CALIBRATION FEATURE
WHAT LEVEL OF CALIBRATION will satisfy my requirements?
Instrument manufacturers are continually developing new and
innovative products. However, even though the technology is advancing and the cost of handheld measuring instruments is decreasing, the
requirement for calibration is still as important. Paul Jordan, business
manager – Service and Calibration – Testo, explains why
O
ne of the more frequent questions we get asked at Testo is: “Why
is the cost of calibration relatively high compared to the price of the instrument?” The answer to this question is not simple as there are different factors which can contribute, but essentially the process of delivering a technically competent calibration doesn’t change and there are inherent costs associated with it. In other words, the cost of the instrument is not directly related to the cost of the calibration. The costs which contribute can vary, such as reference equipment purchase price, other testing apparatus (such as wind tunnel), on-going equipment maintenance including re-calibration, maintaining good quality processes and, of course, manpower. As these costs exist whether we are calibrating a high end reference instrument or a typical budget instrument, the question of cost is more a consideration for the instrument rather than the calibration.
AN IMPORTANT CONSIDERATION Today the issue of compliance is ever present in the workplace. Factors such as temperature, electrical, airflow, humidity, CO, CO2
and light are some
of the common parameters where compliance may be necessary. There may also be a need to comply with
Having good quality instruments calibrated by a reputable
calibration laboratory,
such as Testo’s, greatly enhances a company’s ability to demonstrate compliance and/or
adherence to standards
an internal or external quality standard where having calibrated equipment is an essential requirement. Having good quality instruments
calibrated by a reputable calibration laboratory, such as Testo’s, greatly enhances a company’s ability to demonstrate compliance and/or adherence to standards. In addition, maintaining a calibrated instrument demonstrates due diligence and can greatly help in any Health & Safety or criminal investigation. In these cases, calibration
should be an important consideration. The question should not just be “Is a calibration certificate needed?” but “What level of calibration will satisfy my requirements?” Calibration is often misunderstood.
Yes it is to check the accuracy of one measuring instrument against another, the known reference standard, but the process of producing this measurement is much more than this. In order to perform an accuracy check a number of factors need to be considered. For example, any laboratory carrying out calibration will need to prove the performance of their own reference instruments or
“The consequences of an incorrectly calibrated device can be severe. If an instrument is found to be outside of specification when presented for calibration, numerous inaccurate results could have been recorded”
standards, not only at a single point in time (typically re-calibration) but also throughout the period between calibrations. Any credible laboratory, and particularly one which is accredited to ISO17025:2005, such as Testo’s calibration laboratory, will spend time maintaining calibration reference instruments. This process includes calibration data analysis, measurement uncertainty calculation, on-going performance checks, with further evaluation to monitor drift and, in some cases, inter-laboratory comparison (checks against other laboratory’s reference equipment) to further enhance confidence in the laboratory’s ability to produce good results. Of course, these activities require additional time and documentation which can then demonstrate the laboratory’s competence to perform good measurements during audits. One key element of any calibration
is the laboratory’s ability to assign a measurement uncertainty to any calibration it performs. This is essential – absolute accuracy does not exist, and therefore any measurement will have a degree of error or uncertainty. The amount of uncertainty can vary based upon different factors, such as the reference instruments selected, the apparatus used for the calibration, repeatability and reproducibility to such effects caused by changes in ambient conditions. Many other factors can contribute to the overall measurement uncertainty depending on the type of calibration being carried out, but all of
> 14 INSTRUMENTATION | SEPTEMBER 2016 13
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