INSTRUMENTATION,
INSTRUMENTATION, CONTROL, AUTOMATION COVER Calibration of pressure measuring instruments Necessary evil or quality driver? Calibration is often viewed as a secondary task in plant operation. Against the backdrop of the demands imposed by quality management systems – a new guideline from the German accreditation body (DAkkS) and current efforts to push back the limits of plant efficiency ever further – it is time to view calibration from a different angle. Wika’s multifunctional calibrators can prove traceability up to the national standard for pressure measuring instruments. Proof of traceability In short, calibration means comparing the display of a test item with the value measured by a reference instrument, which has itself been tested against a national or international standard. In Germany, the national metrology institute (PTB) maintains the national standard to which all calibration results can be traced via a chain of comparisons. Regarding proof of traceability, a transformation has taken place. The DAkkS certificate is meanwhile a condition of most QA audits. DAkkS now specifies the requirements in greater detail: factory certificates from accredited laboratories are no longer accepted as proof of traceability. According to the new regulation, they are merely classified as “result reports without an accreditation symbol”, i.e. as a simple proof of functionality – in contrast to DAkkS certificates, which are now officially designated as “calibration certificates with an accreditation symbol”. Mechanical, chemical and thermal influences have an impact on the functionality of measuring instruments. Continued wear can cause an instrument to age and affect the accuracy of the reading. Regular calibrations provide information on the instrument’s metrological characteristics. They either confirm the measured value or they flag up a change in good time. When discussing the importance of calibrating measuring instruments, calibration frequency is a question that automatically arises. When it comes to TÜV or verifications by the Bureau of Standards, German legislation defines clear time periods. However, these do not apply to measuring instruments in industry. The testing intervals generally depend on the type of instrument, its task and the particular application. The following rule of thumb has been adopted for pressure measuring instruments, for example: the more electronics and the higher the accuracy, the more frequently the instrument should be calibrated. Such general statements, which are based on broad practical experience, provide a good starting point for deriving calibration requirements and intervals. The leeway which has traditionally been Author Matthias Bundschuh Head of Product Management Calibration Technology, Wika stretched to the limit for reasons of cost is now much smaller since quality assurance systems came into being. ISO 9001, GMP (Good Manufacturing Practice) and FDA (US Food and Drug Administration) regulations contain detailed rules for calibrating and documenting measuring instruments. However, these standards do not specify particular calibration intervals. They merely require a certain regularity, which the plant operator must define in the QA documentation. The German accreditation body (DAkkS) has issued recommendations for calibration intervals in this context. For pressure measuring instruments, for example, these intervals can range from once a year (e.g. for pressure transmitters with an electrical output and 0.5 % accuracy of the span) to as long as every five years (deadweight tester / pressure balance). To maintain the functionality of measuring instruments, they have to be calibrated regularly The principle of traceability remains unaffected by the change of title. In practice, this means that the chain of comparative measurements up to the national standard must be unbroken. The measurement uncertainty of each reference in the chain must be known in order to calculate the overall uncertainty for a test item. Higherlevel measuring instruments should generally have three to four times better accuracy than lower-level ones. All steps in the chain of comparisons, including the results of the measurements, must be documented. Plant operators are increasingly asking for DAkkS calibration certificates, mainly because of QA requirements. Furthermore, in the wake of agreements between the European co-operation for Accreditation (EA), to which DAkkS is affiliated, and the International Laboratory Accreditation Cooperation (ILAC), the certificate is globally recognised. 20 cpp 1-2017
When pressure measuring instruments are calibrated, Wika can prove traceability up to the national standard the CPC series have down to 0.008 % accuracy and can be used to calibrate both new sensors and the portable calibrators themselves. The pressure ranges, control rise times, sensor changes and accuracy are all designed for maximum flexibility. Two of the CPC series models additionally have a swivelling front to allow the test sensors to be exchanged rapidly and make re-calibration more efficient. Calibration software to produce the certificates automatically rounds off the range of capabilities. The precision instrument at the heart of the calibration technology is a dead-weight tester, also known as a pressure balance, which as a primary standard, for example, can provide up to 0.0015 % accuracy of the measured value purely mechanically. According to the formula Comprehensive calibration service In view of the metrological and documentary effort required for a DAkkS certificate, comparatively few companies can, or indeed want to, have their own accredited calibration laboratory. Most use an external service provider to check their measuring devices. Wika, too, resorts to independent calibration laboratories. These facilities can be accredited for several measurands. As far as the calibration of pressure measuring instruments is concerned, Wika can prove traceability up to the national standard. Several calibration devices are available in-house for this purpose. The CPH7000 portable calibrator measures pressure, temperature, current, voltage and ambient conditions. It includes an integrated hand pump for the test pressure and enables full calibration in the field with just one instrument. The pressure controllers in Wika operates several mobile, DAkkS accredited calibration laboratories in Germany FACTS & FIGURES Multifunctional device for on-site calibration pressure = force/area, the reference measurement is generated using defined mass discs which are placed on a piston by hand. The CPH7000 process calibrator from Wika is a portable, multifunctional instrument. It combines measurements of pressure, temperature, current, voltage and ambient conditions. The CPH7000 is therefore suitable for testing and calibrating analogue pressure measuring instruments as well as pressure and process transmitters. In the version with a built-in hand pump for test pressures from -0.85 to +25 bar, an integrated barometer and an external atmospheric module, the CPH7000 enables on-site calibration with just one instrument. Thanks to its electronic module, the calibrator can supply all standard measuring devices with current and voltage, measure output signals, simulate sensor conditions and carry out pressure switch tests. Furthermore, using a PT100 sensor, it can even measure the temperature of the fluid. With an accuracy of 0.025 % of the span, the The CPH7000 process calibrator is suitable for testing and calibrating analogue pressure measuring instruments as well as pressure and process transmitters CPH7000 is easy to operate. A touch screen with eight application fields enables speedy and safe setting or parameterisation of all functions and tasks. A high-end data logger automatically saves all measured values, which can be transmitted using the wireless function to a final device with the Wika-Cal software for further processing. Mobile calibration laboratories The test items are normally sent in for calibration. Thus, until they have been tested in the laboratory and then returned and re-installed, the process must be offline or fitted with appropriate substitute instruments. Companies are keen to keep downtime as short as possible. These phases can be reduced by using a mobile calibration laboratory. The test items are then calibrated either on site at the plant or in the mobile laboratory vehicle, depending on the task. This solution elimi - nates the logistical effort for the client. Wika operates several such vehicles, which are also DAkkS accredited, in Germany. » www.cpp-net.com Online search: cpp0117wika cpp 1-2017 21
