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Capacitance Humidity Sensor

Published online by Cambridge University Press:  15 February 2011

Andrew R.K. Ralston,
Paul E. Thoma,
Carl F. Klein and
Denice D. Denton
Andrew R.K. Ralston
Affiliation:
University of Wisconsin-Madison, Materials Science Program, Department of Electrical and Computer Engineering and Department of Chemistry, 1415 Johnson Drive, Madison, WI 53706
Paul E. Thoma
Affiliation:
Johnson Controls, Inc., Central Research, P.O. Box 591, Milwaukee, WI 53201-0591
Carl F. Klein
Affiliation:
Johnson Controls, Inc., Central Research, P.O. Box 591, Milwaukee, WI 53201-0591
Denice D. Denton
Affiliation:
University of Wisconsin-Madison, Materials Science Program, Department of Electrical and Computer Engineering and Department of Chemistry, 1415 Johnson Drive, Madison, WI 53706
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Abstract

A capacitance relative humidity (RH) sensor is described that has a design, construction, and material composition that result in an inexpensive and robust sensor. This sensor has a multilayer, free-standing film construction. It consists of a humidity sensitive polyimide (PI) dielectric core and conductive layers consisting of carbon filled polysulfone on each side of the polyirnide film to form a capacitor. The polyimide used is a BPDA-ODA type, and replaces a PMDA-ODA type polyimide used in a previous version of this sensor. The BPDA-ODA sensor has a nominal capacitance of 200 pF and a nominal sensitivity of 13% at 100% RH. The characteristics of this humidity sensor are discussed and compared to the characteristics of the PMDA-ODA type sensor. Characteristics considered include the PI film moisture uptake and water vapor transmission, and the sensors' sensitivity to relative humidity, frequency response, and aging at 85°C/85% RH. The dual-state sorption model and free volume calculations are used to demonstrate that observed differences in the film are due to differences in chemical composition between the films.

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References

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