Measuring and testing – Gas analysis – By thermal property
Patent
1996-08-23
1998-11-17
Williams, Hezron E.
Measuring and testing
Gas analysis
By thermal property
733352, G01N 2556
Patent
active
058378847
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a humidity sensor for detecting the quantity of water vapor in an atmosphere for use in an air conditioner, a dehumidifier, a cooker, a cultivation house, and so on.
BACKGROUND ART
Recently, there has been an increasing demand for detecting and controlling the humidity such as the relative humidity and the absolute humidity in an air conditioner, a dehumidifier, a humidifier, a cooker, a cultivation house, and so on. To meet the demand, various types of humidity sensors have been proposed.
Conventional humidity sensors include an electrical resistance type or a capacitance type which utilizes the change in electric characteristic depending on moisture absorption of a humidity sensing material, a heat conduction type which detects the change in heat conductivity of the air depending on presence or absence of water vapor in the air, and the like. The heat conduction type is excellent in long-term stability because of no moisture absorption.
As shown in FIG. 1, a conventional humidity sensor comprises a Wheatstone bridge circuit formed by a temperature sensing resistor 31 having a resistance value R.sub.4H, a temperature sensing resistor 32 having a resistance value R.sub.4T, and fixed resistors R.sub.41, R.sub.42, R.sub.43, and R.sub.4S, and measures the humidity by utilizing the fact that the heat radiation of the temperature sensing resistors 31 and 32 varies depending upon the humidity. It is noted here that R.sub.4S is not necessary in case where a temperature sensing resistor, such as a platinum resistor, having a positive temperature characteristic is used as each fixed resistor. The temperature-resistance characteristics of R.sub.4T and R.sub.4H must be identical. The resistance values of R.sub.41 and R.sub.42 must be equal, too.
In the above-mentioned humidity sensor, the temperature sensing resistor 31 is exposed in the outside air while the temperature sensing resistor 32 is sealed in a dry atmosphere. In this state, a voltage V.sub.4IN applied to the temperature sensing resistors 31 and 32 makes a current flow through the temperature sensing resistors 31 and 32 which then generate the Joule heat to have temperatures higher than the ambient temperature. The temperatures of the temperature sensing resistors 31 and 32 are determined by electric power applied to the temperature sensing resistors 31 and 32 and heat radiation of the temperature sensing resistors 31 and 32, respectively.
When water vapor is contained in the outside air, the heat radiation becomes large under the influence of the heat conduction of the water vapor, as compared with the case where no water vapor is contained in the outside air. Therefore, the temperature of the temperature sensing resistor 31 becomes lower than that of the temperature sensing resistor 32. As a result, a potential difference V.sub.4OUT is produced across the fixed resistor R.sub.43. By the use of this phenomenon, it is possible to detect the absolute humidity in the air.
The conventional humidity sensor of a heat conduction type has a structure illustrated in an exploded perspective view of FIG. 2 and a perspective view of FIG. 3. Referring to FIGS. 2 and 3, each of the temperature sensing resistors 31 and 32 comprises a platinum thin film formed on an alumina substrate. In place of the platinum thin film, the temperature sensing resistors 31 and 32 may be made of any other material having a resistance value which vary following the temperature change.
The conventional humidity sensor of a heat conduction type is manufactured as follows. As shown in FIG. 2 and FIG. 3, the temperature sensing resistors 31 and 32 are fixed on different stems 34 through supports 314 by means of bonding with an adhesive or welding. Thereafter, connection of terminals is carried out by wire bonding. The stem 34 with the temperature sensing resistor 31 fixed thereto is covered with a cap 33a having ventilation holes 35 by welding. As the adhesive, an inorganic or organic adhesive is selectively used depending on the temperature
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Kimura Mitsuteru
Sato Hiroyuki
Takeda Mituyuki
Kimura Mitsuteru
Politzer Jay L.
Tokin Corporation
Williams Hezron E.
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