ISO 27893:2026

• Abstract:

  This document specifies a method for the determination and reporting of measurement uncertainties arising during vacuum gauge calibration by direct comparison with a reference gauge carried out in accordance with ISO 3567. This document specifies methods for uniform reporting of uncertainties in vacuum gauge certificates. Uncertainties reported in accordance with the guidelines given in this document are transferable in the sense that the uncertainty evaluated for one result can be used as a component in the uncertainty evaluation of another measurement or calibration in which the first result is used. This document specifies two measurement models that are sufficient to cover most practical cases. However, it is possible that the models given cannot be applied to newly developed vacuum gauges. The final uncertainty to be reported in a certificate is evaluated from the uncertainties of the input quantities and influence quantities. The principal quantities that can affect the result of a vacuum calibration are described; however, a complete list of the possible quantities that can have an influence on the final result lies outside the scope of this document.

• Status: Published
• Publication Date: 27-Jul-2026
• ICS: 23.160 - Vacuum technology
• Technical Committee: ISO/TC 112 - Vacuum technology
• Drafting Committee: ISO/TC 112 - Vacuum technology
• Current Stage: 6060 - International Standard published
• Start Date: 28-Jul-2026
• Due Date: 30-Aug-2026
• Completion Date: 28-Jul-2026

• Revises: ISO 27893:2011 - Vacuum technology — Vacuum gauges — Evaluation of the uncertainties of results of calibrations by direct comparison with a reference gauge
• Effective Date: 29-Oct-2022

ISO 27893:2026 is an International Standard from ISO for evaluating the uncertainty of vacuum gauge calibration results when a gauge is calibrated by direct comparison with a reference gauge. It gives a method for calculating and reporting those uncertainties in a uniform way, so calibration laboratories can state results in a consistent certificate form and users can reuse the uncertainty from one result in later measurements or calibrations.

What does ISO 27893:2026 specify?

ISO 27893:2026 specifies uncertainty determination and reporting for vacuum gauge calibrations carried out by direct comparison with a reference gauge in accordance with ISO 3567. The document is about pressure calibration in vacuum technology, and it is written for cases where the calibration result needs to be traceable and reported with an uncertainty statement.

The standard says that its uncertainty reports are transferable, meaning the uncertainty from one calibration result can be used as part of another uncertainty evaluation when that result is used again. It also states that two measurement models cover most practical cases - a sum model for pressure error and a quotient model for calibration factors, sensitivities or similar ratios.

The document is organized into clauses on scope, references, terms, symbols, the two models, uncertainty calculation for each model, combination of the models for error of reading, and reporting in uncertainty budgets and certificates. Annex A gives a procedure for Type A uncertainty analysis when repeated observations matter.

Annex	What it covers
Annex A	Efficient uncertainty analysis when Type A uncertainties are not negligible

What are the key requirements of ISO 27893:2026?

ISO 27893:2026 requires the uncertainty statement to be built from the uncertainty of the input quantities and the influence quantities that affect a vacuum calibration. In practice, that means the laboratory has to identify the main contributors, quantify them, and combine them according to the chosen model.

Choose the right measurement model

Clause 5 sets out the basic concept. ISO 27893:2026 treats vacuum gauge calibration as a pressure calibration traceable to the SI unit pascal, using the corrected reading of the reference gauge to establish the calibration pressure. If the measurand is a pressure difference or reading error, the sum model applies; if the measurand is a correction factor, sensitivity coefficient, effective accommodation factor or gauge constant, the quotient model applies.

This matters because the report must match what the calibration actually delivers. A user reading the certificate needs to know whether the value is an error, a ratio, or another derived quantity.

Account for the main uncertainty sources

Clauses 6 and 7 break the uncertainty into contributions from the reference standard, the unit under calibration (UUC), and the calibration method or conditions. The reference standard contribution includes its indication, offset, drift, certificate correction, long-term instability, laboratory temperature, thermal transpiration, and other effects such as mounting position.

For the UUC, the document treats the indication, offset, drift, thermal transpiration and other device-specific influences in the same way. In practice, this tells the laboratory to check the gauge reading itself and also the effects that shift it away from the true pressure.

Use repeated observations when needed

Where dependencies or values are not known, or cannot be estimated from manufacturer information or laboratory knowledge, ISO 27893:2026 calls for repeat measurements on different days. For the UUC, the uncertainty from repeated values is then taken from repeatability, and Annex A gives a fuller procedure when Type A uncertainty is significant.

