Moisture sensitive element

A moisture sensitive element for detecting humidity variations through sensing variations in electric resistance using a porous metal oxide ceramic. At least either phorphorus or sulfur is incorporated in and supported by the porous metal oxide ceramic with the phosphorus or sulfur being applied in either a pure or oxide form.

BACKGROUND OF THE INVENTION 
The present invention relates to a moisture sensitive element of the type 
which detects variations in humidity with electric resistance. 
Moisture sensitive elements which detect variations in humidity by sensing 
variations of electric signals are used as moisture sensors in automatic 
humidity control apparatus such as room air conditioners. 
One known type of moisture sensor is described, for example, in Japanese 
Pat. Publications No. 31836/79 and No. 49705/80. It is comprised of an 
insulative substrate on which a pair of electrodes are disposed, and 
moisture located between the electrodes will be detected 
This sensor, however, cannot detect delicate variations of humidity in the 
surrounding atmosphere. 
Another type of prior art moisture sensor comprises a porous metal oxide 
ceramic plate and a pair of electrodes formed on the same surface or on 
opposite surface of the ceramic plate. This type of the sensor relies on 
the excellent hygroscopicity generally exhibited by the metal oxide 
ceramic. The metal oxide ceramic exhibits significant variation in 
electric resistance in response to the variation of humidity in the 
surrounding atmosphere. It is supposed that the variation in electric 
resistance of the metal oxide ceramic is caused by the movement of protons 
in the H.sub.2 O molecule adsorbed on the surface and on the wall of 
porosity of the metal oxide ceramic. 
However, the humidity-resistance characteristics of this moisture sensor 
change as time passes because of the adsorption of contaminants such as 
oil mist, powdered dust, miscellaneous gases and so on onto and into the 
ceramic as well as the chemisorption occurring between the metal oxide 
ceramic and part of H.sub.2 O molecule adsorbed thereon. In the worst case 
the moisture sensor would not even exhibit humidity-resistance 
characteristics. 
To eliminate the above-mentioned defect, a method which heats the ceramic 
at a high temperature so as to desorb the oil mist, powdered dust, 
miscellaneous gases and H.sub.2 O molecule from the ceramics was employed. 
In this method, however, the effect of heating is not enough when the 
temperature of heat is less than 400.degree. C. Therefore, a great deal of 
electric power is required to effectively heat the moisture sensor and it 
is not efficient to apply this method to general electronic control 
circuits. Further, the material disposed around or on which the moisture 
sensor is mounted must be comprised of an incombustible material. Still 
further, the moisture sensitive element has to be heated every 30-60 
minutes so as to desorb the contaminants. Further, since the moisture 
sensor cannot be operated during the heating and cooling process, it 
cannot monitor the humidity continuously if heating is used to remove 
contaminants. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a moisture 
sensitive element in which the humidity-resistance characteristics do not 
significantly change as time passes. 
In accordance with the preferred embodiment of this invention, the moisture 
sensitive element includes an improved moisture sensing element which 
exhibits significantly better and more consistent response in electric 
resistance variations in response to variations in relative humidity. A 
predetermined amount of phosphorus or sulfur is incorporated into the 
porous metal oxide ceramic and adhered thereto and is thus supported by 
the ceramic material. The phosphorus or sulfur can be applied either in a 
pure form or an oxide form. Further, the moisture sensitive element made 
according to this composition exhibits stable characteristics even over a 
relatively long lapse of time.