ISO 27894:2009

INTERNATIONAL ISO
STANDARD 27894
First edition
2009-12-15


Vacuum technology — Vacuum
gauges — Specifications for hot
cathode ionization gauges
Technique du vide — Manomètres à vide — Spécifications pour les
manomètres à ionisation à cathode chaude




Reference number
ISO 27894:2009(E)
©
ISO 2009

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ISO 27894:2009(E)
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ISO 27894:2009(E)
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .2
4 Symbols and abbreviated terms .6
5 Principle of hot cathode ionization gauge.7
6 Specifications for hot cathode ionization gauge to be provided by manufacturers.7
7 Additional (optional) specifications for hot cathode ionization gauge to be provided by
manufacturers.10
8 Influences contributing to the measurement uncertainty with hot cathode ionization
gauges .11
Annex A (informative) Typical Bayard-Alpert gauge with a glass envelope.14
Annex B (informative) Typical electrical connection of a Bayard-Alpert gauge .15
Annex C (informative) Problems with ionization gauges.16
Bibliography.18

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ISO 27894:2009(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 27894 was prepared by Technical Committee ISO/TC 112, Vacuum technology.

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ISO 27894:2009(E)
Introduction
Ionization gauges are commonly used in the measurement of high and ultra-high vacua. The collected ion
current in this gauge is proportional to gas density, respectively pressure, at a known temperature in high and
ultra-high vacua. The ionization of neutral gas particles is accomplished by fast electrons. These electrons are
either produced by a self-sustaining discharge or by an emissive cathode. In commercial ionization gauges,
this emissive cathode is provided by a heated wire (“hot cathode”) emitting electrons by thermionic emission.
Since ionization gauges with a self-sustaining discharge by crossed electrical and magnetic fields show non-
linearity in discharge current versus gas density, they are tedious to calibrate. For this reason, ionization
gauges with “hot cathodes” exhibiting a more linear reading are the ones mainly used for the dissemination of
the pressure scale in high and ultra-high vacua.
For the dissemination of the pressure scale and a reliable measurement of high and ultra-high vacuum
pressures by an ionization gauge, the relevant parameters and uncertainties must be given, and are described
in this International Standard. It therefore complements ISO/TS 3567 when using ionization gauges as
reference standards.

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INTERNATIONAL STANDARD ISO 27894:2009(E)

Vacuum technology — Vacuum gauges — Specifications for
hot cathode ionization gauges
1 Scope
This International Standard defines terms relating to hot cathode ionization vacuum gauges, and specifies
which parameters are given by manufacturers of hot cathode ionization gauges and which measurement
uncertainties have to be considered when operating these gauges. The reasons for this are as follows.
a) This International Standard updates some terms and definitions given in ISO 3529-3:1981.
b) This International Standard specifies information for suitable laboratories to correctly calibrate vacuum
gauges under high and ultra-high vacua, since ionization gauges with hot cathodes are often used as
reference standards. This information consists of the relevant parameters and characteristics suitable for
quotation in manufacturers' instructions to users employing ionization gauges for traceable measurement
of pressure under high or ultra-high vacua.
c) This International Standard also lists those uncertainties associated with the measurement of pressure by
the ionization gauge, which are known to be significant, and gives guidelines on how to evaluate them. It
is possible that the list is not comprehensive for some current or future vacuum gauges.
d) This International Standard complements ISO/TS 3567 and ISO/TS 27893 when using ionization gauges
as reference standards.
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/TS 3567, Vacuum gauges — Calibration by direct comparison with a reference gauge
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
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ISO 27894:2009(E)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 Definitions of components
3.1.1
gauge head
gauge tube
part of the gauge that is exposed to the vacuum
NOTE 1 Adapted from ISO 3529-3:1981.
NOTE 2 The gauge head of the hot cathode ionization gauge contains at least a cathode or filament, anode, ion
collector and the corresponding electrical vacuum feedthroughs. See Figure A.1 in Annex A.
3.1.2
control unit
controller
part of the ionization gauge which comprises the electrical circuits necessary to energize the tube, to control
and measure currents or voltages, and, in some cases, to supply power for degassing of tube elements
NOTE 1 See Figure B.1 in Annex B.
NOTE 2 This cancels and replaces the definition for “gauge control unit” in ISO 3529-3:1981.
3.1.3
integrated type
active gauge type
transmitter type
gauge in which the tube and controller form one piece of equipment which may be separated for baking
NOTE See Figure 1 a).
3.1.4
separated type
passive gauge type
gauge in which the tube and gauge controller are separate pieces of equipment connected by a cable
NOTE See Figure 1 b).
3.1.5
single gauge
one gauge in one piece of equipment
NOTE See Figure 2 a).
3.1.6
combined gauge
more than one gauge in one piece of equipment
NOTE See Figure 2 b).
3.1.7
envelope
wall of metal or glass that encloses the operating elements of a vacuum gauge
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ISO 27894:2009(E)

a) Integrated type b) Separated type

Key
1 controller
2 gauge tube
3 control unit
4 cable
Figure 1 — Vacuum gauges; integrated and separated type


a) One gauge in a body (single gauge) b) Two gauges in a body (combined gauge)

