CONFIDENCE under pressure

Throughout the process industries, pressure measurement performs a critical role. Operators rely upon the performance of pressure measurement devices, including electronic pressure transmitters and transducers, switches and gauges, to ensure that essential safety and efficiency criteria are continually met and, wherever possible, exceeded.

The growing market for increased measurement performance, including accuracy and stability, is therefore driving demand for high-tech pressure sensors which are far superior to their equivalents of even five years ago.

As a result of manufacturer innovations and developments, today's devices can offer very high performances to satisfy many applications. However, even before specifying these instruments, operators must think carefully as to how their performance for a specific duty will be checked and verified, both directly and indirectly, against corporate, national and even international quality standards.

At worst, selection of an inappropriate method of calibration and traceability can invalidate critical performance data. At best, the time lost and inconvenience caused will mean considerable disruption to the user.

Traceability

Calibration is formally defined as a set of operations which establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument and the corresponding values realised by reference standards. Any remedial adjustment of the measuring instrument requires subsequent verification to distinguish between `as found' and `as left' calibrations.

Confidence in the results of a calibration and their relationship with other pressure measurements demands that measurements have traceability. Formally, this is said to be the property of the result of a measurement, whereby it can be ultimately related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. The calculation of measurement uncertainties at each link in the chain is therefore vitally important and, if not handled properly, can destroy traceability.

At each stage, the comparison procedure itself must be analysed carefully in addition to the characteristics of the reference standard employed and, moreover, the skill of the personnel actually carrying out the calibration process. Possession of a traceable instrument does not in itself guarantee that measurements will be traceable!

The question of how often should a measuring instrument be calibrated depends upon its recent calibration history and the measurement uncertainty required for the application. If a device has required no adjustment on at least two previous occasions (one recently) or the data shows performance substantially better than the uncertainty required, the `recalibration interval' can probably be quite long, often more than a year. Conversely, regular device adjustment or performance approaching the uncertainty limits will mean more frequent calibration: quarterly, monthly or even weekly.

From lab to line

A variety of equipment is available to address the broad issue of pressure calibration at each stage of the traceability `tree'. Categories of so called `calibrators' extend from basic `working' standards in the field or factory, through production transfer and corporate reference `secondary' standards to the highest level of national `primary' standard.

The features, functionality and packaging of each type varies considerably to reflect the differences in operating environment, performance and throughput required. Typically, automation features (which increase calibration productivity while reducing errors and assuring quality compliance) are growing ever more popular with field, factory and laboratory based users.

The increased use of menu driven, intelligent calibrators which communicate directly with PC-based management systems means that even complex calibration procedures can be implemented easily and quickly, while removing the burden of quality standards such as ISO 9000 from the user. Pre-ordained work procedures are selected and implemented at the calibrator, with subsequent results uploaded and analysed back at the computer. Centralised and highly accurate records can then be maintained, from which Quality Assured reports and certificates can be produced as required.

Full capability

Experienced, specialist manufacturers such as Druck can offer a full capability of products, which can be used at each stage from field working standard to laboratory primary standard. These include fully self-contained hand-held testers for field maintenance and troubleshooting; high-speed pressure controllers for factory production and workshop test purposes; and deadweight testers used as industrial, corporate and national primary standards worldwide. Performance levels range from better than fractional percentages of reading to just several parts per million precision.

The products available embrace a wealth of the latest pressure measuring, control and packaging technologies. For example, precision piston gauge systems; fused quartz bourdon tube sensors; micro-machined resonant silicon sensors; all-digital and micro processor control; and built-in fine pressure generation.

Correctly applied to industrial process and research applications, they offer a smart, convenient and reliable solution to calibration, traceability and ultimately, confident pressure measurement. PE

Tony Cuttill is sales manager of Druck