HR-33T Dew Point Microvoltmeter Instruction/Service Manual ...

4.5 – Effect of Ambient TemperatureThermocouple output as a function of water potential and ambient temperature is easily obtained for the HR-33T.Thermocouple output is simply the thermocouple sensitivity, P ab multiplied by the dew point depression, )T.When the dew point depression is small (as is the case for water potential measurement), )T = )e/S, where )eis the difference between saturation and chamber vapor pressure, and S is the slope of the vapor pressuretemperaturefunction at ambient temperature. The Clausius-Clapeyron equation gives S as a function oftemperature, saturation vapor pressure, (e 0 ), and latent heat of vaporization (8):S =e 0 8RT 2(11)where R is the universal gas constant. A relationship between )e and water potential (8) is obtained using theexpression,e = e 0 expRRT(12)Thus,)e = e 0 – e = e 0 (1 – expRRT) (13)and )E = P ab )T = P ab )e/S =P ab RT 2 (1 – exp R/RT) (14)8Equation (14) is the desired relationship between thermocouple output and water potential. At high waterpotentials where the exponential can be approximated by the first two terms of a series, equation (14) becomesE =-P ab TR8(15)indicating a linear relationship between E and 8 with a sensitivity of about &0.75 µvolts bar -1 . Figure 4 showsthe agreement between theoretical and actual calibration curves. Values of constants used were T = 298°K, R =4.60 bars °K -1 , 8 = 24330 bars and P ab = 63 µvolts °K -1 .22