Figure 3. — Cross section 

 of the ceramic bulb 

 thermocouple psychro- 

 meter proposed by 

 Rawlins and Dalton 

 (1967). 



ACRYLIC TUBING 

 COPPER LEAD WIRES 

 EPOXY RESIN 



COPPER HEAT SINKS 



TEFLON INSERT 

 EPOXY RESIN 



CHROMEL-CONSTANTAN 

 THERMOCOUPLE 



CERAMIC BULB 



psychrometer encased in a wire gauze cage of 100-mesh stainless steel for measurements 

 of soil water potential. The wire cage provides a protective covering for the thermo- 

 couple, and thus would be superior to the model used by Hoffman and Splinter (1968a). 

 However, Lang's psychrometer was especially constructed to be inserted into the wire 

 cage sometime after the cage was buried in the soil. This approach may lead to drying 

 of the soil immediately adjacent to the wire cage, and it may also offer a considerable 

 degree of difficulty in properly inserting the exposed psychrometer into an access hole 

 in the soil . 



The most important recent development for in situ measurements of leaf water 

 potential is the double junction thermocouple psychrometer (Wiebe 1970). This instru- 

 ment consists of two chromel-constantan thermocouple junctions and three reference 

 junctions attached within a Teflon insert. Three copper leads are joined within the 

 Teflon insert by the two thermocouple junctions; the chromel wires of both junctions 

 are attached to two of the copper leads, and the constantan wires of both junctions 

 are attached to the third copper lead. The two sensing junctions produce opposing 

 e.m.f.'s so that the cooling current is passed through one junction and the output is 

 read from the other. The output is then proportional to the temperature difference 

 between both sensing junctions, and since both are in the same thermal environment, 

 one junction compensates for rapid temperature changes which may easily occur on an 

 attached leaf surface. This entire assembly can be placed in a Teflon cup open at the 

 bottom end, and mounted on a leaf surface similar to the method of Hoffman and Splinter 

 (1968a, and 1968b). 



9 



