TechArchive

This article was originally written in the period 1995-2000

Mick Hall, Head of Laboratory at Comark, explains the reasons why you should choose a calibration procedure for an instrument.

Certificates of conformance, calibration certificates, NPL, NAMAS – the plethora of certificates and approvals available to users of instruments is enough to fog the clearest of minds.

Calibration certificates in general provide some comfort that an instrument is performing adequately. The choice depends on the accuracy and traceability required.

So what do they all mean? How can users be sure that their instruments are accurate. Is the law on their side? All is not necessarily as it may appear.

Many current regulations require process and storage conditions (temperature, pressure, flow, etc.) to be measured and for the operator to provide proof of the accuracy and traceability of the measurements taken. Frequently, however, companies leave themselves open to prosecution, not because they fail to take readings or keep comprehensive records, but because they have no definitive proof of the accuracy of their measuring instruments.

So how can these pit-falls be avoided? Obtaining a simple calibration certificate may not be enough to satisfy the authorities. The problem is much more complex than that.

A Certificate of Conformance, for example, is a useful document that confirms that the instrument was manufactured and tested to specification. From 1 January this year, it could also confirm that the instrument conforms with the EMC Directive.

It does not, however, relate to the accuracy demanded by the application:

• It cannot consider what might have happened to it after it left the test bench;
• It has no reference to the environment in which the instrument works;
• Nor does it allow for the accuracy of any probes or accessories with which it might be used.

A Certificate of Calibration is much better. The calibrated instrument has been set up with reference to a known value. There is, however, no guarantee that the reference itself is accurate or that its uncertainty (tolerance) is within acceptable limits. If absolute accuracy is vital, Traceable Certificates will be necessary.

Traceable Certificates are issued by laboratories that have calibration equipment, the accuracy of which can be traced back to the standards held by the National Physical Laboratory. This provides absolute confidence that instruments are accurate within the laboratory’s published uncertainties.

Unfortunately, even this degree of assurance is insufficient to meet many current regulations. Many regulations require proof, not only that the calibration instruments used were accurate, but also that the operators of those instruments were competent and followed an agreed procedure.

The only way to be certain that both the calibrated standard is correct – and that the laboratory has the necessary skills, experience and equipment adequately to perform the task – is to use a NAMAS (National Accreditation for Measurement and Sampling) accredited laboratory.

NAMAS provides a formal recognition of the laboratory’s competence to undertake work, within an agreed measurement range and calibration uncertainties detailed in a published NAMAS schedule. The accreditation ensures that:

• The laboratory’s instruments have been calibrated within the correct time, by competent people;
• The laboratory’s facilities, staff training, environment and documented procedures are adequate for the work to be conducted;
• The uncertainties of the reference standards are low enough to ensure that the calibrated instrument meets the desired specification;
• The reference standards are traceable directly to national standards.

NAMAS calibration certificates may be issued only by a NAMAS-accredited laboratory and may be signed only by one of the laboratory’s approved signatories.

NAMAS is widely recognised internationally, through bilateral and multilateral mutual recognition agreements and through its membership of EAL (European Cooperation for Accreditation of Laboratories), the organisation that works to achieve a uniform approach to laboratory accreditation throughout Europe. This international recognition further underscores the value of NAMAS certification.

To make sure that the laboratories maintain their high standard, NAMAS makes its own checks, twice during the first year, then annually thereafter. Calibration certificates, measurements and records are checked for procedural correctness and accuracy and to ensure that results have been checked for abnormalities. Customers must have been alerted to any deviation that might adversely affect the performance of an instrument.

It can take a company up to three years to satisfy NAMAS that its laboratory can consistently achieve the standard. The process is both lengthy and expensive. Comark spent over (UK pounds)75,000 setting up its laboratory and achieving NAMAS accreditation for the extra facilities.

• Comark
• Tel: 01707 331051
• Fax: 01707 331202