35 I 1 1 1 1 r 



30 - 



Bars • screen psychrometers 



Figure 10. --Relationship of the response of the ceramic cup psychrometers to that of the 

 screen psychrometers under high evaporation conditions (r 2 = 0.9 S3). 



Potential Uses of Thermocouple Psychrometers 



The screen cup psychrometer discussed here is particularly well suited for measure- 

 ments of soil water potential. However, certain precautions must be observed because 

 the copper lead wires are excellent heat conductors, and thermal gradients within the 

 thermocouple may result if these are not damped out of the system. This damping can 

 easily be accomplished by burying a short portion of the lead wire (several inches are 

 usually enough) behind the thermocouple in the soil. For water potential measurements 

 near the soil surface, the psychrometer should be buried in a position parallel to the 

 surface so that a short length of the lead wire can also be buried for insulation 

 against thermal gradients. Because the psychrometer output varies with temperature, a 

 copper-constantan thermocouple (26 gage or smaller) should be attached to the psychrom- 

 eter within the epoxy resin just above the screen cup so that psychrometer temperature 

 at the time of reading can be determined. 



The screen cup psychrometer is also well adapted for measuring the water potential 

 gradients in stems of trees or large shrubs, using a technique similar to Wiebe and 

 others (1970). Under this method, the sensing unit of the psychrometer is placed in 

 carefully drilled holes to a depth of about 1 cm. or less inside the cambium layer. The 

 hole is then sealed with a waterproof compound to prevent evaporation and entrance of 

 disease. The psychrometer and a short segment of the lead wires should be insulated 

 with polyurethane foam or other insulating material to damp out temperature gradients. 

 The smallest diameter stems that have been studied with this size of a psychrometer is 

 2 cm. , but with miniaturization of the psychrometer assembly it should be possible to 

 work with much smaller stems. 



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