Humidity sensor with differential thermal detection and method o

Details Humidity sensor with differential thermal detection and method o Humidity sensor with differential thermal detection and method o

1998-01-12

2000-06-13

Williams, Hezron

Measuring and testing

Gas analysis

Moisture content or vapor pressure

7333505, G01N 1910

Patent

active

060734809

ABSTRACT:
A method of moisture measurement wherein a moisture responsive element is subjected to the measuring environment and its thermal capacitance or resistance temperature coefficient is measured. A system operates by cycling a sensor element between a first temperature and a second temperature and determining the sensor capacitance at each temperature. The sensor capacitance difference, corresponding to its temperature coefficient, is then compared to a previously compiled table of gas humidity or dew point versus capacitance increment values. The differential measurement thus made automatically corrects for systemic error originating in equipment drift, cable capacitance change and various aging and slow hysteresis or sensor capacitance variations. In one embodiment, a capacitive type relative humidity sensor is placed in thermal contact with an electric heater, a thermoelectric cooler, or both, and the heater and/or cooler are operated to drive the sensing element from a first temperature to a second temperature, under control of a microprocessor. Depending on the ambient temperature, the device may return to its initial temperature passively, or be actively driven by the heater or cooler, respectively. The frequency or cycle time of these temperature changes is selected to correspond to the time constant of the sensor, which in turn, may be a function of the relative humidity. In illustrative embodiments, a platinum film may be employed as the heater and a Peltier device may be employed as the cooler, and these are driven to change the sensor temperature and effect readings at upper and lower temperatures with time constant or cycle time of approximately 1-3 minutes. Preferably, a fixed cycle time is employed, and capacitance is measured at the upper and lower temperatures of each cycle with the sensor immersed in the sample environment, until the capacitance difference measurement stabilizes. This difference is then compared to a previously compiled table of differences for various dew points. In another embodiment or mode of operation, the thermal source is operated in two cycles or at two different power levels to drive the sensor to a first temperature T.sub.1 away from ambient temperature, and to a second more divergent, temperature T.sub.2. The temperature then returns to T.sub.1 by passive interaction with the surroundings. By identifying the dew point curve by reference to capacitance difference between two fixed temperatures, errors and uncertainty at the high humidity/high temperature range are also reduced. In another embodiment, an occasional differential measurement is performed to detect errors in a set of stored curves and update the tables used for single-point humidity measurements, thus obviating the need for protocols involving reference gases or recalibration.

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