This matters for low pressures, high-resolution gauges, and other cases where random scatter is large enough to affect the budget. The standard is telling the laboratory not to rely only on judgment when measured data are needed.

Combine uncertainties correctly

For both models, the standard combines the separate standard uncertainties into a total standard uncertainty, then expands it with a coverage factor. ISO 27893:2026 says the coverage factor is chosen according to the confidence needed, normally in the range from 2 to 3, and it points to Annex A for the determination of k. If the effective degrees of freedom are clearly greater than 10, k = 2 may be taken for about 95 % confidence.

This matters because the certificate has to show an uncertainty with a meaningful confidence level, not just a raw scatter estimate.

Handle the error of reading carefully

Clause 8 covers the error of reading, e, when the sum and quotient models are combined. ISO 27893:2026 gives formulas for the uncertainty of e depending on whether the calibration method term is present or zero.

In practice, this helps when a certificate needs to present the reading error rather than only a pressure value or a ratio. The document also warns against combining the models casually in one formula, because that can create sensitivity coefficients that are harder to manage.

Report uncertainty in a usable certificate form

Clause 9 requires an uncertainty budget in quality documentation and gives summary-table formats for both models. The certificate should state the model of the measurand so that correction factors, sensitivity factors and their inverses are not confused. It should also show the calibrated value with its expanded uncertainty, and if one value is used over a pressure range, that value should cover the range and reflect the largest uncertainty where agreed.

That matters because the certificate is the user-facing record. ISO 27893:2026 is not only about calculation - it is also about making the result understandable and usable.

What terms does ISO 27893:2026 define?

• Corrected reading - the gauge reading after systematic errors have been corrected.
• Long-term instability - a possible change in calibrated value after a long period, including changes caused by transport.
• Offset / zero error - the reading when the measured quantity should be zero.
• Deviation of offset - the change in offset between measuring the offset and taking the pressure reading.
• Reference standard / reference gauge - the higher-quality standard used as the basis for the calibration and traceability.
• Calibration pressure - the pressure derived from the corrected reference reading and the necessary corrections at the UUC port.
• Type A and Type B uncertainties - Type A from statistical analysis of observations, Type B from other means such as prior knowledge or judgment.
• UUC - unit under calibration, meaning the vacuum gauge being calibrated.

Who uses ISO 27893:2026?

ISO 27893:2026 is used by calibration laboratories that calibrate vacuum gauges by direct comparison with a reference gauge. It is also relevant to metrology staff, quality managers and technical assessors who review uncertainty budgets and calibration certificates.

Manufacturers of vacuum gauges can use it when they specify calibration methods or prepare certificates for their instruments. Buyers and users of calibrated gauges can use it to judge whether a certificate states the measurand clearly and whether the uncertainty statement fits the way they will use the gauge.

What changed in ISO 27893:2026 from the previous edition?

ISO 27893:2026 is the second edition and it cancels and replaces ISO 27893:2011. The document says the edition is technically revised, and the main stated change is the addition of Annex A, which describes how to evaluate Type A uncertainties.

Which standards are used with ISO 27893:2026?

ISO 27893:2026 cites these normative references:

• ISO 3567 - the direct-comparison calibration procedure that this standard builds on.
• ISO/IEC Guide 98-3 - the GUM, used for expressing uncertainty of measurement and for the uncertainty framework.
• ISO/IEC Guide 99:2007 - the VIM, used for the metrology vocabulary and basic concepts.

The bibliography also points to ISO/IEC 17025 for laboratory competence and ISO 24477:2025 for related spinning rotor gauge calibration and uncertainty practice.

What does the ISO 27893:2026 document contain?

ISO 27893:2026 contains the basic uncertainty models, the equations for uncertainty propagation, and the guidance needed to turn calibration observations into a reported uncertainty. It lays out the sum model, the quotient model, and the combined treatment of error of reading, then shows how to build uncertainty budgets for certificates.

It also includes a detailed annex for Type A analysis, including when repeated calibrations are needed and how the effective degrees of freedom affect the coverage factor. The document uses summary tables for uncertainty budgets and recommends that certificates state the measurand model and a formula for converting gauge output into true pressure.

For practical work, the document is a calculation and reporting guide rather than a product specification. It tells the laboratory what to evaluate, how to combine the contributions, and what to show on the calibration certificate so the result can be used with confidence.

