18 1 — or 



16 - 



14 



2 12 



is io 



8 



-.o 



fc> ^ ceramic cup 



-. ^ screen 



-A 



k *v- «, 



^•'jlji ■/■■Vjfli<<iii>t ■■■■■■■■■■ ■)(■ ii •■■■>i -.on......... . . . • • rri HUtyHUttMH i n i rr <• w>W WViV/^Vi Vm Vi Vi'iViV " 



bare psychrometer 



20 40 60 80 100 120 140 160 180 200 



Time, minutes 



Figure 9. --Lag in response of thermocouple psychrometers to vapor equilibrium over a 0.3 

 m KCl solution at 25°C. Note the equilibrium time required for the screen and ceramic 

 cup psychrometers . 



resistance to vapor transfer, but this appears to be negligible and may even be reduced 

 by use of a larger mesh screen. In standard KCl solutions, vapor equilibrium for the 

 three psychrometers was achieved somewhat sooner in solutions having a lower water 

 potential because of the slightly reduced water potential gradient between the solutions 

 and the psychrometers. However, the relative magnitudes of lag, as shown in figure 9 

 for 0.3 m KCl, remained about the same between the screen and ceramic cup psychrometers 

 for different water potential gradients. 



A more meaningful comparison of the relative responses of the screen and ceramic 

 cup psychrometers to changing water potential was conducted in soil under severe evapo- 

 ration conditions. Screen and ceramic psychrometers were taped together so that the 

 sensors remained exposed, yet were adjacent to each other. These were buried in the 

 soil of pots containing month-old corn plants that were rapidly transpiring. A copper- 

 constantan thermocouple was buried with each pair of sensors to measure soil temperature. 

 The pots were placed in an environmental growth chamber with a programed environment 

 for air temperature at 25°C, 0.5 cal. cm.- 2 min.- 1 radiation (0.3 to 3.0 y) , windspeed 

 at 45 cm. sec.- 1 , and a vapor pressure deficit of 19 mm. Hg. The soil was brought to 

 saturation (\p = bars) initially, and then allowed to dry until severe wilting occur- 

 red 3 days later. During this period the psychrometers were read every few hours; 

 the assumption was made that the soil water potential adjacent to the pair of psychrom- 

 eters (a screen and a ceramic unit taped together) would be the same for both psy- 

 chrometers. Outputs were converted to 25°C. using equation (5) above. 



17 



