Figure 5. — The screen 

 thermocouple psy- 

 chrometer. 



INSULATED LEAD WIRE 

 EPOXY RESIN 



COPPER LEAD WIRES 

 TEFLON INSERT 



CHROMEL-CONSTANTAN 

 THERMOCOUPLE WIRES 



SCREEN CAGE 



criteria could be accomplished without imposing a resistance to vapor exchange, at least 

 the accuracy of the instrument would be improved; this improvement would result from 

 reduction of a lag in response to a change in water potential where water transfer is in 

 the vapor phase. Even though a rigid ceramic cup imposes a substantial lag to vapor 

 exchange, a fine mesh noncorrosive screen cup appears to meet the essential criteria 

 quite well, and permits heat and vapor exchange with negligible resistance. These 

 features were incorporated into a psychrometer built by the author, quite independent of 

 Lang's (1968) instrument, and considerably simpler in design and of greater versatility. 

 This psychrometer somewhat resembles the instrument used by Wiebe and others (1970) , 

 except that the ceramic cup was replaced with a screen cup. A large number of these 

 psychrometers that were used over the last 2 years in the laboratory at Logan for mea- 

 surements of soil water potential have proven this design to be stable and reliable. 



Construction and Equipment 



The essential parts of the pyschrometer (figure 5) consist of a screen cup con- 

 structed of fine-mesh stainless-steel wire and attached to a Teflon insert. Within this 

 insert the copper lead wires are attached to the thermocouple. After cutting the Teflon 

 insert (see Appendix 4 for sources of supply) to the proper length (0.475 cm. diameter 

 rod cut to 0.635 cm.), make two holes approximately 0.025 cm. in diameter through the 

 long axis of the Teflon with a fine dissecting needle. Then insert the chromel 



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