GUIDANCEILAC-G24

INTERNATIONALOIML D 10

SERIES

Edition 2022 (E)

DOCUMENT

Edition 2022 (E)Guidelines for the determination of recalibrationintervals of measuring equipment

Guide pour la détermination des intervalles de réétalonnagedes équipements de mesure

INTERNATIONALLABORATORYORGANISATIONINTERNATIONALE
DE METROLOGIE LEGALE

ACCREDITATIONCOOPERATIONINTERNATIONAL ORGANIZATION
OF LEGAL METROLOGYILAC-G24:2022 / OIML D 10:2022

Contents

Foreword (ILAC) ...................................................................................................................................3
Foreword (OIML) ..................................................................................................................................51

Introduction ..................................................................................................................................62

Scope..............................................................................................................................................63

Terms and definitions ..................................................................................................................64

General ........................................................................................................................................11

5

Initial choice of recalibration intervals .................................................................................... 12

6

Methods of reviewing recalibration intervals ..........................................................................13

7

6.1

General principles...................................................................................................................13

6.2

Method 1: Automatic adjustment or “staircase” (calendar-time) ...........................................14

6.3

Method 2: Control chart (calendar-time) ................................................................................14

6.4

Method 3: “In-use” time .........................................................................................................15

6.5

Method 4: In service checking, or “black box” testing ..........................................................15

6.6

Method 5: Other statistical approaches ..................................................................................16

6.7

Comparison of methods for reviewing recalibration intervals ...............................................16
Bibliography ...............................................................................................................................17

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Foreword (ILAC)
ILAC is the global association for the accreditation of laboratories, inspection bodies, proficiency testing
providers, reference material producers and biobanks, with a membership consisting of accreditation bodies
and stakeholder organisations throughout the world.
It is a representative organisation that is involved with:






the development of accreditation practices and procedures,the promotion of accreditation as a trade facilitation tool,
supporting the provision of local and national services,
the assistance of developing accreditation systems,
the recognition of competent testing (including medical) and calibration laboratories, inspection
bodies, proficiency testing providers, reference material producers and biobanks around the world.

ILAC actively cooperates with other relevant international organisations in pursuing these aims.
ILAC facilitates trade and supports regulators by operating a worldwide mutual recognition arrangement – theILAC Arrangement - among Accreditation Bodies (ABs). The data and test results issued by laboratories,
inspection bodies, proficiency testing providers and reference material producers collectively known as
Conformity Assessment Bodies (CABs), accredited by ILAC Accreditation Body members are accepted
globally via this Arrangement. Thereby, technical barriers to trade, such as the re-testing of products each timethey enter a new economy is reduced, in support of realising the free-trade goal of “accredited once, accepted
everywhere”.
In addition, accreditation reduces risk for business and its customers by assuring that accredited CABs arecompetent to carry out the work they undertake within their scope of accreditation.
Further, the results from accredited facilities are used extensively by regulators for the public benefit in theprovision of services that promote an unpolluted environment, safe food, clean water, energy, health and social
care services.
Accreditation Bodies that are members of ILAC and the CABs they accredit are required to comply withappropriate international standards and the applicable ILAC application documents for the consistent
implementation of those standards.
Accreditation Bodies having signed the ILAC Arrangement are subject to peer evaluation via formallyestablished and recognised regional cooperation bodies using ILAC rules and procedures prior to becoming asignatory to the ILAC Arrangement.
The ILAC website provides a range of information on topics covering accreditation, conformity assessment,
trade facilitation, as well as the contact details of members. Further information to illustrate the value ofaccredited conformity assessment to regulators and the public sector through case studies and independent
research can also be found at www.publicsectorassurance.org.
For more information, please contact:
The ILAC Secretariat
PO Box 7507
Silverwater NSW 2128
Australia
Phone: +61 2 9736 8374
Email: ilac@nata.com.au
Website: www.ilac.org@ILAC_Official
https://www.youtube.com/user/IAFandILAC

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ILAC-G24:2022 / OIML D 10:2022© Copyright ILAC 2022
ILAC encourages the authorised reproduction of its publications, or parts thereof, by organisations wishing touse such material for areas related to education, standardisation, accreditation, or other purposes relevant to
ILAC’s area of expertise or endeavour. The document in which the reproduced material appears must containa statement acknowledging ILAC’s contribution to the document.