International
Standard
ISO 27893
Second edition
Vacuum technology — Vacuum
2026-07
gauges — Evaluation of the
uncertainties of results of
calibrations by direct comparison
with a reference gauge
Technique du vide — Manomètres à vide — Évaluation de
l'incertitude des résultats des étalonnages par comparaison
directe avec un manomètre de référence
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 3
5 Basic concept and model . 3
5.1 General .3
5.2 Sum model .4
5.3 Quotient model .4
5.4 Combination of the two models .5
6 Calculation of uncertainty in the sum model . 5
6.1 Total uncertainty — Sum model .5
6.2 Uncertainty contributions due to reference standard .5
6.3 Uncertainty contributions due to unit under calibration .7
6.4 Uncertainty contributions due to calibration method or calibration conditions .8
6.5 Coverage factor .8
7 Calculation of uncertainty in the quotient model . 8
7.1 Total uncertainty — Quotient model .8
7.2 Uncertainty contributions due to reference standard .9
7.3 Uncertainty contributions due to the unit under calibration .10
7.4 Uncertainty contributions due to calibration method or calibration conditions .11
7.5 Coverage factor . 12
8 Combination of the sum and quotient model for error of reading.12
9 Reporting uncertainties .13
9.1 Uncertainty budget . 13
9.2 Calibration certificate .14
Annex A (normative) Efficient uncertainty analysis when Type A uncertainties are not
negligible .15
Bibliography . 17

iii
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
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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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
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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 27893:2011), which has been technically
revised.
The main changes are as follows:
— added Annex A, which describes how to evaluate Type A uncertainties.
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
International Standard ISO 27893:2026(en)
Vacuum technology — Vacuum gauges — Evaluation of the
uncertainties of results of calibrations by direct comparison
with a reference gauge
1 Scope
This document specifies a method for the determination and reporting of measurement uncertainties arising
during vacuum gauge calibration by direct comparison with a reference gauge carried out in accordance
with ISO 3567.
This document specifies methods for uniform reporting of uncertainties in vacuum gauge certificates.
Uncertainties reported in accordance with the guidelines given in this document are transferable in the
sense that the uncertainty evaluated for one result can be used as a component in the uncertainty evaluation
of another measurement or calibration in which the first result is used.
This document specifies two measurement models that are sufficient to cover most practical cases. However,
it is possible that the models given cannot be applied to newly developed vacuum gauges.
The final uncertainty to be reported in a certificate is evaluated from the uncertainties of the input quantities
and influence quantities. The principal quantities that can affect the result of a vacuum calibration are
described; however, a complete list of the possible quantities that can have an influence on the final result
lies outside the scope of this document.
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, Vacuum gauges — Calibration by direct comparison with a reference gauge
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3567, ISO/IEC Guide 98-3,
ISO/IEC Guide 99 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
corrected reading
value resulting after the reading of the gauge has been corrected for systematic errors
EXAMPLE For the results given in the calibration certificate of the reference standard.

3.2
long-term instability
possible change of calibrated value after long periods of time
EXAMPLE Change resulting from transportation of the device.
Note 1 to entry: Long-term instability is different from reproducibility as defined in ISO/IEC Guide 99:2007, 3.7.
3.3
model
〈uncertainty of measurement〉 mathematical model set out in ISO/IEC Guide 98-3
3.4
offset
zero error
〈measuring instruments〉 datum measurement error where the specified measured quantity value is zero
Note 1 to entry: Adapted from ISO/IEC Guide 99:2007, 4.28.
EXAMPLE The reading when there is no pressure (absolute or differential) or a pressure far below the resolution
limit applied to a vacuum gauge.
3.5
deviation of offset
possible difference of an offset (3.4) value between the time of the measurement of the offset (3.4) and the
time when a pressure reading is taken
3.6
reference standard
reference gauge
standard, generally having the highest metrological quality available at a given location or in a given
organization, from which measurements made there are derived
Note 1 to entry: Adapted from ISO/IEC Guide 99:2007, 6.6.
EXAMPLE The gauge or standard that gives traceability to the SI unit in the calibration apparatus in accordance
with ISO 3567.
3.7
calibration pressure
〈vacuum gauges〉 pressure evaluated from the corrected reading (3.1) of the reference standard (3.6) and all
necessary corrections at the gauge port of the unit under calibration
EXAMPLE Necessary corrections can be for known differences between gauge ports.
3.8
Type A and Type B uncertainties
classifications used to indicate the two different ways of evaluating uncertainty components, which are
intended only for convenience of discussion
Note 1 to entry: The classification is not meant to indicate any difference in the nature of the uncertainty components.
Note 2 to entry: Type A uncertainties are evaluated by the statistical analysis of series of observations.
Note 3 to entry: Type B uncertainties are evaluated by means other than the statistical analysis of series of
observations.
Note 4 to entry: For more information, see ISO/IEC Guide 98-3.

