Abstract
In order to precisely measure insensible perspiration, low drift, precision near-infrared hygrometer is designed. Basically, this hygrometer comprises two light beams (both absorbing band 2.6μm) and two PbS photocell detectors kept at constant temperature by an electronic control unit. Those detertors are connected in parallel and used as one of the branches of detecting bridge, since this reduces the resistance of the branch, thus increasing the sensitivity of the bridge. The intensity of light sources is decreased as lower as possible to keep detectors from decay, and to decrease the drift of the instrument. The method to increase the sensitivity of the bridge circuit with very high resistance is discussed. In order to decrease in 0-point drift due to slight change in temperature of detectors, a compensation method is also discussed that a thermistor to detect the temperature of the photocell is used as a reference of the bridge.
As a result of those countermeasures, the drift of the instrument decreases within ±0.01μA/h and the sensitivity is approximately 10 times more than that of one beam, one detector type near-infrared hygrometer.
The apparatus for measuring insensible perspiration using this hygrometer is sensitive to perspiration levels as low as 0.05mg/10cm2·10min. The accuracy of this apparatus better than 2% is obtained.