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ILAC-G24:2022 / OIML D 10:2022

Foreword (OIML)The International Organization of Legal Metrology (OIML) is a worldwide, intergovernmentalorganisation whose primary aim is to harmonise the regulations and metrological controls applied by
the national metrological services, or related organisations, of its Member States.
The main categories of OIML publications are:International Recommendations (OIML R), which are model regulations that establish themetrological characteristics required of certain measuring instruments and which specify
methods and equipment for checking their conformity. OIML Member States shall implement
these Recommendations to the greatest possible extent;International Documents (OIML D), which are informative in nature and which are intended
to harmonise and improve work in the field of legal metrology;International Guides (OIML G), which are also informative in nature and which are intended
to give guidelines for the application of certain requirements to legal metrology; andInternational Basic Publications (OIML B), which define the operating rules of the various
OIML structures and systems.

OIML Draft Recommendations, Documents and Guides are developed by Project Groups linked toTechnical Committees or Subcommittees which comprise representatives from OIML Member States.
Certain international and regional institutions also participate on a consultation basis. Cooperative
agreements have been established between the OIML and certain institutions, such as ISO and the IEC,
with the objective of avoiding contradictory requirements. Consequently, manufacturers and users ofmeasuring instruments, test laboratories, etc. may simultaneously apply OIML publications and thoseof other institutions.
International Recommendations, Documents, Guides and Basic Publications are published in English(E) and translated into French (F) and are subject to periodic revision.
Additionally, the OIML participates in Joint Committees with other Institutions for the development of
Vocabularies (OIML V) and Joint Guides and periodically commissions legal metrology experts towrite Expert Reports (OIML E). Expert Reports are intended to provide information and advice, andare written solely from the viewpoint of their author, without the involvement of a Technical Committee
or Subcommittee, nor that of the CIML. Thus, they do not necessarily represent the views of the OIML.
This publication – reference ILAC-G24 / OIML D 10, Edition 2022 – was developed by the ILAC
Accreditation Committee and by OIML TC 4 Measurement standards and calibration and verification
devices. It was approved for final publication by ILAC in December 2022 and by the International
Committee of Legal Metrology at its 57th meeting in October 2022 and will be submitted to theInternational Conference on Legal Metrology for formal sanction. This edition of D 10 supersedes theprevious edition dated 2007.
OIML Publications may be downloaded from the OIML website in the form of PDF files. Additional
information on OIML Publications may be obtained from the Organisation’s headquarters:
Bureau International de Métrologie Légale
11, rue Turgot - 75009 Paris - France
Telephone: 33 1 48 78 12 82
Fax: 33 1 42 82 17 27
E-mail: biml@oiml.org
Internet: www.oiml.org

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1

Introduction

1.1

This guidance Document was developed by the OIML (International Organization of Legal
Metrology) and ILAC (International Laboratory Accreditation Cooperation) as a joint venture andis published as such.

1.2

It is important to point out thata) it is the responsibility of each laboratory to choose to implement any or none of themethods described in this Document based on its individual needs and risk assessments,
andb) it is also the responsibility of each laboratory to evaluate the effectiveness of theimplemented method(s). The laboratory should also take responsibility for theconsequences of the choice of the method(s).

2

Scope

2.1

The purpose of this Document is to provide guidance to laboratories on methods to determine andreview the recalibration intervals of measuring equipment under their control as part ofestablishing the calibration program of their laboratory. This Document is also applicable to other
Conformity Assessment Bodies (e.g. Inspection Bodies and Certification Bodies) and otherparties (e.g. manufacturers) that utilise measuring equipment.

3

Terms and definitions
Unless otherwise stated in the following subclauses, the terminology used in this Document
conforms to the VIM3 [1], ISO/IEC 17000 [12], ISO/IEC 17020 [13], ISO/IEC 17025 [3],ISO/IEC 17065 [17] and CIPM MRA-G-13 [2].
For the purpose of this Document, the definitions and abbreviations given below apply. Some of
the terms in clause 3 are listed with alternative terms which are considered to have an identicaldefinition. The text “for D 10” marks text which is not part of the definition found in thereferenced documents (e.g. additional explanatory notes that specifically concern terms used inthis Document).

