Are pharmaceutical companies a necessary evil

Calibration of measuring devices under pressure of efficiency and quality

Calibration is a must: To ensure the accuracy of measuring devices, regular comparison measurements are necessary. (Image: Gunnar Assmy - Fotolia)

Against the background of the regulations of quality management systems, the new guideline of the German Accreditation Service (DAkkS) and an ever increasing exhaustion of the system efficiency, calibration is increasingly viewed from a different point of view. Mechanical, chemical and thermal influences act on measuring devices during operation and cause them to age - the accuracy drops. Calibrations provide information about the metrological properties of the measuring device. They confirm the measured value or can draw attention to a change in good time. In this way, these examinations secure the metrological monitoring and control system of a plant. They are a decisive factor when it comes to the safety of people and machines, the profitability of production processes and the optimization of processes.

No legally stipulated calibration cycles

Given the importance of the calibration of measuring instruments, the question of frequency automatically arises. The legislator has set clear deadlines for the general inspection of vehicles or the calibration, a task related to calibration. There is no such thing for the control of measuring devices in industry. In general, test cycles depend on the type of measuring device, its task and the respective application. For pressure measuring devices, for example, the rule of thumb has become established: the more electronics and the higher the accuracy, the more often it has to be calibrated. Calibration requirements and cycles can definitely be derived from such general statements, which are based on broad practical experience. The leeway that was often exhausted in the past for cost reasons has been narrowed in the age of quality assurance systems. The guidelines of ISO 9001, Good Manufacturing Practice (GMP) or the US Food and Drug Administration (FDA) contain detailed regulations for the calibration of measuring devices and their documentation.

Example of a typical calibration cycle. (Image: WIKA)

Complex DAkkS certificate becomes a condition for audits

However, these standards do not specify any specific test cycles either. They only require a certain regularity, which the system operator has to specify in the QA documentation. In this context, the German Accreditation Body (DAkkS) has issued recommendations for the sequence of calibrations. In the case of pressure measuring devices, for example, the intervals range from once a year (e.g. for pressure transducers with an electrical output and an accuracy of 0.5% of the measuring span) to every five years (pressure balance / piston manometer). In short, the calibration itself means comparing the display of a device under test with the measured value of a reference device, which in turn has been tested by a national or international standard. In Germany, the Physikalisch-Technische Bundesanstalt maintains the national standard to which all calibration results can be traced back via a comparison chain.

A change has taken place in the verification of traceability. In the past, the 3.1 calibration certificate was also used as confirmation, similar to the DKD / DAkkS calibration certificate. However, creating it requires significantly more effort, both in terms of measurement (more points and rows) and in terms of documentation. In addition, this certificate may only be issued by appropriately accredited calibration laboratories whose competence is continuously monitored by DAkkS. The DAkkS certificate is now made a condition for most QS audits. In this context, the DAkkS has specified the requirements once again: Work certificates from accredited laboratories are no longer accepted as evidence of return. According to the new regulation, they are only classified as a "result report without an accreditation symbol", ie as a simple proof of function - in contrast to the DAkkS certificates, which are now officially referred to as "calibration certificates with accreditation symbols".

Multipurpose devices are increasingly being used for calibration. In the picture: portable calibrator for pressure, temperature, current, voltage and ambient conditions. (Image: WIKA)

Chain of comparative measurements up to the national standard

The principle of traceability remains unaffected by the change in title. In practice this means: The chain of comparative measurements up to the national standard must not be interrupted. The measurement uncertainty must be known for every link in the chain in order to be able to calculate the total uncertainty of the test item. As a rule, higher-level measuring devices should have a measuring accuracy three to four times as high. All steps in the comparison chain including the measurement results must be documented.

It is true that plant operators are increasingly asking for DAkkS calibration certificates, primarily because of the QS requirements. But not only: The certificate is globally recognized on the basis of agreements between the European co-operation for Accreditation (EA), to which the DAkkS is affiliated, and the International Laboratory Accreditation Cooperation (ILAC).

In view of the metrological and documentary effort required for a DAkkS certificate, only comparatively few companies can and want to afford their own accredited calibration laboratory; An exception are, for example, large production facilities in the chemical industry with thousands and thousands of measuring points. Most companies use an external service provider to check their measuring devices. Manufacturers of measurement technology such as Wika also have supplier-independent calibration laboratories. These facilities can be accredited for several measured variables at the same time.

Test items are usually sent in. Until they are back and reinstalled after the inspection in the laboratory, the process must rest or be equipped with appropriate replacement devices. Companies strive to keep downtime as short as possible. These phases can be reduced by requiring a mobile calibration laboratory. The test items are calibrated on site, depending on the task, in the system or in the laboratory vehicle. This solution eliminates the logistical effort for the client.

Profitability through automated processes

The demand for qualified and at the same time economical calibration solutions is also increasing regardless of the requirements of the QA systems. In order to be able to operate plants efficiently and safely at the same time, processes are increasingly automated, especially for sensitive and critical applications. Against this background, there is a trend towards multifunctionality in the development of calibration devices. One example of this is a new generation of pressure controllers such as the CPC series from Wika. These devices with an accuracy of up to 0.008% are designed both for applications in production, for example in the mass production of sensors, and for calibration tasks in the laboratory. The pressure ranges, control times, sensor changes and accuracy are designed for maximum flexibility. Two models of the CPC series also have a swiveling front for a quick change of the test sensors and thus enable efficient recalibration. Calibration software for the automatic generation of certificates rounds off the range of services.

The trend towards the multi-purpose device is also manifested in a new portable calibrator. The CPH7000 measures pressure, temperature, current, voltage and ambient conditions. Including an integrated hand pump for the test pressure, it enables complete calibration in the field with just one device. All measured values ​​can also be transferred to an end device via wireless function, for example for documentation. When it comes to operation, the new generation of devices benefits from the findings of smartphone technology. All functions are controlled via initiative application fields on a touchscreen, which speeds up the calibration process and makes it safer at the same time. Incorrect calibrations can have costly consequences as well as neglect.

Conclusion: A new awareness of calibration has grown under the influence of QA systems and increasingly delicate process efficiency. The control of measuring equipment no longer plays a minor role in the structuring of industrial processes. This is reflected, among other things, in the development of new, above all multifunctional calibration devices, the concept of which is based on the triad of quality, safety and economy. The checking of measuring devices is already largely taken over by external service providers. Since calibration processes also have to be designed economically, the demand for further supporting services will also increase.

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