Key
1 gauge tube
2 gauge tube (gauge 1)
3 gauge tube (gauge 2)
Figure 2 — Vacuum gauges; single and combined gauge
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ISO 27894:2009(E)
3.2 Definitions of physical parameters
3.2.1
sensitivity
sensitivity coefficient
S
quantity given by
II−
cc0
S = (1)
I pp−
()
e0
where
I is the emission current;
e
I is the ion current, measured at pressure p;
c
I is the ion current, measured at pressure p ;
c0 0
p is the pressure;
p is the residual pressure.
0
NOTE This definition cancels and replaces the definition of “ionization gauge coefficient” given in ISO 3529-3:1981.
This quantity was formerly also referred to as “gauge constant”.
3.2.2
ionization sensitivity
S
+
quantity given by
I − I
cc0
SS==I (2)
+ e
p − p
0
where
I is the emission current;
e
I is the ion current, measured at pressure p;
c
I is the ion current, measured at pressure p ;
c0 0
p is the pressure;
p is the residual pressure;
0
S is the sensitivity (3.2.1).
NOTE This definition cancels and replaces the definition of “sensitivity coefficient”, synonym “sensitivity”, given in
ISO 3529-3:1981.
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ISO 27894:2009(E)
3.2.3
relative sensitivity factor
r
x
quantity given by
S
x
r = (3)
x
S
N
2
where
S is the sensitivity for a specified gas species “x”;
x
S is the sensitivity for nitrogen for the same gauge at the same pressure and the same operating
N
2
conditions.
NOTE 1 Adapted from ISO 3529-3:1981.
NOTE 2 The pressure reading p of a gauge which is correct for nitrogen has to be divided by the relative sensitivity
ind
factor r of a gas species to obtain the correct pressure p of the gas, when it is measuring that gas.
x x
p
ind
p = (4)
x
r
x
3.2.4
correction factor
f
c
factor by which a pressure reading of a gauge has to be multiplied to obtain the correct pressure according to
a calibration
p=⋅fp (5)
cind
NOTE In a calibration, f is determined by the quotient of the pressure standard p and the indicated reading p
c std UUC
of the unit under calibration; f may depend on pressure.
c
p
std
f = (6)
c
p
UUC
3.2.5
relative correction factor
f
c x/N
2
quantity given by
f
cx
f = (7)
cx/N
2
f
cN
2
where
f is the correction factor for a specified gas species “x”;
c x
f is the correction factor for nitrogen for the same gauge at the same pressure and the same
c N
2
operating conditions.
NOTE 1 The pressure reading p of a gauge which is correct for nitrogen has to be multiplied by the relative
ind
correction factor of a gas species to obtain the correct pressure of the gas, when it is measuring that gas.
pf=⋅p (8)
c x /N ind
2
NOTE 2 f may depend on the pressure.
c x/N
2
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ISO 27894:2009(E)
3.2.6
warm-up time
time after which the ion gauge reading is stable within a specified value (e.g. 2 %) at a constant pressure after
switching on the gauge
NOTE There should be no trend in the gauge reading at constant pressure after warm-up time.
3.2.7
residual current
smallest ion collector current that can be obtained when the gauge is operated at its normal operating
conditions and at a pressure that is zero or negligible compared with the lower measurement pressure limit of
the gauge
NOTE The residual current can be measured in a baked-out ultra-high vacuum system with the ionization gauge in
the baked-out and degassed condition. The residual pressure is defined as the ion current obtained when the vacuum
system has returned to normal room temperature < 30 °C, 48 h after stopping bake-out. The residual current is mainly
composed of the X-ray effect, the inverse X-ray effect, the electron-stimulated desorption effect, outgassing and leakage
currents from other potentials.
3.2.8
residual current-equivalent pressure
equivalent pressure of nitrogen to the residual current (3.2.7)
NOTE The residual current-equivalent pressure is given in pascals (Pa).
3.2.9
internal volume
〈vacuum gauges〉 volume inside the envelope up to the sealing plane minus the volume of the electrodes
reaching out of the sealing plane
NOTE The internal volume is the volume of the gauge tube exposed to a vacuum system. For a nude gauge, in
extreme cases, the internal volume may be negative, when the electrode volumes exceed the volume below the sealing
plane.
4 Symbols and abbreviated terms
Symbol Designation Unit
f
correction factor
1
c
p pressure Pa
p residual pressure
Pa
0
p indicated pressure of a gauge
Pa