3.1

accreditation body (ISO/IEC 17000, 4.7)
authoritative body that performs accreditation
Note:

3.2

The authority of an accreditation body can be derived from government, publicauthorities, contracts, market acceptance, or scheme owners.

adjustment of a measuring system (VIM3, 3.11)
adjustment
set of operations carried out on a measuring system so that it provides prescribed indications
corresponding to given values of a quantity to be measured
Note 1:

Types of adjustment of a measuring system include zero adjustment of a measuringsystem, offset adjustment, and span adjustment (sometimes called gain adjustment).

Note 2:Adjustment of a measuring system should not be confused with calibration, which isa prerequisite for adjustment.

Note 3:After an adjustment of a measuring system, the measuring system must usually berecalibrated.

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3.3

calibration (VIM3, 2.39)
operation that, under specified conditions, in a first step, establishes a relation between thequantity values with measurement uncertainties provided by measurement standards andcorresponding indications with associated measurement uncertainties and, in a second step, uses
this information to establish a relation for obtaining a measurement result from an indication

3.4

Note 1:A calibration may be expressed by a statement, calibration function, calibration diagram,calibration curve, or calibration table. In some cases, it may consist of an additive or
multiplicative correction of the indication with associated measurement uncertainty.

Note 2:

Calibration should not be confused with adjustment of a measuring system, often
mistakenly called “self-calibration”, nor with verification of calibration.

Note 3:

Often, the first step alone in the above definition is perceived as being calibration.

calibration and measurement capability (CIPM MRA-G-13)(CMC)
calibration and measurement capability available to customers under normal conditions:a)as published in the BIPM key comparison database (KCDB) of the CIPM MRA (International
Committee for Weights and Measures Mutual Recognition Arrangement); orb) as described in the laboratory’s scope of accreditation granted by a signatory to the ILAC
Arrangement
3.5

certification body (ISO/IEC 17065, 3.12)
third-party conformity assessment body operating certification schemes
Note:

3.6

A certification body can be non-governmental or governmental (with or without aregulatory authority).

certified reference material (VIM3, 5.14)
CRM
reference material, accompanied by documentation issued by an authoritative body and providing
one or more specified property values with associated uncertainties and traceabilities, using valid
procedures
ExampleHuman serum with assigned quantity value for the concentration of cholesterol andassociated measurement uncertainty stated in an accompanying certificate, used as acalibrator or measurement trueness control material.

Note 1:‘Documentation’ is given in the form of a ‘certificate’ (see ISO Guide 31:2000).

Note 2:

Procedures for the production and certification of certified reference materials are
given, e.g. in ISO Guide 34 and ISO Guide 35.

Note 3:In this definition, “uncertainty” covers both ‘measurement uncertainty’ and‘uncertainty associated with the value of a nominal property’, such as for identity andsequence. “Traceability” covers both ‘metrological traceability of a quantity value’and ‘traceability of a nominal property value’.

Note 4:

Specified quantity values of certified reference materials require metrologicaltraceability with associated measurement uncertainty (Accred. Qual. Assur.:2006).

Note 5:

ISO/REMCO has an analogous definition (Accred. Qual. Assur.:2006) but uses themodifiers “metrological” and “metrologically” to refer to both quantity and nominal
property.

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3.7

combined standard measurement uncertainty (VIM3, 2.31)
combined standard uncertainty
standard measurement uncertainty that is obtained using the individual standard measurement
uncertainties associated with the input quantities in a measurement model
Note:

3.8

In the case of correlations of input quantities in a measurement model, covariances
must also be taken into account when calculating the combined standard measurement
uncertainty; see also GUM:1995, 2.3.4.

conformity assessment body (ISO/IEC 17000, 4.6)
body that performs conformity assessment activities, excluding accreditation

3.9

inspection body (ISO/IEC 17020, 3.5)
body that performs inspection
Note:

An inspection body can be an organisation, or part of an organisation.

3.10 instrumental drift (VIM3, 4.21)
continuous or incremental change over time in indication, due to changes in metrologicalproperties of a measuring instrument
Note:

Instrumental drift is related neither to a change in a quantity being measured nor to achange of any recognised influence quantity.

