Figure 4. — Cross section 

 of the oeramio cup 

 thermocouple psy chro- 

 me ter used by Wiebe 

 and others (1970). 



NSULATED LEAD WIRE 



EPOXY RESIN 



COPPER LEAD WIRES 



TEFLON INSERT 



CHROMEL-CONSTANTAN 

 THERMOCOUPLE WIRES 



CERAMIC CUP 



IMPROVED DESIGN CRITERIA FOR 

 THERMOCOUPLE PSYCHROMETERS 

 Considerations of Instrument Design 



The thermocouple psychrometers described by Rawlins and Dalton (1967) and Wiebe 

 and others (1970) would appear to offer excellent opportunities for the measurement of 

 soil and plant (tree trunks) water potentials under natural conditions. However, the 

 question of resistance offered by the ceramic cup to the transfer of water, particularly 

 water vapor, has not been adequately investigated. Rawlins and Dalton (1967) concluded 

 that in order to maintain a water potential difference of less than 0.1 bar between 

 the chamber and the soil, the conductivity of the ceramic wall must be at least 4 yg. 

 cm." 2 bar hour for each degree per hour change in temperature. They indicated that the 

 saturated conductivity of porous ceramic is about six orders of magnitude greater than 

 this, and that resistance to water transfer was not limiting. However, they did not 

 discuss the implications resulting when liquid contact between the soil particles and 

 the ceramic cup is broken, wherein water transfer between the chamber and the soil will 

 occur only through vapor exchange. If a resistance to vapor exchange between the 

 ceramic chamber and the surrounding medium results, significant errors in estimates 

 of water potential will result. 



The primary function of the ceramic bulb or the ceramic cup is to maintain a fixed 

 dimension in the soil, and to offer protection for the thermocouple. If these same 



10 



