Abstract
Developments in the field of sensing elements for frost point hygrometers are briefly reviewed. The rod type geometry connecting a refrigerant at one end with the mirror and heater at the other is analyzed mathematically to show the response and temperature distribution limitations of this geometry. A new sensing element is then described which consists of a hemispherical silver (or copper) thermal reservoir attached to a low mass mirror, heat distributor, and resistance heater via a thin layer of Invar. The latter confines most of the thermal gradient to itself due to its low thermal conductivity and optimum design of the hemisphere and refrigerant flow path. The new sensing element was simulated on a Pace 221R analog computer as well as being subjected to transient analysis which showed that the element responded as a first order lag of time constant 1.67 seconds. Actual construction and tests with step changes of input to the heater confirmed the first order response but with a time constant of 1.3 sec which corresponds to an average full scale response of 14.5 C°/sec with a maximum of 74 C°/sec. Subsequent tracking tests revealed smooth response particularly through 0°C where the ice-water transition takes place.
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