3.11 laboratory (ISO/IEC 17025, 3.6)
body that performs one or more of the following activities:───

testing;
calibration;
sampling, associated with subsequent testing or calibration

3.12 material measure (VIM3, 3.6)
measuring instrument reproducing or supplying, in a permanent manner during its use, quantities
of one or more given kinds, each with an assigned quantity value
Examples: Standard weight, volume measure (supplying one or several quantity values, with orwithout a quantity-value scale), standard electric resistor, line scale (ruler), gauge
block, standard signal generator, certified reference material.
Note 1:

The indication of a material measure is its assigned quantity value.

Note 2:A material measure can be a measurement standard.

3.13 maximum permissible measurement error (VIM3, 4.26)
maximum permissible error
limit of error
extreme value of measurement error, with respect to a known reference quantity value, permittedby specifications or regulations for a given measurement, measuring instrument, or measuringsystemNote 1:

Usually, the terms “maximum permissible errors” or “limits of error” are used where
there are two extreme values.

Note 2:The term “tolerance” should not be used to designate ‘maximum permissible error’.

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3.14 measurement result (VIM3, 2.9)
result of measurement
In the context of this Document, result is defined as:
set of quantity values being attributed to a measurand together with any other available relevant
information
Note 1:

A measurement result generally contains “relevant information” about the set ofquantity values, such that some may be more representative of the measurand thanothers. This may be expressed in the form of a probability density function (PDF).

Note 2:A measurement result is generally expressed as a single measured quantity value anda measurement uncertainty. If the measurement uncertainty is considered to be
negligible for some purpose, the measurement result may be expressed as a single
measured quantity value. In many fields, this is the common way of expressing ameasurement result.

Note 3:In the traditional literature and in the previous edition of the VIM, measurement result
was defined as a value attributed to a measurand and explained to mean an indication,or an uncorrected result, or a corrected result, according to the context.

3.15 measurement standard (VIM3, 5.1)
etalonrealisation of the definition of a given quantity, with stated quantity value and associatedmeasurement uncertainty, used as a referenceNote:

For examples and notes see VIM3, 5.1.

3.16 measurement uncertainty (VIM3, 2.26)
uncertainty of measurement
uncertainty
non-negative parameter characterising the dispersion of the quantity values being attributed to ameasurand, based on the information used
Note 1:

Measurement uncertainty includes components arising from systematic effects, suchas components associated with corrections and the assigned quantity values ofmeasurement standards, as well as the definitional uncertainty. Sometimes estimatedsystematic effects are not corrected for but, instead, associated measurement
uncertainty components are incorporated.

Note 2:The parameter may be, for example, a standard deviation called standard measurement
uncertainty (or a specified multiple of it), or the half-width of an interval, having astated coverage probability.

Note 3:

Measurement uncertainty comprises, in general, many components. Some of these may
be evaluated by Type A evaluation of measurement uncertainty from the statistical
distribution of the quantity values from series of measurements and can be characterised
by standard deviations. The other components, which may be evaluated by Type B
evaluation of measurement uncertainty, can also be characterised by standard deviations,evaluated from probability density functions based on experience or other information.

Note 4:

In general, for a given set of information, it is understood that the measurement
uncertainty is associated with a stated quantity value attributed to the measurand.
A modification of this value results in a modification of the associated uncertainty.

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3.17 measuring equipment
equipment (including, but not limited to, measuring instruments, software, measurement
standards , reference materials, reference data, reagents, consumables or auxiliary apparatus) thatis required for the correct performance of laboratory activities and that can influence the results
Note 1:In the context of this Document, a measuring instrument is a component of themeasuring equipment which plays an important role for measurement. Somemeasuring instruments can be used independently to complete a measurement process
or to realise a physical quantity.

Note 2:In the context of this Document, measuring equipment may be considered as being
equivalent to measuring system.

3.18 measuring instrument (VIM3, 3.1)
device used for making measurements, alone or in conjunction with one or more supplementarydevices
Note 1:

A measuring instrument that can be used alone is a measuring system.

Note 2:A measuring instrument may be an indicating measuring instrument or a material
measure.

3.19 measuring system (VIM3, 3.2)
set of one or more measuring instruments and often other devices, including any reagent andsupply, assembled and adapted to give information used to generate measured quantity values
within specified intervals for quantities of specified kinds
Note:

A measuring system may consist of only one measuring instrument.

