National Physical Laboratory

Relative Humidity: 1 %rh to 98 %rh at temperatures from -40 °C to +100 °C

Relative humidity

Relative humidity calibrations are offered at primary level (calibration directly against a national standard humidity generator) or at a secondary level (against a traceably calibrated transfer standard hygrometer).

Relative humidity results are derived from dew-point temperatures, together with air temperatures, allowing calculation of vapour pressures and hence values of relative humidity.

Temperatures are closely controlled, monitored and reported as part of the humidity calibration, since relative humidity is strongly dependent on temperature. As an additional service, instrument temperature sensors can be calibrated in air.

Calibration against an NPL primary or transfer standard

Range: 1 %rh to 98 %rh, at temperatures from -40 °C to +100 °C

Calibration against an NPL primary standard, uncertainties in applied condition:

Up to 80 °C:
Relative humidity uncertainty ±(0.5 % of reading + 0.1 %rh)

Calibration against an NPL transfer standard, uncertainties in applied condition:

Above 80 °C:
Relative humidity uncertainty ±(0.6 % of reading + 0.1 %rh)

Calibration against an NPL transfer standard, uncertainties in applied condition:

Relative humidity uncertainty ± 1.0 % of reading
Temperature uncertainty ±0.1 °C

Uncertainties stated on certificates are the combination of the values due to the applied condition together with a contribution from the hygrometer under test.

Note: A recommended general calibration could consist of 5 relative humidities in the range 10 %rh to 90 %rh at 20 °C and two additional temperatures (e.g. 15 °C and 25 °C) at 50 %rh. This calibration would give some information on the linearity and temperature dependence of the instrument under test.

NPL can advise on selecting the range and number of calibration values appropriate for a particular instrument and application.

Contact

Customer Service tel: +44 20 8943 8681
E-mail: humidity_enquiries@npl.co.uk

Last Updated: 1 Mar 2012
Created: 11 Sep 2007

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