Some other calibration issues

It is acceptable for testing laboratories to be self-sufficient in calibration and not to use any external calibration services for their equipment. They will still, of course, need to interact with the international metrology system to achieve traceability. This will usually mean holding traceable calibrated references to be used as the basis of the in-house calibration system.

For example, a testing laboratory might well hold a master reference thermometer which has a calibration certificate issued by an accredited calibration laboratory. The laboratory could then use this to calibrate all of its working thermometers according to a documented procedure. The body assessing the laboratory as a testing facility would include in its assessment an evaluation of the internal calibration system for thermometers and would need to be satisfied on the following points:-

ƒ That the reference thermometer was acceptably and traceably calibrated by a body recognised by the assessment body as providing adequate calibration.

ƒ That a sound and documented procedure was in place for carrying out calibration of working thermometers and that adequate quality controls were applied.

ƒ That staff carrying out the internal calibrations were properly trained and seen to be competent.

ƒ That there were auditable records showing that all of these criteria were being met on a routine basis.

Note, too, that the internal calibration would also have to be subject to an evaluation of its uncertainty by the laboratory just as though it were carried out by an external accredited calibration service.

In practice, laboratories typically carry out some calibrations in-house; thermometers, spectrometers and simple equipment, such as pH meters and conductivity meters, are examples. However, more complex calibrations, and especially those requiring expensive references which are themselves expensive to recalibrate, are typically carried out by outside services. For example, balance calibration is quite complex, needs only to be done fully once a year and needs expensive weights which have themselves to be calibrated annually. It is, therefore, more economical in most situations to use a commercial calibration service.

Another point to consider is the question of adjustment of equipment found out of calibration. This can be a skill in itself and may well be outside the capability of the laboratory. Modern balances are a case in point. In these circumstances, the use of a service which calibrates and can, if necessary, service and adjust is likely to be an attractive option.

9.5.2 Intervals between calibrations

Each laboratory must have a policy for determining calibration intervals, the object being to ensure that re-calibration takes place before the previous calibration has deteriorated to the point where the validity of the measurements is called into question.

To some extent, routine quality control checks will provide information which could reveal instruments drifting out of calibration. However, wherever practicable, the laboratory should institute a regime for verification of calibration of equipment between formal calibration intervals in order to detect unexpected drift or malfunction. This is particularly important for key items such as balances where a loss in calibration would have far reaching consequences.

Where it is not possible to carry out any simple routine verification to confirm that calibration is maintained, the laboratory will have to rely on setting specific calibration intervals. In the first instance, the laboratory sets an initial calibration interval based on the manufacturer's recommendations, the heaviness of use of the instrument, the accuracy required, the perceived risk of a loss of calibration and the magnitude of the impact, and local experience of similar instruments. The calibration is checked at the end of this interval and, if it is still correct, the interval is confirmed as adequate. Alternatively, the interval is reduced by 50% if the check shows that re-calibration is required.

This process is continued until an appropriate and adequate interval is arrived at. Records must be kept so that, if necessary, the laboratory can justify the interval chosen.

The process may, conversely, provide evidence of the stability of equipment and hence a justification for reducing the frequency of calibration relative to the original estimate. Whatever the strategy for deciding calibration intervals, it must be consistent and documented. It should also take into account that it is not unusual for accreditation bodies to insist on minimum calibration intervals for some equipment, for example balances, which are typically calibrated annually. In some cases, the accreditation body is prepared to consider relaxations to longer calibration intervals, but the laboratory would have to provide convincing evidence that data validity is not being compromised by extending the interval.

It should also be borne in mind that some test methods, for example construction materials testing, include requirements for verification of equipment calibration at specific intervals. These intervals must be complied with or the test method will be invalid.

9.5.3 Relaxation of calibration requirements

Some relaxation on the traceability of calibrations is apparently permitted in ISO 17025 by paragraph 5.6.2.2.1, but only with testing laboratories. If it can be established that calibration uncertainty ‘contributes little to the total uncertainty of the test result’, then traceable calibration is not required and other methods of demonstrating confidence in the equipment may be adopted.

In practice, this is not really a dispensation at all since there is still a requirement to show that the equipment provides the necessary uncertainty of measurement, and it is difficult to see how this can be done without at least some calibration.

In any case, accreditation bodies have a tendency to take a narrow view of the traceability of calibrations, and it is not recommended that this clause be relied upon as an easy excuse to avoid traceable calibration.