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Ensuring that your calibration remains useful

A calibration certificate provides the user with an assurance that the instrument will be reliable in use.

This assurance is based on the calibration laboratory’s knowledge and expertise on the behaviour of similar instruments, but is conditional upon reasonable care and use of the instrument.

Proof that the instrument performance continues to be consistent with its calibration history rests with the user.

Care of the instrument

To demonstrate adequate care of an instrument ISO Guide 25 recommends a log for the instrument that contains procedures for use, records of calibrations and checks, records of repair and servicing, and most importantly, restrictions on the use of the instrument to approved staff and sites. This ensures that you can:

  1. demonstrate through regular checks and calibrations that the instrument is stable, and
  2. demonstrate that the instrument has not been exposed to conditions or use that may adversely affect its performance.

Adjustments, other than front panel offset or scale adjustments that are a part of the normal measurement procedure, will cause an irreversible change in the instrument’s behaviour. This effectively breaks the link between your measurements and the SI, so immediately invalidating the certificate. For this reason non-routine adjustments (e.g. those requiring screwdriver or password access to the instrument) should not be carried out unless that instrument has had to be repaired, or because the inconvenience of having to make corrections is intolerable.

If you must adjust the instrument it is best to co-ordinate the adjustment with the calibration laboratory. To ensure that your history of stability is not broken and that the certificate can be used retrospectively, the calibration laboratory will carry out measurements before and after the adjustment.

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Placing the burden of proof of validity of the certificate with the user clarifies the question of recalibration. Traditionally a certificate was treated like a dog licence with an expiry date.

Recalibration is required as soon as you no longer have confidence in the results because you are unable to demonstrate that the measurements are traceable

As a general guide MSL recommends that instruments be calibrated:

  • when they are first purchased,
  • after one year’s service (depending on use) to confirm the stability, then
  • as necessary up to a maximum interval of five years.

Appendix: The information required on a calibration certificate

Information Required Why?
Title: "Calibration Certificate." To establish what the document claims to be.
Name and address of calibration laboratory. So you know who to contact for further information.
Unique identification of certificate e.g a report number. So you and the laboratory can trace all work relating to the calibration and use of the instrument.
Each page should be numbered and the total number of pages given. So you know that the information is complete.
Unambiguous identification of the instrument, including make, model, serial number and software version numbers. So you know that this is the correct report and that the instrument is complete.
Date of calibration So you know how current the results are.
Identification of calibration method used or unambiguous description of any non-standard method used, and/or departures from standard methods. How were the results obtained? How will they relate to measurements I make? This and the next information category is the "fine print" of the report: read with care!
Conditions under which the calibration is carried out e.g. ambient temperature, loading conditions, instrument options. This ensures that you can reproduce the calibration conditions and ensure that the results are applicable to your situation.
Measurements and derived results. These form the heart of the report, what you have paid your money for!
A statement of the estimated uncertainty of the calibration results. To enable you to calculate the uncertainty in your measurements.
A signature and title of the person(s) accepting responsibility for the report contents This indicates that the report has gone through the quality system of the laboratory.
A statement that the certificate shall not be reproduced in full, without the written approval of the laboratory. This ensures that all relevant information is always available to the user of the certificate, and that the data used in the certificate cannot be used to mislead.
ISO Guide 25 accreditation endorsement. An assurance that the laboratory is competent to provide all of the above information.

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