To ensure the accuracy of humidity measurement instruments it is essential that the functioning of even a very stable instrument is checked against a reference from time to time. Vaisala's humidity calibrator HMK15 makes calibration and spot checking of humidity probes and transmitters easy and reliable. The operating principle of the HMK15 is based on the fact that a saturated salt solution generates a certain relative humidity in the air above it. The reading of the humidity probe or transmitter can then be adjusted accordingly. Usually two or three different salt solutions, which are chosen according to the application, are used.