3.20 reference material (VIM3, 5.13)
RM
material, sufficiently homogeneous and stable with reference to specified properties, which has
been established to be fit for its intended use in measurement or in examination of nominal
properties
Note: For notes see VIM3, 5.13.
3.21 reference quantity value (VIM3, 5.18)
reference value
quantity value used as a basis for comparison with values of quantities of the same kind
Note 1:

A reference quantity value can be a true quantity value of a measurand, in which case
it is unknown, or a conventional quantity value, in which case it is known.

Note 2:

A reference quantity value with associated measurement uncertainty is usuallyprovided with reference toa) a material, e.g. a certified reference material,b) a device, e.g. a stabilised laser,c) a reference measurement procedure, ord) a comparison of measurement standards.

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4

General

4.1

An important aspect for maintaining the capability of a laboratory to produce traceable
measurement results is to determine the maximum period that should be permitted between
successive calibrations (recalibrations) of the measuring equipment used. Various international
standards dealing with measurement activities take this aspect into account, e.g. ISO/IEC 17025[3] and ISO 15189 [15]. In addition, this aspect is also included in international standards
applicable to conformity assessment bodies and other parties operating according to e.g. ISO/IEC
17020 [13], ISO/IEC 17043 [14], ISO/IEC 17065 [17], ISO 9001 [11], ISO 17034 [16] or ISO
22870 [18].
Note:

Establishing and maintaining traceability of measurement results can be done bymeans such as, but not limited to
‒
‒
‒

4.2

defining calibration periodicity,
defining process control measures,
defining intermediate checks.

The purposes of calibrating measuring equipment as a measure of maintaining metrologicaltraceability are:a)to provide an estimate of the deviation between a reference value and the value obtained using
the measuring equipment, and the uncertainty in this deviation, at the time the measuring
equipment is actually used;b) to support the validation of the required or declared measurement uncertainty that can beachieved with the measuring equipment; andc)to confirm whether or not there has been any alteration of the measuring equipment which
could introduce doubt about the results delivered in the elapsed period.

4.3

One of the most significant decisions regarding the calibration of measuring equipment is thetiming and frequency of its implementation. The frequency between calibrations is a critical issue
and is influenced by many factors that need to be taken into account by the laboratory. The mostimportant of these factors are provided in 5.1.

4.4

The calibration records may be used to determine recalibration intervals, when calibrations areprovided by, but not limited to:a)national metrology institutes and designated institutes that have been subject to appropriate
peer review processes under the CIPM MRA; orb) laboratories that have been accredited by an accreditation body which is a signatory to the
ILAC (International Laboratory Accreditation Cooperation) Arrangement or to Regional
Arrangements recognised by ILAC; orc)calibration provided by national metrological institutes, designated institutes or laboratories
not fulfilling conditions a) or b) and whose services are suitable for the intended use, provided
that conditions a) or b) could not be met for other than economic reasons (i.e. are notavailable). Also refer to ILAC P10 [19].

The recommendations mentioned above do not preclude involvement of other parties, provided
that sufficient evidence of metrological traceability is available.
4.5

It is acknowledged that the costs associated with performing recalibrations may be higher whenincreased recalibration frequencies are applied. However, these costs need to be balanced against
increased measurement uncertainties or a higher risk in decreased measurement reliability which
may occur with longer recalibration intervals.

4.6

There is no universally applicable single best practice for establishing and adjusting therecalibration intervals. This has created the need for a better understanding of the recalibration

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interval determination. As no single method is ideally suited for the whole range of measuring
equipment, some of the simpler methods of assigning and reviewing the recalibration interval andtheir suitability for different types of measuring equipment are covered in this Document.
Note:

The methods have been published in more detail in certain standards by reputable
technical organisations (e.g. [6], [7], [8]), or in relevant scientific journals.

4.7

Methods for determining recalibration intervals developed by or adapted by the laboratory may
also be used if they are appropriate and validated.

4.8

The laboratory should select the appropriate methods for determining recalibration intervals andshould document those methods used. Calibration results should be collected and retained as thehistorical data, in order to form the basis of future decisions for recalibration intervals of themeasuring equipment.

4.9

The laboratory should have an appropriate system of intermediate checks to ensure the correctfunctioning and calibration status of the measuring equipment used between calibrations (e.g. see
ISO/IEC 17025 [3]).