4 Symbols and abbreviated terms
Symbol or Designation Unit
abbreviated term
UUC unit under calibration (vacuum gauge) —
e error of reading in relative units
k coverage factor to expand standard uncertainty, u 1
p pressure indication of a UUC corrected for known deviations Pa
UUC
p pressure indication of a UUC not corrected for any deviation Pa
ind,UUC
p pressure indication of reference gauge (reference standard) Pa
std
corrected for known deviations
p pressure indication of reference gauge (reference standard) not Pa
ind,std
corrected for any deviation
r quantity determined by a calibration in the quotient model for the any unit
UUC
UUC
r quantity determined by a calibration in the quotient model for the any unit
std
reference standard
S sensitivity of the output of a vacuum gauge any unit
u standard uncertainty any unit
U expanded uncertainty any unit
x indication of a UUC any unit
UUC
x indication of a reference gauge any unit
std
x (often unknown) input quantities and corrections of gauge any unit
i
X (often unknown) input quantities and corrections of calibration any unit
i
method or condition
Δp error of pressure reading in absolute units Pa
δp deviations in the pressure unit (often unknown) Pa
i
δx (often unknown) deviations in x any unit
i i
σ effective accommodation factor of a spinning rotor gauge 1
eff
5 Basic concept and model
5.1 General
In a vacuum gauge calibration carried out in accordance with ISO 3567, the corrected reading of a reference
gauge gives the value of the quantity that is traceable to the SI. All vacuum gauges shall be calibrated in
terms of pressure. This means that the user of the vacuum gauge calibrated in accordance with ISO 3567 and
this document obtains a clear assignment of the output quantity of the gauge to the SI unit of pressure, the
pascal.
The value of pressure obtained from the corrected reading of the reference standard output can be used
to determine the pressure at the entrance port of the unit under calibration (UUC). This is referred to
as calibration pressure value. Often the corrected reading of the reference standard is identical to the
calibration pressure value and valid for all gauge ports.
The calibration pressure value can be used to determine an error of the reading, Δp, of the unit under
calibration. In this case, a sum model gives an adequate description of the measurement.
The calibration pressure value can also be used to determine a correction factor, a sensitivity coefficient,
an effective accommodation factor or a gauge constant, in which case a quotient model gives an adequate
description of the measurement.
In both models it can be assumed that all the input quantities are uncorrelated.
5.2 Sum model
In the sum model, the difference between the reading of the UUC, p , and the “true” calibration pressure
UUC
traceable to the SI units is taken as the measurand, Δp. The calibration pressure is given by the reference
standard pressure value, p , and possibly by a correction term, δp , due to the calibration method
std m
considering known effects like height correction, thermal transpiration, and pressure non-uniformity. The
general sum model thus becomes
pp pp � (1)

UUC stdm
The first term refers to the UUC, the second to the reference standard, and the third to the calibration
method. The sum of the last two terms gives the calibration pressure value. All quantities shall be expressed
in the SI unit of pressure, the pascal.
Each of these terms is again expressed by another model equation, which makes all necessary corrections
due to offsets, temperature corrections, deviation of indication from the SI value in accordance with the
calibration certificate, etc.
5.3 Quotient model
In the quotient model, the ratio of the reading of the UUC, x , and the standard pressure value, p , is
UUC std
taken as the measurand, r . The general quotient model thus becomes
UUC
x
UUC
r  X � (2)
UUC  i
p
std
i
The numerator refers to the UUC, the denominator to the reference standard, and the product to the
calibration method and conditions. The latter can also be defined by the vacuum gauges under calibration,
e.g. the emission current in a hot cathode ionization gauge. It is possible to express x in any reasonable
UUC
unit, e.g. that of pressure, voltage or current. The X can be expressed in any meaningful physical unit or can
i
be without dimension.
Each of these factors is expressed by another model equation, which makes all necessary corrections due to
offsets, temperature corrections, deviation of indication in accordance with calibration certificate, etc.
Examples of r are
UUC
-1
a) f the reciprocal of a dimensionless correction factor, where x = p and X = 1;
c UUC UUC i
b) S a sensitivity of the analogue output, V , of a capacitance diaphragm gauge, where x = V ;
UUC UUC UUC
c) S a sensitivity of the analogue output, V , of a thermal conductivity gauge, where x = V ;
UUC UUC UUC
d) σ the effective accommodation factor of a spinning rotor gauge, where x = p , when σ = 1 was
eff UUC UUC eff
entered into the controller;
e) S a sensitivity of a Bayard-Alpert gauge with a hot cathode, where x = I is the positive ion current
UUC UUC
of the collector and X = 1/I , where I is the emission current.
1 e e
5.4 Combination of the two models
It is possible to evaluate some of the input quantities in each model by either of the two models. First, for
example, p as well as its uncertainty can be evaluated by the quotient model, thus
std
x
std
p = � (3)
std
r
std
The result can then be used in Formula (1). This is unavoidable if r is given in the certificate applying
std
Formula (2) (e.g. the sensitivity of an analogue output), where r is replaced by r .
UUC std
It is, however, not recommended to combine the sum and quotient model in one Formula. This task should be
left to experts, since complicated sensitivity coefficients can appear that are not covered in this document
for reasons of clarity. The relative error of reading, e, however, is a common case, where an easy-to-handle
combination of the two methods is possible.
The error of reading, e, can be expressed mathematically as
ppp p

