Figure 7. -A commonly used 

 microvoltmeter (Keithley 

 model 155) and a switch- 

 box used for measuring 

 psychrometer output. 



The yv output of the thermocouple for each KC£ standard solution can be plotted as 

 a function of water potential to give a calibration curve for each psychrometer (figure 

 8) . It can be seen from the curve in figure 8 that thermocouple output varies about 

 0.5 yv bar -1 on the lower portion of the curve (below -40 bars) but decreases at lower 

 water potentials. If the cooling time were extended beyond 15 seconds (to about 30 

 seconds), the linearity of the curve would be extended to lower water potentials (more 

 negative), and the usable range of the psychrometer would be expanded to near -100 bars. 

 However, even when using a 60-second cooling time there appears to be a lower limit 

 between -75 and -100 bars for these psychrometers . This lower limit for chromel- 

 constantan thermocouples results because the Peltier effect is a somewhat inefficient 

 cooling method, allowing a temperature depression of only about 0.6°C. (Rawlins 2 ). 



Recalibration of the psychrometer should be performed after each extended period 

 of use. Calibration points have been observed to change by ±1 yv over a period of 3 

 months, indicating that errors as great as 3 bars can result. The thermocouple junction 

 will age somewhat due to corrosion, and adherence of foreign matter to the junction will 

 also contribute to a change in the calibration. I have used several exceptionally well 

 made psychrometers here at Logan that showed no sign of calibration drift even after 

 3 months of use. However, after each use of the psychrometer it is strongly recommended 

 that a fairly vigorous cleaning routine be followed. For instance, I use the following 

 procedure for routine cleaning: 



1. Thoroughly wash the psychrometer in tap water to remove soil or other 

 particles from the screen cup. 



2. Then rinse in distilled water. 



3. Boil for 10 minutes in distilled water. 



4. Rinse in acetone. 



5. Rinse in warm distilled water, let dry. 



S.L. Rawlins, personal communication, 1970. 



15 



