Measurement and Calibration Systems (Metrology)

3.13.
For big factories it is desirable to establish and maintain an effective system for the control
and calibration of measurement standards and measuring equipment employed. The calibration
system should be designed to ensure that all measurement resources have the capability of making
measurements within the specified limits. Such a system should provide for prevention of inaccuracy
by prompt detection of deficiencies and timely action for their correction.
The calibration system should be periodically and systematically reviewed to ensure its
continued effectiveness and the results should be recorded. All measurements should take into
account all the errors and uncertainties in the measurement process that are attributable to the
measurement standard or measuring equipment, as appropriate, those contributed by personnel,
procedures and environment.
Documented calibration procedures should be available for reference and for the calibration
of all measurement standards and measuring equipment used.
All measuring equipment and measurement standards should have records in regard to
(i) the description of equipment and unique identification, (ii) the date on which each calibration
performed, (iii) the results of calibration, (iv) the planned calibration interval, (v) the designated
permissible limits of error, (vi) the reference to calibration procedures, (vii) environmental condition
for calibration and corrections for reference condition, (viii) statement of cumulative effect of
uncertainties on the data obtained in calibration, (ix) details of any maintenance (servicing,
adjustment, repairs) or modifications that could affect the calibration status, (x) any limitations in
use. All measurement standards and measuring equipment should be labelled, coded or otherwise
identified to indicate their calibration status. Any limitation of calibration or restriction of use
should be clearly indicated on the equipment.
Access to adjustable devices provided on measurement standard and measuring equipment
should be sealed after fixing at the time of calibration. Seals should be so designed that tampering
will destroy them.
The interval between periodic calibrations depends on the basis of stability, purpose and
usage. Depending upon the results of preceding calibrations, intervals of calibration may be
shortened, if necessary, to ensure continued accuracy.
The measurement standard or measuring equipment that has failed in operation or shows
evidence of physical damage that may affect its accuracy, or that is suspected of being outside its
designated limits, or which is outside its designated calibration period should be removed or
identified to prevent their use. Details of such invalidation of calibration requiring corrective action
should be recorded.
The system for handling, transporting and storing of measurement standards and measuring
equipment to prevent their abuse, misuse, damage or change in dimensional/functional charac-
teristics should be established and maintained.
All measurement standards and measuring equipment should be calibrated using measure-
ment standards that are traceable to national or international measurement standards, except
where they have been derived from acceptable values of natural physical constants or by the ratio
type of self-calibration techniques.
All measurement standards used in the calibration system should be supported by certifi-
cates, reports or data sheets attesting to the date, accuracy, and conditions under which the results
were obtained and are valid.
The cumulative effect of the errors in each successive stage of a calibration chain should be
taken into account for each measurement standard or item of measuring equipment calibrated.
Action should be taken when the total uncertainty is such that it significantly compromises the
ability to make measurements within the required limits.
Measurement standards and measuring equipment should be calibrated and used in an
environment controlled to the extent necessary to ensure valid measurements. Due consideration
should be given to temperature, rate of change of temperature, humidity, lighting, vibration, dust
control, cleanliness and other factors affecting measurement. Where pertinent, these factors should
be continuously monitored and recorded, and when necessary, compensating corrections should be
applied to measurement data. Records should contain both the original and the corrected data.