A completely automated frost-dew point hygrometer with ultra fast response capability includes the hygrometer, refrigerant circulation system, integral two-stage low-temperature (-100°t o -125°F ) refrigerator, a cascaded controller of unconventional design, as well as ancillary automata. The continuous operating and recording range of the instrument is -100°to +200°F. The unique sensing element of the hygrometer combines a thin Invar buffer layer and a hemispherical silver thermal reservoir to permit reduction of the mass of the temperature-tracking element. A state of the art design is characterized by a first-order transfer function and time constant of 1.3 seconds. The internal control loop of the cascade control is temperature-activated and its design is based on final-value control principles. The outer loop is activated by a photoelectric deposit thickness monitor. The system tracks through the32°F transition temperature very smoothly without any indication of upset, instability, or tendency to hunt. The hygrometer and cascaded controller are completely compensated for nonlinearity to produce uniform response over the entire instrument range and the response speed is so great that thermodynamic equilibrium between
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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