Quality starts with device calibration

Quality standard ISO 9001-2000 proposes many requirements for monitoring and measuring devices, as Richard Sanders, director of regulation at the National Weights and Measures Laboratory, explains

Organisations wishing to comply with the Quality Management Standard ISO 9001-2000 must (among other things) meet the requirements of section 7.6: "Control of monitoring and measuring devices". This part of the standard ensures that processes involving the use of monitoring or measurement devices provide confidence that the results are accurate and reliable.

The standard requires that where necessary to ensure valid results, measuring equipment shall:

a) be calibrated or verified at specified intervals, or prior to use, against measurement standards traceable to international or national measurement standards; where no such standards exist, the basis used for calibration or verification shall be recorded. The measuring instrument or standard (for example a thermometer or weight) should be sent to a test/calibration laboratory for periodic re-calibration or re-test. The laboratory should preferably be accredited to IEC/ISO 17025 (in the UK accreditation of laboratories is carried out by UKAS and this will guarantee traceability to national standards or equivalent methods).

b) be adjusted or re-adjusted as necessary. After the test or calibration results have been obtained, the organisation (or test house if requested to do so by the client) should check that the measuring equipment remains within the required tolerance. For example, a thermometer may need to be accurate to within ± 0.5 degrees of reading, or a weight within ± 10 mg of the nominal value. If the device is outside the required tolerance then it should be adjusted if this is possible or replaced. Typically, the uncertainty of measurement for the test or calibration should not exceed 1/3 to 1/5 of the tolerance specified for the measuring equipment. A calibration certificate must always state the uncertainty of measurement associated with the calibration result. This requirement is optional for test certificates although the issuing laboratory should have and make available the uncertainty data to the client if requested.

c) be identified to enable the calibration status to be determined. Typically the measuring instrument or standard should have some way of letting the user know if and when a re-calibration or re-test is due. Usually this is achieved by placing a sticker with "re-calibration due" or "calibration expires" on the device (note: for weight standards the sticker should be placed on the storage box and not the weight itself!).

d) be safeguarded from adjustments that would invalidate the measurement result. If the reading of the measuring equipment is adjustable then this should be disabled or sealed to prevent unauthorised or accidental adjustment. Typically this is done following calibration or testing.

e) be protected from damage and deterioration during handling, maintenance and storage. This is a common sense requirement to protect the often fragile items of measuring equipment. Damage could happen if the measuring equipment is exposed to heat, cold, dampness, mechanical shock etc.

In addition, companies should assess and record the validity of the previous measuring results when the equipment is found not to conform to requirements and take appropriate action on the equipment and any product affected.

If the measuring equipment is found to be erroneous, e.g., out of tolerance or calibration period expired, the processes that have taken place using the erroneous measuring equipment must be evaluated to assess whether the errors can be tolerated. The equipment must be fixed and recalibrated

Records of the results of calibration and verification must be maintained. This is to ensure the calibration or test results of the measuring instrument are securely maintained. The data should be analysed so that a long-term picture of the performance can be built up. The rate of "drift" may be used to decide a suitable recalibration period.

When used in the monitoring and measurement of specified requirements, the ability of computer software to satisfy the intended application must be confirmed prior to initial use and reconfirmed as necessary.

If results from monitoring and measuring devices are fed into a computer and the program mathematically processes the results to provide an end result (for example using Microsoft Excel), the program should be "validated" prior to use. Typically, validation is achieved by comparing the results of the program with the results calculated manuallyto ensure they tally. If the calculation algorithm of the program is modified then the validation ex-ercise should be repeated.

NWML is accredited by UKAS to certify quality management systems to ISO9001-2000.