Vacuum encapsulated resonators for humidity measurement

This study uses surface resistance of silicon dioxide and a charge-biased, microshell encapsulated, single-anchored double-ended tuning fork resonator (DETF) to measure relative humidity (RH). As relative humidity increases, the surface resistance of silicon dioxide decreases due to increasing amounts of water adsorbed on the surface. This decrease in surface resistance leads to a faster charge decay from a capacitor. The DETF resonator converts the amount of charge on the capacitor to a frequency shift, via the electrostatic spring softening term. This frequency shift can be characterized by either exponential fitting or slope fitting. The hysteresis was shown to be less than 0.15% RH. The relative humidity is determined from the slope or the decay time constant, with the precision of 1.3 × 10−3% RH for slope fitting and 1.3 × 10−4% RH for exponential fitting.