Innovations on a Quartz Crystal Micronbalance Frost-Point Hygrometer

Abstract The authors describe a hygrometer based on a quartz crystal (QC) microbalance. The hygrometer is based on frost accumulation rather than adsorption. Previous designs are improved in two ways. First, a radial flow geometry makes it possible to quantitatively remove water from an airstream and therefore measure water vapor by measuring the rate of frost accumulation as well as by measuring the frost point. Second, a thin-film copper-constantan electrode on the QC measures the temperature of the front surface of the QC while still allowing measurement of the oscillation frequency. The hygrometer design is appropriate for frost points between about −1 00° and −40°C, and a future implementation could be used on a lightweight balloon or unmanned aerial vehicle payload.