric measurements,
corrections with the factor T /T are necessary if the temperature, T , of the flow meter and T of the test dome
D f f D
are different.
NOTE Thermoelectric mass flow meters do not measure the throughput but the volume flow rate, q , at standard
Vstd
reference conditions for gases in accordance with ISO 3529-1 (p = 101 325 Pa and T = 273,15 K). To obtain the
0 0
throughput, q is multiplied by the factor T p /T . Consequently, q will be calculated by means of the following
Vstd D 0 0 V
equation:
qpT
Vstd 0 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 in litres per
Vstd V
−3
second by inserting [q = (q /sccm) × 10 l / 60 s], [p = 101 325 Pa] and [T = 273,15 K] in Equation (3), as
Vstd Vsccm 0 0
follows:
−3
(qT/ sccm)××10 l 101325 Pa×
Vsccm D
q = l/s (4)
V
60 s××273,15 K (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 , p , the
1 1 3
ambient temperature and the test dome temperature, T , and the admitted throughput, Q. If the throughput
D
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
1
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.
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ISO 21360:2007(E)
Measurements shall be made at a minimum of three points per pressure decade of p . If the throughput is
1
increased to the maximum allowed value, Q , the maximum inlet pressure is obtained whose values may be
max
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 pressure 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, calculated by
V
means of Equation (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 as to show the size of
VB std 3 3
the backing pump. The range of abscissa shall cover the whole range of pressures p and p . The base
1 3
pressures of the vacuum pump, p , and of the backing pump, p , shall be indicated.
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 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) DN (size of the test dome and flange type);
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 with 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.
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ISO 21360:2007(E)
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 (L/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 to ensure that
d e
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 equal to or greater than D = 100 mm, the nominal diameter, D , of
N N
the dome shall be equal to the actual diameter of the inlet flange.
For pumps with an inlet flange diameter of less than D = 100 mm, the diameter of the dome shall correspond
N
to D = 100 mm. In this case, the transition to the pump inlet flange shall be made through a 45° taper fitting
N
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
L 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
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ISO 21360:2007(E)
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.

Key
1 test dome
2 backing pump
3 test pump
4 gas inlet valve
5 gas inlet valve
6 vacuum gauge to measure p
3
7 vacuum gauge to measure p
d
8 vacuum gauge to measure p
e
9 heating jacket
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.
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ISO 21360:2007(E)
The base pressures, p and p , in the upper and lower chamber of the test dome are measured after baking
bd be
(see 5.4) and before admission of the gas. The conductance of the orifice with diameter, d, and thickness, L,
may be calculated using the following equation:
πRT⎛⎞1
D 2
Cd= (6)
⎜⎟
32ML1+ /d
⎝⎠
1
The term is a correction factor (only valid for L  d) that can be defined as the average transition
1/+ L d
probability through the orifice.
Take care that the equation is used with consistent units. Specific values such as [R = 8,314 J/(mol⋅K)],
−3
[M = 28,97 × 10 kg/mol] and [T = 293 K (20 °C)]
air D
will give, in cubic metres per second,
2
91d
C = (7)
air
L
1+
d
or, in litres per second,
2
91000d
C = (8)
air
L
1+
d
where L 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, test gas is admitted to
bd be
valve (4) to check the sensitivity of the gauges (7) and (8).
Because the gas flows directly to the pump inlet, the actual pressures (pp− ) and ()pp− are equal at
dbd ebe
a constant gas flow through the valve.
CAUTION — Use only dry gases (99,9 %) 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 threshold
e
value of twice that of the base pressure p .
be
pp−
dbd
Calculate the ratio for every couple of pressure values which should be equal to 1. If there are
pp−
ebe
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 will be pumped down to almost the base pressure and the measurement
of the volume flow rate can start.
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ISO 21360:2007(E)
5.2.7 Measurement of the volume flow rate
The gas is admitted to the test dome through the adjustable valve (5). Take measurements with increasing
pressures, starting from a threshold value of twice that of the base pressure, p . When the required pressure,
be
p , is obtained and remains stable for the following minute to within ± 3%, this point may be regarded as valid.
be
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 pressure
e
at which the mean free path (see Reference [2], Ch.3, pp.32-41) 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
d e
and p are recorded at each measurement.
3
Calculate the volume flow rate, q , with Equation (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 beginni
...