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Calibration-whats up?
16 Feb 2000, Test & Measurement

Accurate measurement is one of the fundamentals of both human life and the world's economy. A bolt made to a certain specification in South Africa must be compatible with the matching nut produced by another manufacturer in Korea or North America. When your blood pressure or temperature is measured in hospital you want to be assured that it is a true measurement so that the medical staff can make the correct diagnosis of your illness. If you are stopped at a police road block and a breathalyser test shows that you are over the legal alcohol limit when you only have had one beer, you would be very annoyed, and even more so if your car's speedometer showed 60 km/h when you were actually doing 90.

Today all measurements can seemingly be made with a very high degree of accuracy. After all the digital LED display on most electronic meters allows you to read to one or more digits after the decimal point. But is this really accuracy? The answer is of course no, as all measuring instruments must first be calibrated, and then the uncertainty of the measurement taken into account. A discussion of uncertainty is well beyond the scope of this short article, but depending on the type of measurement, even normal environmental conditions such as temperature, pressure and humidity could all have an influence. Not all new instruments are calibrated, and the onus is on the buyer to ask for a recognised certificate of calibration. A manufacturer's own calibration may be inadequate if his facilities have not been accredited.

How can you ensure that your electronic or other measuring equipment has been correctly calibrated? How can you ensure that your customer who is ISO 9000 certified will believe your measurement results? The easiest and most straightforward way is to ensure that you make use of a laboratory that has been accredited by SANAS to calibrate your instrument for the specific parameter you want to measure. Some accredited laboratories are multi-disciplinary, while others focus on specific areas of expertise, but their competence and best measurement capability has been assessed by SANAS, and their measurements are traceable to both national and international standards. You will find laboratories in the SANAS Directory which have been accredited in virtually every field of measurement, dimensional, mass, electrical, time and frequency, force, flow and so forth. The onus is on the user of the services offered by a calibration laboratory to ensure that they are accredited for the specific parameters required. Ask to see their accreditation schedule and find out what their 'Best Measurement Capability' is.

An accredited laboratory will issue you with a calibration certificate for each instrument, and this will bear the official SANAS logo for this area of accreditation. The instrument itself should also carry a stick-on label, again bearing the logo, which also has the date of the calibration. As use of this logo is limited to currently accredited laboratories for use on such documentation, its presence provides you, your customers and ISO 9000 auditors with confidence that the instrument has been properly calibrated and that its measurement is traceable in terms of international standards.

Calibration is not something which lasts for ever, and all measuring instruments, from simple balances to complex digital oscilloscopes must be re-calibrated at an interval which is dependent on many factors, including frequency of use (and abuse) of the device, the level of uncertainty permissible in the measurements performed, and a large number of other factors. The issue of when to recalibrate is a complex one, but is the responsibility of the user, and cannot be defined by either the manufacturer or the calibration laboratory. Both of these can however provide advice to the user, and companies certified to standards such as ISO 9000 (or QS 9000, VDA-6 etc.) must also take note of the onus placed on them in regard to traceability of measurement, proof of adequate calibration being required by auditors during the periodic assessments. A major benefit of using SANAS-accredited laboratories for calibration and testing is that the certificates and reports issued bearing the accreditation body's logo, are recognised internationally by a large number of similar national bodies which operate in our major trading nations. SANAS has existing mutual recognition agreements (MRAs) with the EA (European accreditation) which covers 15 European countries, Australia, New Zealand and the Republic of China. SANAS is also a member of the International Laboratory Accreditation Co-operation (ILAC), and the CEO of SANAS, Mike Peet was recently elected as Vice Chairman of this international body. ILAC is in the process of formulating the requirements for a MRA which will result in all member bodies recognising certificates and reports issued by each others accredited laboratories. This 'global' MRA would through the present membership of ILAC, include North America and much of the Far East, including Japan and Hong Kong.

SANAS is the sole South African accreditation body and is recognised as such by government through the Department of Trade and Industry. Besides the accreditation of laboratories SANAS also accredits certification bodies (to issue ISO 9000, ISO 14000 and HACCP certificates), inspection bodies and personnel certification bodies. SANAS is also a member of the International Accreditation Forum (IAF) and as a signatory to the IAF multilateral agreement.

While this article has focussed on calibration laboratories a major growth area for SANAS is in the accreditation of testing laboratories. The same principles as above apply, but in this case the end product would be a test report (bearing the recognised logo) attesting for example that a specific piece of equipment complies with the European Directive regarding the CE Mark.

SANAS

(012) 349 1441/2


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