4.10 The laboratory should check whether the results of external calibration and/or intermediate checks
fall within predetermined limits prior to approving the measuring equipment for further use.
Note 1:For some kinds of measuring equipment, each measuring instrument or device which
composes the equipment may be calibrated separately. In this case, a combined
standard measurement uncertainty of the measuring equipment is calculated from theuncertainties arising from all the measuring instruments and devices.

Note 2:It may be necessary to re-evaluate calibration intervals of whole measuring equipment, or
its measuring instruments and devices based on data obtained from previous calibrations.

5

Initial choice of recalibration intervals

5.1

The initial decision in determining the recalibration interval is based mainly on a risk assessment
analysis and should take into account, but not limited to, the following factors:a) measurement uncertainty required and evaluated by the laboratory;b) type of measuring equipment and its components;c) risk of the measuring equipment exceeding the predetermined limits (e.g. maximum
permissible error), or accuracy requirements in use;d) manufacturer’s recommendations regarding the measuring equipment (e.g. when themeasurement uncertainty is required and evaluated by the laboratory based on the accuracyof the instrument);e) tendency to wear and drift;f) expected extent and severity of use;g) environmental conditions (e.g. climatic conditions, vibration, ionising radiation);h) influence of the measured quantity (e.g. high temperature effect on thermocouples) on
measurement results;i) pooled or published data about the same or similar devices;j) frequency of comparisons with other measurement standards or measuring instruments;k) frequency, quality and results of intermediate checks;l) transportation arrangements of the measuring equipment and associated risks;m) degree to which the operating staff are trained and extent to which the established procedures
are implemented; andn) legal requirements.

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5.2

The decision should be made by personnel having the relevant technical competence. An estimate
should be made for each piece (or a group of pieces) of measuring equipment as to the time period
in which the piece(s) is (are) likely to remain within the prescribed limits (i.e. maximum
permissible error, accuracy requirements) after a calibration.

6

Methods of reviewing recalibration intervals
Note:

6.1

The methods described in this section may also be used to review the type andfrequency of intermediate checks.

General principles

Once calibration has been conducted on a routine basis (based on a defined number of consecutive
results), adjustment of the recalibration intervals should be possible in order to optimise the balanceof risks and costs as stated in the general aspects. It will probably be found that the intervals initiallyselected do not give the desired optimum results due to a number of reasons, for example:a)measuring equipment may be more or less reliable than expected;b) the extent of usage and care in maintenance may not be as anticipated;c)

for certain measuring equipment it may be sufficient to carry out a partial calibration insteadof a full calibration; andd) the instrumental drift determined by the recalibration of the measuring equipment may showthat shorter calibration intervals are required or longer calibration intervals may be possible
without increasing risks, etc.
Several different methods are available for reviewing the recalibration intervals. The method
chosen differs according to whethera)measuring equipment is treated individually or as groups (e.g. by the manufacturer’s modelor by the type),b) the measuring equipment’s performance fails to meet prescribed limits (e.g. maximum
permissible error, accuracy requirements) due to drift over time or by usage,c)the measuring equipment shows different types of instabilities,d) the measuring equipment undergoes adjustments, ande)data are available and the history of calibration of the measuring equipment (e.g. trend dataobtained from previous calibration records, recorded history of maintenance and servicing of
the measuring instrument, data from intermediate checks) can be analysed.

New measuring equipment should be calibrated more frequently to identify any trend in its
performance characteristics which may indicate that a change to the recalibration interval may be
warranted. Ongoing review of the recalibration interval and equipment performance is necessary
and for this reason, fixed recalibration intervals are not recommended unless the interval has been
specified in a normative document such as a reference measurement procedure, specified method
or a consensus standard.

