Humidity Calibration Systems for National Standards and Industry

The validity of measurements made by industrial sensors and instruments is ensured by regular calibration, providing traceability through to a National Standard. In other words, a field sensor must be calibrated against a reference system that itself has an un-broken chain of calibration through to some primary system, such as operated by National Standards laboratories like NPL in the UK, NIST in the USA and PTB in Germany. Alternatively, large independent companies may operate their own primary standards in certain measurement fields such as electrical parameters, pressure etc.

Therefore there is a need for high quality, reliable and repeatable calibration reference equipment to effect the link between an industrial sensor and the primary standard. In the field of humidity calibration, Michell Instruments has developed a variety of calibration systems that are suitable for use as a stand-alone facility to provide a high performance level and act as a transfer-standard between a primary humidity standard and the field sensor, or some other reference hygrometer.

Michell has developed an expertise in providing such humidity calibration equipment to cover the full humidity range, from trace moisture measurement at ppm levels through to relative humidity measurement at elevated temperatures. Each system is built to customers' specific requirements. Normally a calibration system will comprise an integrated compressor/dryer to produce a zero gas ( no moisture) feed into a humidity generator and sensor housing that are designed for the user's specific needs. The final element of calibration system is always a precision cooled mirror reference hygrometer that provides direct traceability to a primary humidity standard.

A good example is at Justervesenet, the Norwegian standards institute in Oslo. In the absence of a primary standard within Norway, this institute decided to use two humidity calibration systems supplied by Michell Instruments, one covering the range -90 to +20 oC dew point and the other for the high temperature/high humidity range up to +80 °C dew point. The S4000 reference hygrometers used in each system are directly calibrated at NPL. These systems provide Justervesenet the capability to calibrate other reference hygrometers or field humidity sensors with direct traceability to a primary standard, giving the end user added confidence in humidity measurement.

Find out more about the calibration products and services we offer.

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