NIST Calibration confirms Michell's dewpoint excellence

A recent calibration performed at the National Institute for Standards and Technology, Washington DC, USA, has shown that the S4000 Precision Dewpointmeter from Michell Instruments gives extremely high levels of accuracy across a wide range of dewpoints, from -75 to +20 ºC .

The National Institute for Standards and Technology in Washington DC and London's National Physical Laboratory are premier organisations World-wide in the field of fundamental scientific measurements. Each organisation holds it's own country's highest level, fundamental national standard for humidity measurement.

As part of an on-going programme to maintain direct traceability of measurements to international standards, Michell Instruments has a number of cooled mirror dewpointmeters calibrated each year by both NIST and NPL, which are then used either in Michell's own laboratory or in that of their American distributor, Kahn Instruments. Traceability of calibration for the thousands of industrial humidity sensors produced each year is thus maintained.

Michell has maintained continuous traceabiltiy to NIST since 1982, when the first Michell cooled mirror dewpointmeter was built under contract to the EEC Bureau of Reference, and to NPL since the introduction of their National Humidity Standard in London in 1985.

The chart shown following illustrates the level of accuracy achievable with a standard, commercially available S4000 dewpointmeter. Data is extracted from the NIST Calibration Reports dated 21st March, 1997 and 9th March, 1999 and relates to S4000 serial number 101028/29, which was built in 1995.

Not only do the results show excellent accuracy for a four year old instrument, with a maximum error in the 1999 calibration of 0.07 ºC dewpoint, they also show superb repeatability over a two year period. This confirms the long-term stability of the S4000 even when in continuous use and regularly transported by commercial freight carrier across the Atlantic Ocean.

The figure below illustrates the traceability path from NPL and NIST to end user instruments and includes the more direct route to Standards offered by Michell's UKAS (NAMAS) calibration facility.

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