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6.2

Method 1: Automatic adjustment or “staircase” (calendar-time)

Each time a piece of measuring equipment is calibrated on a routine basis, the subsequent
recalibration interval is extended (or kept unchanged) if the deviation from the reference value isfound to be within an appropriately defined percentage of the range between the maximum
permissible errors. Otherwise, the recalibration interval is reduced when the deviation from thereference value is outside this percentage of the range. The maximum permissible errors may bereplaced with any other set of limits as required. It is recommended that appropriate decision
criteria for extension or reduction of the recalibration interval of measuring equipment are
specified for typical individual cases. This “staircase” response may produce a rapid adjustment
of intervals and is easily carried out without administrative effort. When the records of calibration
are maintained and utilised, future issues with a group of measuring equipment become
predictable because the records indicate the need for technical modifications or preventive
maintenance.
A disadvantage of systems dealing with measuring equipment individually may be that it isdifficult to keep the calibration workload smooth, relatively stable and balanced between risks
and costs, and that it requires detailed advanced planning.
It would be inappropriate to set an extremely long recalibration interval using this method. Sucha case may lead to risks associated with withdrawing large numbers of reported measurement
results, or repeating a significant amount of work, and such risks may ultimately become
unacceptable.
6.3

Method 2: Control chart (calendar-time)

Control charting is one of the most important tools of Statistical Quality Control (SQC) and iswell described in various publications (e.g. [4], [5], [9]). In principle, it works as follows:Significant calibration points are chosen and the results are plotted against time. From these plots,
both the dispersion of the results and the instrumental drift are calculated. The instrumental drift
is the mean drift normally over one recalibration interval, although several intervals may be taken
into account in the calculation for very stable measuring equipment. From these figures, theoptimum interval may be calculated.
Considerable knowledge of the variability properties of the measuring equipment is required touse this method. A considerable variation of the recalibration intervals from those prescribed ispossible, because the performance of a control chart can be calculated and in theory at least gives
the efficient recalibration interval. Furthermore, the calculation of the dispersion of the results
will indicate whether the manufacturer’s specification limits are reasonable and the analysis of
the instrumental drift found may indicate the cause of the drift.
Note:This method is not suitable for calibrations of measuring equipment without an
instrumental drift. This method is suitable, for example, for a material measure with asingle assigned quantity value, e.g. calibration of a gauge block or a standard
resistance.

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6.4

Method 3: “In-use” time

Method 3 is a variation of Method 1 and Method 2. The basic method remains unchanged but therecalibration interval is expressed in hours of use, rather than in calendar time, e.g. months. The
measuring equipment is equipped with a device which indicates the actual “in service” time and isreturned for calibration when the indication reaches a specified value. Such measuring equipment
are for example thermocouples used at extreme temperatures, standard lamps of which the drift issubject to their burning time, and dead weight testers for gas pressure or length gauges (i.e.
measuring equipment that may be subject to mechanical wear). The major advantage in principle ofthis method is that the number of calibrations performed and therefore the cost of the calibration
varies directly with the length of time that the measuring equipment is used. Another advantage ofthis method is that an automatic timer for the hours of use of the measuring equipment may beavailable.
Nevertheless, this method also has the following practical disadvantages:a) it is not suitable for measuring equipment containing passive (not requiring additional energy
input source for providing output) measuring instruments (e.g. attenuators) or passive
measurement standards (e.g. resistance, capacitance);b) it is not suitable for measuring equipment known to have a drift or deteriorate when not in use(e.g. it is on the shelf) or when handled or subjected to a number of short on-off cycles;c) the initial cost of providing and installing suitable timers for measuring the “in-service” time
may be high if the time is not recorded manually. Since users may interfere with the timers,
additional supervision may be required which will increase the costs; andd) the planning of recalibration work is more difficult in comparison with the procedures of
Methods 1 and 2 since is it not possible to predict the precise date on which the next calibrationis required.
6.5

Method 4: In service checking, or “black box” testing

Method 4 is also a variation of Method 1 and Method 2, and is especially suitable when aquick/easy check of the measuring equipment or one of its components is possible. Criticalparameters are checked frequently (e.g. once a day or even more often) by portable calibration
gear, or preferably, by a “black box” designed specifically to check the selected parameters. If themeasuring equipment is found to be outside the maximum permissible error (or any other set oflimits as required) by the “black box” or portable calibration gear, it is returned for a fullcalibration and adjustment if necessary. Method 4 may prove to be more effective than evaluating
the original measuring equipment’s interval.
Note:

Measuring equipment suitable for this method are for example density meters(resonance type), Pt-resistance thermometers (in combination with calendar-time
methods), dosimeters (source included), or sound level meters (source included).

