Calibration Know-How
What's Calibration?
During the calibration in measurement technology calibrators detcet and document reliably and reproducibly how large the deviation of a measurement device to a normal is. Measuring control, calibration and traceability is a central topic of most quality assurance systems.
Why should you calibrate?
In modern production processes, the need for precision monitoring of the ambient conditions during the production process, and to regulate these within a specific range where necessary, is becoming increasingly common. Sectors typically having this requirement include semi-conductor production or manufacturing in the food and pharmaceutical industry. With industrial production processes as well (e.g. drying processes), it is often necessary to monitor the ambient conditions during the process with precision. "Ambient conditions" generally used to refer to monitoring and regulating the ambient temperature, but increasingly also relates to the ambient or process humidity (e.g. relative humidity). Where values such as the temperature or relative air humidity ffect production processes (otherwise, monitoring would simply be a luxury), the need for periodic, documented calibration of the relevant measuring equipment as part of a measuring equipment monitoring system is immediately apparent. To calibrate temperature measurement accuracy, there is an established procedure and a variety of devices on the market (temperature calibrators) that can be used for calibrating temperature measuring equipment.
The situation for calibrating measurement values for "relative humidity" or gas humidity in general is rather different. On the one hand, the potential measurement uncertainties are considerably greater than would intuitively be expected in comparison to temperature measurement uncertainties, and on the other hand, the options for humidity calibration are much more important in terms of usage or they often do not provide the anticipated level of accuracy.
Calibration Procedure
Upon receipt of the measuring equipment an initial calibration is carried out in accordance with customer‘s specifications. The measured values are documented on a certificate.
If requested, the measuring results will be compared with the specified tolerances of the measuring equipment as part of the initial calibration. If the recorded values for the measuring equipment fall outside the permissible tolerances, the customer may use the information to validate the results of previous quality tests and eventually initiate appropriate corrective action.
At the request of the customer, a quotation will be issued for the cost of repair, respectively the adjustments. If E+E is not able to repair the equipment, it will be either returned to the customer or shipped directly to the manufacturer. Once the equipment has been repaired and adjusted, the measuring values will be documented during the second calibration routine.
Separate certificates are issued for the first and second calibration confirming the measured values ‚as received‘ and ‚as delivered‘.
What's Tracebility?
The term "traceability" describes a process through which a measurement taken by a measuring instrument can be compared, in one or more steps, with a national standard for the physical value.
These steps must form an uninterrupted chain of calibrations. In each step, a measuring instrument is compared with a standard whosemeasurement features themselves have been determined by comparison with a higher ranking standard. This process creates a calibration hierarchy or a hierarchy of the testing equipment. The bodies performing the comparisons within the chain must also have their technical expertise verified, e.g. through accreditation by a calibration authority.
Calibrations performed by other bodies not accredited as calibration authorities are not considered traceable in line with EN ISO 9001 or EN ISO/IEC 17025 (even if they hold certification in accordance with this standard),as in this case, the body's technical expertise is not proven.
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What Does Measurement Uncertainty Mean?
The measurement uncertainty is a feature of an accredited procedure. It describes to what extent a measurement can be traced to national standards and ultimately to International System of Units (SI units). Usually, an extended measuring uncertainty is specified, which is calculated from the standard uncertainty multiplied with the extension factor k=2.
Each measuring value is associated with a (measurement) uncertainty. In the calibration hierarchy, testing equipment on a higher level has a lower measuring uncertainty that that on a lower level.
Accreditation in line with EN ISO/IEC 17025 requires that measurement uncertainties are calculated according to EA-4/02 „Guide to Expression of the Uncertainty of Measurement in Calibration, GUM".