ISO 19685:2017

INTERNATIONAL ISO
STANDARD 19685
First edition
2017-09
Vacuum technology — Vacuum
gauges — Specifications, calibration
and measurement uncertainties for
Pirani gauges
Technique du vide — Manomètres à vide — Spécifications, étalonnage
et incertitudes de mesure pour les jauges Pirani
Reference number
ISO 19685:2017(E)
©
ISO 2017

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ISO 19685:2017(E)

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© ISO 2017, Published in Switzerland
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ISO 19685:2017(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 4
5 Principle of Pirani gauge . 4
6 Specifications for Pirani gauge . 5
6.1 General . 5
6.2 Measuring principle . 5
6.3 Type of output . 5
6.4 Range of output and output display resolution . 5
6.5 Measurement range and full scale . 5
6.6 Expected measurement uncertainty . 5
6.7 Material exposed to gas . 5
6.8 Connecting flange . 5
6.9 Interface and pin-connections . 6
6.10 Warm-up period . 6
6.11 Compatibility between gauge tube and controller . 6
6.12 Normal operating (environment) conditions . 6
6.13 Electrical power supply . 6
6.14 Cable length . 6
6.15 Maximum bake-out temperature . 6
6.16 Dimensions of gauge head and controller . 6
6.17 Mounting orientation . 6
7 Additional (optional) specifications for Pirani gauge . 6
7.1 Long-term instability . 6
7.2 Relative sensitivity factor . 7
7.3 Hysteresis . 7
7.4 Response time. 7
7.5 Inspection record and calibration certificate . 7
7.6 Configuration . 7
7.7 Weight . 7
7.8 Storage and transport condition . 7
7.9 Photographs . 7
7.10 Maximum power consumption . 7
7.11 Set point of pressure . 7
8 Calibration and other measurement procedures. 7
8.1 General . 7
8.2 Conditions for calibrations . 8
8.3 Assembly and adjustments of calibration item . 8
8.4 Calibration . 9
8.5 Measurement uncertainties during calibration . 9
8.6 Calibration certificate . 9
8.7 Measurement procedure for hysteresis .10
8.8 Measurement procedure of response time.10
9 Measurement uncertainties at use .10
Annex A (informative) Typical Pirani gauge .12
Bibliography .13
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ISO 19685:2017(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
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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
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URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 112, Vacuum technology.
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ISO 19685:2017(E)

Introduction
ISO 3567 and ISO 27893 are basic standards with no specific guidelines of a special type of vacuum
gauge and are generally applicable. Detailed guidance for a specific gauge is intended to be given in
separate technical specifications for the calibration of special types of gauges.
This document complements ISO 3567 and ISO 27893 when characterizing or calibrating Pirani gauges
or using them as reference gauges.
Pirani gauges are widely used to measure pressures in the medium vacuum up to atmospheric pressure.
The relevant parameters, calibration guidelines and uncertainties for the dissemination of the pressure
scale and measurement of low and medium vacuum pressures by a Pirani gauge are described in this
document.
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INTERNATIONAL STANDARD ISO 19685:2017(E)
Vacuum technology — Vacuum gauges — Specifications,
calibration and measurement uncertainties for Pirani
gauges
1 Scope
This document identifies parameters of Pirani gauges, their calibration procedure, and describes
measurement uncertainties to be considered when operating these gauges.
This document applies to Pirani vacuum gauges operating over a pressure range of 0,01 Pa to 150 kPa.
This document complements ISO 3567 and ISO 27893 when calibrating Pirani gauges and using them as
reference standards.
In addition, this document defines procedures to characterize Pirani gauges for response time and
hysteresis.
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 3567:2011, Vacuum gauges — Calibration by direct comparison with a reference gauge
ISO 27893, Vacuum technology — Vacuum gauges — Evaluation of the uncertainties of results of
calibrations by direct comparison with a reference gauge
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1 Definitions of components
3.1.1
thermal conductivity gauge
vacuum gauge in which the pressure is determined in relation to the transfer of thermal energy between
the surfaces of two fixed elements maintained at different temperatures
Note 1 to entry: This gauge is based on the thermal conductivity of a gas being pressure dependent.
[SOURCE: ISO 3529-3:2014, 2.4.2.2, modified — Example has been deleted.]
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ISO 19685:2017(E)

