24 1 1 1 1 1 1 r 



Figure 8. — Calibration curve for a chromel -const ant an thermocouple psychrom- 

 eter (output in yu vs. osmotic potential of KCl solutions in bars) at 25°C. 



Psychrometer Response to Changing Water Potentials 



If the screen psychrometer is to give accurate estimates of water potential, it 

 should offer a negligible resistance to vapor and heat flux. To determine the magni- 

 tude of psychrometer lag in response to actual water potential, laboratory experiments 

 were conducted in which the response of the screen psychrometers , ceramic cup psychrom- 

 eters , and bare unshielded psychrometers were compared. The ceramic cup psychrometers 

 used were of the same design as shown in figure 4 (Wiebe and others 1970), and the un- 

 shielded psychrometers were constructed as described above, except that the screen cup 

 was not used. The exposed thermocouple was assumed to offer no resistance to vapor 

 transfer, hence to yield accurate estimates of water potential. 



In an attempt to determine the relative magnitude of the resistance imposed by 

 various psychrometers to the transfer of water vapor, psychrometer response to various 

 equilibrium vapor pressures was measured under isothermal conditions. The bare 

 unshielded, screen , and ceramic cup psychrometers were sealed in test tubes containing 

 standard KC £ solutions of known vapor pressures at 25°C. Summarized in figure 9 are 

 the relative magnitudes of psychrometer response with time, and the length of time 

 required for vapor equilibrium in a 0.3 molal (m) KC£ solution. Time in minutes began 

 when the psychrometers were placed in the water bath ; temperature equilibrium as measured 

 by thermocouple output was achieved within 20 minutes for all the psychrometers. How- 

 ever, vapor pressure equilibrium required a longer period of time for both the screen 

 and ceramic cup psychrometers. The unshielded psychrometers reached vapor equilibrium 

 at about the same time that temperature equilibrium was reached (20 minutes). The 

 screen psychrometers reached vapor equilibrium some 13 minutes later, but the ceramic 

 cup psychrometers did not reach equilibrium until about 2-1/2 hours later. 



From the data in figure 9 it is quite apparent that the ceramic cup psychrometers 

 offer a far greater resistance and lag to' vapor exchange than do the screen psychrom- 

 eters. It is interesting to observe that the screen does offer a relatively small 



16 



