**9.0 Measurement Traceability and Calibration**

9.1 Policy

> 9.1.1 Standards and measuring and test equipment significantly
> affecting the measurement integrity of tests conducted by the
> laboratory are monitored for stability as part of the measurement
> control program. Standards and equipment are calibrated and/or
> verified before use to ensure the recall or removal from service of
> any equipment or standards that are unreliable or that have exceeded
> the calibration interval if established.

9.2 Measurement Traceability

> 9.2.1 The established program of calibration and verification of
> measuring and test equipment is designed to ensure that the
> measurements made by the laboratory are traceable to national
> standards of measurement. All laboratory reports contain a statement
> that the standards used are traceable to the National Institute of
> Standards and Technology.
>
> 9.2.2 To provide external evidence of traceability, the laboratory
> participates in measurement control programs, proficiency tests, and
> other interlaboratory and collaborative experiments (see Appendices L
> and K).

9.3 Calibration/Verification (See Procedure for Calibration Intervals,
AP No. 13)

> 9.3.1 Standards (Calibration)
>
> 9.3.1.1 Primary standards are calibrated by the National Institute of
> Standards and Technology or by a NIST-accredited laboratory.
>
> 9.3.1.2 Primary standards are continuously monitored by the
> measurement control programs established in the laboratory and are
> under the custody of trained metrologists (Note: According to State
> requirements, if any, document here who maintains custody). These
> standards are generally stable and not subject to change. Any changes
> that occur are generally identified by the laboratory and the NIST OWM
> State audit program in ample time for corrective action.
>
> 9.3.1.3 Standards are recalibrated if there is damage to the standards
> or any significant change is observed in the measurement control
> program.
>
> 9.3.2 Standards (Verification)
>
> 9.3.2.1 Continuous verification of standards, through the measurement
> control program, ensures required measurement integrity of test and
> includes:

A. Statistical data from check standards and/or control charts (see
Appendix J); and

B. Results from interlaboratory comparisons and/or proficiency tests
(see Appendix K).

> 9.3.2.2 Measurement assurance procedures for verification of standards
> are maintained in the laboratory files (see Appendix H, AP No. 13)
>
> 9.3.3 Measuring and Test Equipment (M &TE) (Calibration)
>
> 9.3.3.1 Equipment used with nominal values and corrections is
> calibrated by NIST or by a calibration laboratory whose traceability
> to NIST has been validated through an accreditation process. A
> calibration interval is established for the equipment (i.e.,
> environmental equipment, balances).
>
> 9.3.3.2 When appropriate, each item of equipment is labeled, marked,
> or otherwise identified to indicate its calibration status.

A. All equipment used with nominal values and corrections is labeled
indicating the calibration status. Examples of this equipment include
thermometers, barometers, hygrometers, and balances.

B. All equipment used to facilitate comparison between a standard
artifact and an unknown is labeled \"Used for Comparisons Only - No
Calibration Required.\"

> 9.3.3.3 Procedures for setting and changing M&TE calibration intervals
> are maintained in the laboratory files (see Appendix H, AP No. 14).
>
> 9.3.3.4 Calibration of equipment is conducted at a frequency to ensure
> that the equipment remains in tolerance during its use in the
> laboratory. Frequency of calibration is based on a review of
> calibration, maintenance, and repair history. Reviews are conducted by
> the technical manager and records are maintained with internal audit
> records in the laboratory files.

9.4 Uncertainty of Measurement

1.  Where applicable, the *ISO Guide to the Expression of Uncertainty in
    Measurement*, 1993; the G. Harris paper, *Implementation of the
    Guide to the Expression of Uncertainty in Measurement for
    Calibration Laboratories*, 1994; and NIST HB 145, 1986, are used as
    the basis for expression of uncertainty in measurement (see Appendix
    I, Assessment of Uncertainties). Note: NIST HB 145 is being revised
    to meet the new ISO requirements for the expression of uncertainty
    in measurement. The revised document will be adopted by the
    laboratory upon completion.

