The piezoelectric sorption instrument compares the changes in frequency of hydroscopically coated quartz oscillators. As the mass of the crystal changes due to adsorption of water vapor, the frequency of the oscillator changes. The sensor is a relative measurement, so an integrated calibration system with desiccant dryers, permeations tubes and sample line switching is used to correlate the system on a frequent basis.
The system has success in some applications. However, in natural gas, interference from glycol, methanol, and damage from hydrogen sulfide result in erratic readings. The sensor itself is relatively inexpensive and very precise. The required calibration system is not as precise and adds to the cost and mechanical complexity of the system. The labor for frequent replacement of desiccant dryers, permeation components, and the sensor heads greatly increase the operational costs. Additionally, slugs of water render the system nonfunctional for long periods of time as the sensor head has to “dry-down.”