3.1.2
Pirani gauge
thermal conductivity gauge in which the heated element is part of a Wheatstone bridge that supplies
the energy to the element and by which the electrical resistance or the dissipated power of the element
is being measured
Note 1 to entry: Some types of Pirani gauges have an extended upper limit of measurement range by using the
heat convection inside the tube. This type is often called convection-enhanced Pirani gauge.
Note 2 to entry: See Annex A.
[SOURCE: ISO 3529-3:2014, 2.4.2.2.2, modified — Note 1 to entry has been replaced.]
3.2 Definitions of physical parameters
3.2.1
internal volume
volume of the gauge that is exposed to the vacuum system where it is attached to, counted from the
connecting flange plane
3.2.2
thermal accommodation coefficient
ratio of the absolute temperature of the molecule after a hit with the heated element in a Pirani gauge
and the absolute temperature of the heated element
3.2.3
long-term instability
p pp−
D
UUC
quantity characterizing the typical change of relative measurement error e== over time
p p
where the period needs to be specified
Note 1 to entry: Measurement errors to determine long-term instability of this gauge type are taken for nitrogen
at a pressure of p=±()10 1 Pa (nitrogen) after a specified period. This quantity δ shall be a relative quantity
t
and may be determined in two ways:
a) as the relative standard deviation of measurement error e obtained from at least three calibrations each
i
being separated by the specified period
2
n
1
δ = ee−
ti∑( )
n−1
i=1
with n being the number of calibrations i, and
n
1
e= e
∑ i
n
i=1
b) as the mean of absolute (non-negative) changes of measurement error Δp between recalibrations
i
separated by the specified period
n−1
ee−
∑ ii+1
i=1
δ =
t
n−1
and n as described above
Note 2 to entry: The formula in a) is recommended when the measurement error does not show a significant drift
but random variations, and the formula in b) when the measurement error shows a systematic and monotonic drift.
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ISO 19685:2017(E)

Note 3 to entry: If the output signal of the gauge is not pressure (e.g. voltage or current), this signal shall be
converted to pressure based on the manufacturer's specification, before the measurement error is calculated.
Note 4 to entry: Long term instability can be determined by recalibrations with a more accurate gauge or a
primary standard. This often requires a transport which itself can lead to an instability of the calibrated value.
For this reason, it is not reasonable to assume a linear relationship of instability with time (e.g. δ for a period of
t
2 years is not 2 times δ for a period of 1 year).
t
Note 5 to entry: If not specified otherwise, it is recommended to determine δ over a period of 1 year. This is
t
usually a reasonable compromise between costs and influence of transport on the one hand and a possible drift
and lowest possible measurement uncertainty on the other hand.
EXAMPLE If δ was determined to 6,5 % according to the formula in b), where the period of recalibration
t
was 1 year, the report will be “δ = 6,5 % for a period of one year”.
t
3.2.4
response time
τ
time that elapses when the Pirani gauge is exposed to a sudden pressure change of nitrogen between
1 kPa to 100 Pa (up or down to be specified), until the Pirani shows 90 % of the pressure change
difference
Note 1 to entry: See Figure 1.
Figure 1 — Illustration to explain definition of response time τ
3.2.5
hysteresis
relative difference of indication of a Pirani gauge when exposed to the same pressure in a rising and
decreasing sequence
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ISO 19685:2017(E)

4 Symbols and abbreviated terms
Symbol Designation Unit
e Relative measurement error 1 or in %
p Pressure, traceable to SI Pa
Measurement error (measured pressure p
UUC
Δ p Pa
minus reference pressure p)
Measured pressure by unit under calibra-
p Pa
UUC
tion (UUC)
δ Long-term instability as defined in 3.2.3 1 or in %
t
τ Response time ms
UUC Unit under calibration
5 Principle of Pirani gauge
The Pirani gauge is one type of a thermal conductivity vacuum gauge. In the Pirani gauge, gas molecules
conduct heat away from a heated element to a surrounding wall that is usually at room temperature
(see Figure A.1). In a certain pressure range, the heat taken away by the molecules is proportional to
molecular density and hence pressure. The corresponding temperature change of the heated element,
which is often a filament, causes a change in resistance. The resistance is part of a Wheatstone type
bridge circuit that supplies the energy to the element and by which the electrical resistance or the
dissipated power of the element is being measured. The bridge circuit may keep constant either the
temperature of the heated element, or the heating power, or voltage or current. The linear measurement
range is highest, when the temperature is kept constant. More detailed information i
...