Automatic Multipoint Field Calibration Method for RH

New Whitepaper: (Cleanrooms & Chambers Spotlight) By-Lars Stormbom, Vaisala Oyj, Vantaa, Finland


An automatic Relative Humidity (RH) calibration method suitable for field use is described. In this method a humidity source producing a known water vapor pressure is used.The device to be calibrated is placed in a closed chamber containing this humidity source. An automatic method using only one known vapor pressure is demonstrated to be a viable alternative to multipoint field calibration.

In many applications it is necessary to frequently check and adjust relative humidity instruments in the field. One point calibrations can be easily performed using a reference instrument.

But checking/calibration at a minimum of two different RH levels is necessary to correct for both offset and gain type errors.

There are several solutions available for this need, ranging from simple but inherently manual methods using salt solutions to portable humidity generators.

Manual methods using saturated or unsaturated salt solutions require low investments, but require the presence of an operator during the whole procedure to i.e. move the probe from chamber to chamber.

Portable humidity generators are more expensive and may require facilities (for instance pressurized air) that are not readily available.

Topics covered in this white paper:

1. The effect of humidity sensor response time
2. Heating methods and water vapor source
3. Laboratory & field salt calibrators
4. Two pressure systems
5. Comparison of results in lab conditions and office conditions
6. Temperature dependence of the instruments
7. Error sources

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