The major advantage of this method is that it provides maximum availability for the user of themeasuring equipment. It is very suitable for measuring equipment which is geographically distantfrom the laboratory, since a complete calibration is only performed when it is known to berequired. The difficulty is in deciding on the critical parameters and designing the “black box”.
Although the method is in principle very reliable, this is slightly ambiguous, since the measuring
equipment may be failing on some parameter that is not measured by the “black box”. In addition,
the characteristics of the “black box” itself may not remain constant, thus requiring a choice andperiodic review of the recalibration interval of the black box.

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6.6

Method 5: Other statistical approaches

Methods based on statistical analysis of individual measuring equipment or groups of measuring
instruments can also be a possible approach. These methods are gaining more and more interest,
especially when used in combination with adequate software tools. An example of such a softwaretool and its mathematical background is described by A. Lepek [10].
When large numbers of identical measuring equipment (i.e. groups of measuring equipment) areto be calibrated, the recalibration intervals can be reviewed with the help of statistical methods(see e.g. [8]). Detailed examples are presented for example in the publication of the National
Conference of Standards Laboratories (NCSL) International - Recommended Practice RP-1
Establishment and Adjustment of Calibration Intervals [7].
6.7Comparison of methods for reviewing recalibration intervals

No single method described in 6.2 to 6.6 is ideally suited for all situations, for all measuring
equipment, and for all laboratories (see Table 1). The laboratory may choose the most appropriate
method for each case while considering a variety of factors as discussed in 4, 5 and 6.1. There
may also be additional factors that will affect the laboratory’s choice of method. It should be notedthat the choice of method will be affected by whether the laboratory intends to introduce a planned
maintenance schedule for the equipment. It should also be noted that the method chosen willcertainly affect the recalibration records that are kept.
For comparison of methods, see Table 1.
Table 1 - Comparison of methods of reviewing recalibration intervals

Performance

MethodMethod 1
Method 2
Method 3 Method 4 Method 5 1)
“staircase” control chart “in-use” time “black
other
box”
statistical
approaches

Reliability

mediumhighmediumhighmediumEffort of application

low

high

mediumlow

highWork-load balanced
between risks and costs

mediummediumlow

mediumlow

Applicability with respect
to particular devices

medium

low

high

high

low

Availability of measuring
equipment

mediummediummediumhighmedium1)

Better grading is achieved when an appropriate software tool is used

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7

Bibliography[1]

OIML V 2-200 International Vocabulary of Metrology – Basic and General Concepts andAssociated Terms (VIM), 3rd edition, Edition 2012 (E/F), (Edition 2010 with minor corrections),
JCGM 200:2012(E/F)[2]

CIPM MRA-G-13:2021 Calibration and Measurement Capabilities in the context of the CIPM
MRA (Version 1.1)

[3]

ISO/IEC 17025:2017 General requirements for the competence of testing and calibrationlaboratories[4]

Montgomery, D. C.: Introduction to Statistical Quality Control, John Wiley & Sons, 7th ed., 2012

[5]

ANSI/ASQC B1-B3-1996: Quality Control Chart Methodologies[6]

Methods of reviewing calibration intervals, Electrical Quality Assurance Directorate Procurement
Executive, Ministry of Defence United Kingdom (1973)[7]

Establishment and Adjustment of Calibration Intervals, NCSL Recommended Practice RP 1, 2010[8]

AFNOR FD X07-014:2006 Métrologie - Optimisation des intervalles de confirmation
métrologique des équipements de mesure[9]

Garfield, F.M.: Quality Assurance Principles for Analytical Laboratories, AOAC Int., 3rd Edition,
2000[10] Lepek, A.: Software for the prediction of measurement standards, NCSL International
Conference, 2001[11] ISO 9001:2015 Quality management systems – Requirements[12] ISO/IEC 17000:2020 Conformity assessment – Vocabulary and general principles[13] ISO/IEC 17020:2012 Conformity assessment – Requirements for the operation of various typesof bodies performing inspection[14] ISO/IEC 17043:2010 Conformity assessment – General requirements for proficiency testing[15] ISO 15189:2012 Medical laboratories – Requirements for quality and competence[16] ISO 17034:2016 General requirements for the competence of reference material producers[17] ISO/IEC 17065:2012 Conformity assessment – Requirements for bodies certifying products,
processes and services[18] ISO 22870:2016 Point-of-care testing (POCT) – Requirements for quality and competence[19] ILAC-P10:07/2020 ILAC Policy on Metrological Traceability of Measurement Results

17