Principle of measuring humidity:
The oscillation frequency of a quartz crystal coated with hygroscopic material is a very sensitive detector of the weight of absorbed water because very small changes in frequency can be measured.
Working of Piezoelectric meter:
Two quartz crystal oscillators are used, alternating between moist and dry gas, generally for 30 seconds at a moment.
The frequency of crystal oscillation is about 9.106 Hz and that of the crystal exposed to the wet gas will be lowered and that of the crystal exposed to the dry gas will rise.
The resultant audio frequency difference is extracted, amplified, and converted to voltage to give a meter response of which the maximum value on each 30-second cycle is a measure of the moisture level.
The range of applicable concentrations is 1–3,000 vppm, and at
lower levels the fact that the value after a certain time is measured rather than an equilibrium value means that the instrument can have a more rapid response than alternative methods (sample lines, however, often determine response time).
Because the crystals see the sample gas for equal times, contamination of the two crystals should be similar and the frequency difference little affected, resulting in stability.