UUC stdm UUC
e   1� (4)
pppp

stdm stdm
or, if δp = 0
m
pp p
UUC std UUC
e 1� (5)
p p
std std
See Clause 4 for the designations of p , p , and δp . The uncertainty of e is described in Clause 8.
UUC std m
6 Calculation of uncertainty in the sum model
6.1 Total uncertainty — Sum model
The total uncertainty in the sum model, u(Δp), is given by
22 2
up  up up up � (6)
   
UUC stdm
where
u(p ) is the standard uncertainty of the corrected pressure indication of the vacuum gauge under
UUC
calibration;
u(p ) is the standard uncertainty of the corrected pressure indication of the reference gauge;
std
u(δp ) is the standard uncertainty of the deviations due to the calibration method.
m
6.2 Uncertainty contributions due to reference standard
The corrected pressure indication of the reference standard p is given by
std
ppppp 
stdind,,stdoffs stddrfts,,td calstd
(7)
pp pp
term,std TT,,stdthermal stdels,std
where
p is the indication of the reference standard;
ind,std
p is the offset (zero deviation) of the reference standard;
offs,std
δp is the deviation of offset due to drift (in most cases, δp = 0);
drft,std drft,std
δp is the correction in accordance with the calibration certificate;
cal,std
δp is the deviation due to long-term instability (in most cases, δp = 0);
term,std term,std
δp is the deviation due to temperature at the calibration laboratory;
T,std
p is the deviation due to the effect of thermal transpiration caused by the temperature difference
thermal,std
between reference standard and the unit under calibration;
δp is the deviation due to other influences, e.g. inclination of device (in most cases, δp = 0).
els,std els,std
All quantities in Formula (7) refer to the reference standard gauge.
NOTE If the offset is deducted or adjusted to zero in the device itself, p = 0.
offs,std
The standard uncertainty of the corrected pressure indication of the reference standard u(p ) then is
std
given by
22 2 2
up up upup 
   
ind,stdoffs,std drft,std cal,sttd
up  � (8)

std
22 2 2
upup up up
   
term,std T,stdthermal,std els,sstd
where
u(p ) is the uncertainty originating from the dispersion of measurement values, including
ind,std
dispersion due to digitizing, resolution scatter, etc.;
u(p ) is the uncertainty of the offset values at measurement of the offset [without reproduc-
offs,std
ibility of the offset covered by u(δp )];
drft,std
u(δp ) is the uncertainty of the offset values at time of calibration due to offset drift or other
drft,std
systematic dependencies, e.g. due to the frequency dependence of spinning rotor gauges;
u(δp ) is the uncertainty of the standard in accordance with the calibration certificate;
cal,std
u(δp ) is the uncertainty component of the long-term instability;
termstd
u(δp ) is the uncertainty component due to the temperature influence under the conditions of
T,std
the calibration laboratory;
u(p ) is the uncertainty component due to the effect of thermal transpiration caused by the
thermal,std
temperature difference between the reference standard and the unit under calibration;
u(δp ) is the uncertainty due to the specific conditions at the calibration laboratory, e.g. dif-
els,std
ferent mounting position of built-in devices.
For a p that has not been obtained from repeated observations, estimate u(p ) from scientific
ind,std ind,std
judgement based on all of t
...