Figure l.-The Spanner 

 (Peltier) thermocouple 

 psychrometer and a 

 psyohrometer chamber 

 are shown with the soil 

 sample. The entire 

 assembly is sealed and 

 immersed in a water bath 

 to maintain constant 

 temperature . 



A 



EPOXY RESIN 



COPPER LEAD WIRES 

 "O" RING 



TEFLON INSERT 



BRASS SAMPLE CHANGER 



THERMOCOUPLE JUNCTION 



CHAMBER ATMOSPHERE 

 SOIL SAMPLE 



by passing a small electric current through the junction for a short time in a direction 

 which causes it to cool below the dew point of the atmosphere (Peltier effect) . After 

 this cooling, the current is immediately stopped, allowing the condensation to evaporate 

 back to the atmosphere in the chamber. The rate of evaporation is a function of the 

 vapor pressure in the chamber, hence of the water potential of the sample. The evapora- 

 tion cools the junction as a function of the vapor pressure, and the difference in tem- 

 perature between the sensing junction and the reference junctions causes a minute voltage 

 output from the thermocouple. The magnitude of this voltage output is also a function 

 of the sample water potential, and is recorded with a microvoltmeter . 



Richards and Ogata (1958) suggested a modification of the Spanner psychrometer, 

 consisting of a small silver ring attached to the ends of the chromel-constantan 

 thermocouple wires (figure 2). A small water drop (3-5 u liter) is placed on the ring, 

 and then is sealed in the chamber containing the sample. Readings are made when the rate 

 of evaporation from the water droplet reaches a steady value, and hence when the tempera- 

 ture depression of the thermocouple is constant. The primary difference between the 

 Spanner and the Richards and Ogata types of psychrometers is that the Spanner thermo- 

 couple permits measurements of both dry and wet-bulb temperatures, whereas with the 

 Richards and Ogata psychrometer the dry bulb temperature can only be inferred from 

 bath temperature. 



Considerable attention has been given in recent years to the relative performance 

 of these two psychrometers for measuring water potentials of soil and plant tissue. 

 Rawlins (1966) rigorously reviewed the performances of these two psychrometers and sug- 

 gested sources of possible errors and design criteria for reducing the magnitude of 

 inaccuracies of water potential determinations. Barrs (1965) found that because of 

 the permanently wet junction in the Richards and Ogata psychrometer, the observed tem- 

 perature depression was between the reference junction temperature and wet bulb temper- 

 ature, and this led to low estimates of water potential. Zollinger and others (1966) 



6 