**17025**

**5.4.6** **Estimation of uncertainty of measurement**

**5.4.6.1** A calibration laboratory, or a testing laboratory performing
its own calibrations, shall have and apply a procedure to estimate the
uncertainty of measurement for all calibrations and types of
calibrations.

**5.4.6.2** Testing laboratories shall have and apply procedures for
estimating uncertainty of measurement. In certain cases the nature of
the test method may preclude rigorous, metrologically and statistically
valid, calculation of uncertainty of measurement. In these cases the
laboratory shall at least attempt to identify all the components of
uncertainty and make a reasonable estimation, and ensure that the form
of reporting of the result does not give a wrong impression of the
uncertainty. Reasonable estimation shall be based on knowledge of the
performance of the method and on the measurement scope and shall make
use of, for example, previous experience and validation data.

**NOTE 1** The degree of rigor needed in an estimation of uncertainty of
measurement depends on factors as:

-   requirement of the test method;

-   requirement by the client;

-   there are narrow limits on which decisions on conformance to a
    specification are based.

**NOTE 2** In those cases where a well-recognized test method specifies
limits to the values of the major sources of uncertainty of measurement
and specifies the form of presentation of calculated results, the
laboratory is considered to have satisfied this clause by following the
test method and reporting instructions (see 5.10).

**5.4.6.3** When estimating the uncertainty of measurement, all
uncertainty components which are of importance in the given situation,
shall be taken into account using appropriate methods of analysis.

**NOTE 1** Sources contributing to the uncertainty include, but are not
necessarily limited to, the reference standards and reference materials
used, methods and equipment used, the environmental conditions, the
properties and the condition of the item being tested or calibrated and
the operator.

**NOTE 2** The predicted long-term behaviour of the tested and/or
calibrated item is not normally taken into account when estimating the
measurement uncertainty.

**NOTE 3** For further information see ISO 5725 and the Guide to the
Expression of Uncertainty in Measurement (see bibliography).

**5.6 Measurement traceability**

**5.6.1 General**

All equipment used for tests and/or calibrations, including equipment
for subsidiary measurements (e.g., for environmental conditions) having
a significant effect on the accuracy or validity of the result of the
test, calibration or sampling shall be calibrated before being put into
service. The laboratory shall have an established programme and
procedure for the calibration of its equipment.

**NOTE -** Such a programme should include a system for selecting,
using, calibrating, checking, controlling and maintaining measurement
standards, reference materials, reference standards used as measurement
standards, and measuring and test equipment used to perform tests and
calibrations.

**5.6.2 Specific requirements**

**5.6.2.1 Calibration**

**5.6.2.1.1** For calibration laboratories, the programme for
calibration of equipment shall be designed and operated so as to ensure
that calibrations and measurements made by the laboratory are traceable
to the SI (Système International) units of measurement.

A calibration laboratory establishes traceability of its own measurement
standards and measuring instruments to the SI by means of an unbroken
chain of calibrations or comparisons linking them to relevant primary
standards of the SI units of measurement. The link to SI may be achieved
by reference to national measurement standards. National measurement
standards may be primary standards, which are primary realizations of
the units of the SI or agreed representations of SI units based on
fundamental physical constants or they may be secondary standards which
are standards calibrated by another national metrology institute. When
using external calibration services traceability of measurement shall be
assured by the use of calibration services from laboratories that can
demonstrate competence, measurement capability and traceability. The
calibration certificates issued by these laboratories shall contain the
measurement uncertainty and/or a statement of compliance with an
identified metrological specification (see also 5.10.4.2).

**NOTE 1** -- Calibration laboratories fulfilling the requirements of
this International Standard are considered to be competent. A
calibration certificate bearing an accreditation body logo from a
calibration laboratory accredited to this International Standard for the
calibration concerned, is sufficient evidence of traceability of the
calibration data reported.

