An Accurate Temperature Correction Model for Thermocouple Hygrometers

Abstract

Numerous water relation studies have used thermocouple hygrometers routinely. The accurate temperature correction of hygrometer calibration curve slopes seems to have been largely neglected in psychrometric and dewpoint techniques. In the case of thermocouple psychrometers, 2 temperature correction models are proposed, each based on measurement of the thermojunction radius and calculation of the theoretical voltage sensitivity to changes in water potential. The 1st model relies on calibration at a single temperature and the 2nd at 2 temperatures. Both these models were more accurate than the temperature correction models currently in use for 4 psychrometers calibrated over a range of temperatures (15.degree.-38.degree. C). The model based on calibration at 2-temperatures is superior to that based on only 1 calibration. The model proposed for dewpoint hygrometers is similar to that for psychrometers. It is based on the theoretical voltage sensitivity to changes in water potential. Comparison with empirical data from 3 dewpoint hygrometers calibrated at 4 different temperatures indicates that these instruments need only be calibrated at, e.g. 25.degree. C, if the calibration slopes are corrected for temperature.