Developing an appropriate calibration strategy means looking at all instruments and assessing whether the most efficient use of calibrations is being made. The following points contain suggestions as to how an appropriate calibration strategy may be devised.

Consult with an accredited calibration laboratory

An accredited calibration laboratory should be able to offer advice as to which calibration is the most appropriate for your needs. If the user can describe the type of measurements taken and the relative level of importance of their measurements, a calibration laboratory should be able to recommend:

• The required level of calibration traceability
• Appropriate calibration standards and procedures for each instrument
• A recommended interval between calibrations

Consider which instruments are used for each activity

In the section Measurement Instruments we consider which instruments are used for each measurement activity. Instruments used to make measurements for high-importance activities will require a higher level of calibration.

• It can be costly to maintain a high level of calibration traceability on all your instruments. Is it necessary?
• It may be possible to reserve a subset of instruments for making important measurements. These instruments must be maintained at a high level of calibration traceability, but other instruments may be calibrated to a lower level of traceability and / or accuracy.

Develop appropriate in-house procedures

Many procedures to maintain instrument calibration levels can be carried out in-house.

• If you have an in-house laboratory, it may be possible to maintain reference instruments, calibrated to Level 1 traceability. Other instruments can be cross-checked against the reference instruments, giving a Level 2 equivalent traceable calibration.
• It should be possible to carry out regular cross-checks between instruments to monitor instrument sensitivities and drift.
• A simple database can then be maintained to keep track of calibrations and instrument drift, and thus choose the most appropriate calibration interval for each instrument.
• The use of a portable calibrator before and after measurements will increase the confidence in your measurements. Note that a portable calibrator should generally be calibrated at least once a year, according to guidance in the UKAS Lab 23 document.

How to Use your Calibration

Once an instrument has been calibrated, the following points will ensure that the maximum improvement to measurement quality is obtained, and will ensure that future calibrations are made as appropriate.

• Has the instrument passed the calibration tests? If the equipment has failed, it is outside specification and should not be used.
• Remember to use the newly calibrated sensitivity when taking measurements. If the calibration results or corrections do not feature in your measurements, it is possible that the chosen calibration is not appropriate for your requirements or that your measurement procedures are inappropriate and require checking.
• How does the new calibrated sensitivity compare to the previously used figure? By comparing sensitivity figures for successive calibrations, drift can be monitored.
• An instrument which shows frequent drift may need to be calibrated more often. Frequent drift may also be indicative of an underlying problem.

Alternatives to Calibration

In certain circumstances, there are alternatives to regular calibration that may be more cost-effective.

• If an instrument is only used rarely, but requires a high level of calibration traceability, it may not be cost-effective to maintain it by calibrating regularly. It might be better to hire the required measurement equipment whenever it is required. Ensure that the hired equipment holds a valid and appropriate calibration. It may even be more appropriate to employ a specialist subcontractor to carry out the required measurements and to report upon the results.
• If an item of equipment has a relatively low cost to buy, but a relatively high cost to calibrate, then it may be more cost-effective to buy a new item every time a calibration is required, rather than attempt to calibrate the existing item. Ensure that the new instrumentation is supplied with appropriate certification for your needs.

Formally define your calibration procedures

It is a good idea to develop a written procedure which defines:

• Which instruments should be calibrated
• What type of calibration is to be carried out
• How often calibrations are to be carried out

This procedure may form a part of your organisation's quality management procedures. Developing such a procedure will guide the user through the necessary thought processes and decisions in order to form an appropriate calibration strategy for all instruments.

The Calibration Selection Guide >>