**NOTE 2** Traceability to SI units of measurement may be achieved by
reference to an appropriate primary standard (see VIM:1993, 6.4) or by
reference to a natural constant the value of which in terms of the
relevant SI unit is known and recommended by the General Conference of
Weights and Measures (CGPM) and the International Committee for Weights
and Measures (CIPM).

#  NOTE 3 Calibration laboratories that maintain their own primary standard or representation of SI units based on fundamental physical constants can claim traceability to the SI only after these standards have been compared, directly or indirectly, with other similar standards of a national metrology institute.

**NOTE 4** The term \"identified metrological specification\" means that
it must be clear from the calibration certificate against which
specification the measurements have been compared by including the
specification or by giving an unambiguous reference to the
specification.

**NOTE 5** When the terms \"international standard\" or \"national
standard\" are used in connection with traceability, it is assumed that
these standards fulfil the properties of primary standards for the
realization of SI units.

**NOTE 6** Traceability to national measurement standards does not
necessarily require the use of the national metrology institute of the
country in which the laboratory is located.

**NOTE 7** If a calibration laboratory wishes or needs to obtain
traceability from a national metrology institute other than in its own
country, this laboratory should select a national metrology institute
that actively participates in the activities of BIPM either directly or
through regional groups.

**NOTE 8** The unbroken chain of calibrations or comparisons may be
achieved in several steps carried out by different laboratories that can
demonstrate traceability.

**5.6.2.1.2** . There are certain calibrations that currently cannot be
strictly made in SI units. In these cases calibration shall provide
confidence in measurements by establishing traceability to appropriate
measurement standards such as:

-   the use of certified reference materials provided by a competent
    supplier to give a reliable physical or chemical characterization of
    a material;

-   the use of specified methods and/or consensus standards that are
    clearly described and agreed by all parties concerned.

Participation in a suitable programme of interlaboratory comparisons is
required where possible.

**5.6.2.2 Testing**

**5.6.2.2.1** For testing laboratories, the requirements given in
5.6.2.1 apply for measuring and test equipment with measuring functions
used, unless it has been established that the associated contribution
from the calibration contributes little to the total uncertainty of the
test result. When this situation arises, the laboratory shall ensure
that equipment used can provide the uncertainty of measurement needed.

**NOTE** The extent to which the requirements in 5.6.2.1 should be
followed depends on the relative contribution of calibration uncertainty
to the total uncertainty. If calibration is the dominant factor, the
requirements should be strictly followed.

**5.6.2.2.2** Where traceability of measurements to SI units of
measurement is not possible and/or not relevant, the same requirements
for traceability to e.g. certified reference materials, agreed methods
and/or consensus standards are required as for calibration laboratories
(see 5.6.2.1.2).

**5.6.3 Reference standards and reference materials**

**5.6.3.1 Reference standards**

The laboratory shall have a programme and procedure for the calibration
of its reference standards. Reference standards shall be calibrated by a
body that can provide traceability as described in 5.6.2.1. Such
reference standards of measurement held by the laboratory shall be used
for calibration only and for no other purpose, unless it can be shown
that their performance as reference standards would not be invalidated.
Reference standards shall be calibrated before and after any adjustment.

**5.6.3.2 Reference materials**

Reference materials shall, where possible, be traceable to SI units of
measurement, or to certified reference materials. Internal reference
materials shall be checked as far as is technically and economically
practicable.

**5.6.3.3 Intermediate checks**

Checks needed to maintain confidence in the calibration status of
reference, primary, transfer or working standards and reference
materials shall be carried out according to defined procedures and
schedules.

**5.6.3.4 Transport and storage**

The laboratory shall have procedures for safe handling, transport,
storage and use of reference standards and reference materials in order
to prevent contamination or deterioration and in order to protect their
integrity.

**NOTE** Additional procedures may be necessary when reference standards
and reference materials are used outside the permanent laboratory for
tests, calibrations or sampling.