ind
pressure of a primary or reference
p
Pa
std
standard
indicated pressure of a unit (gauge)
p
Pa
UUC
under calibration
r relative sensitivity factor
1
x
I emission current
A
e
I ion current at pressure p
A
c
I ion current at pressure p
A
c0 0
−1
S sensitivity (coefficient)
Pa
S −1
ionization sensitivity
+ A⋅Pa
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ISO 27894:2009(E)
5 Principle of hot cathode ionization gauge
Electrons emitted from the cathode are accelerated by the anode (grid) potential to ionize gas molecules that
are within their way, which then produce an ion current collected by the ion collector. The ion current I is
c
proportional to the gas density, or pressure p at constant temperature T; I is given by
c
p
I=∆Ilσ (9)
ce
kT
where
I is the emission current;
e
σ is the ionization cross-section area;
∆l is the mean path length of the electron;
k is the Boltzmann constant.
There may be additional electrodes for different purposes. The number of electrodes, their configuration and
their shape depend on the specific type of hot cathode ionization gauge.
6 Specifications for hot cathode ionization gauge to be provided by manufacturers
The features and specifications given in 6.1 to 6.20 shall be provided by the manufacturer, in order to enable
users of their gauges to estimate the measurement uncertainty and/or to disseminate the pressure scale.
6.1 Type of gauge
The manufacturer shall specify the gauge type, such as triode gauge, Bayard-Alpert gauge, extractor gauge,
to mention just a few common types.
For combined gauges, all types of gauges including the non-ionization gauge type shall be specified.
6.2 Display and measurement signal output
The display of the gauge shall show the SI unit of pascal (Pa). Other units of pressure are also allowed.
If the gauge or control unit has a different measurement signal output than pressure, e.g. voltage, a clear
assignment of this value to pressure shall be made by an equation, table or graph.
6.3 Measurement range
The measurement range depends generally on the accepted measurement uncertainty. For this reason, the
manufacturer shall define measurement uncertainty limits. The measurement range is the range between
minimum and maximum pressure where the reading of the gauge is within the defined measurement
uncertainty limits. The pressure range and the pressure reading shall be given in pascals. Equivalent
pressures in other units may also be given.
6.4 Measurement uncertainty or accuracy
The total relative measurement uncertainty (accuracy) u of the gauge shall be specified in percent of the
reading and/or full scale for the measurement range described in 6.3. The relative standard uncertainty in
accordance with ISO/IEC Guide 98-3 shall be given. The relative measurement uncertainty can also be given
by a formula with a constant and pressure-dependent term, e.g.
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ISO 27894:2009(E)
−8
up() 2 ×10
=+ 0,15 (10)
pp
It is understood that this uncertainty is valid for a batch of gauges. If an individual gauge is calibrated, the
uncertainty of the calibration supersedes the uncertainty of the batch.
6.5 Residual current-equivalent pressure
The residual current-equivalent pressure as defined in 3.2.8 shall be given in pascals (Pa). Additionally, other
units may be used.
6.6 Fitting to chamber
The fitting type and size of gauge tube should be specified; e.g. Conflat flange, KF/NW, O-ring, etc.
6.7 Type of envelope
The envelope types shall be specified, such as glass, metal and nude tube, etc.
6.8 Maximum bake-out temperature
The maximum temperatures shall be specified respectively for the gauge tube and cables. If the control unit of
an integrated gauge can be removed, this shall be stated and the maximum bake-out temperature for either
the gauge head or control unit shall be given.
6.9 Filament material and emission current
The number and material of filament(s) should be specified. Also, if designed to be constant in some pressure
range, the emission current, including its possible fluctuation and drift, shall be given. When the emission
current is changed with pressure (or ion current reading) by the control unit, the pressure switch point shall be
given. There may be different switch points for increasing and decreasing pressures. Either shall be given. If
the emission current is changed continuously with the ion current, respectively pressure, this shall be stated
and the range of the emission current shall be given.
NOTE Typically, emission current is measured from anode to ground or from anode to filament cathode. The
emission current is adjusted by the gauge controller unit. The emission current is typically between 0,1 mA and 10 mA.
The number of electrons emitted is normally proportional to the emission current. The stability and disturbance of electrons
emitted and hence emission current are very significant for accurate pressure measurement using h
...