Figure 2. -The Richards and Ogata 

 thermocouple psychrometer show- 

 ing the silver ring used to 

 support the water droplet. 



T 



1 cm 



1 



2ZZZZZ2ZZZZZ2ZZ 



HANDLE 



LEAD WIRES 



COPPER LEADS 



SAMPLE TUBE 

 CHROMEL-CONSTANTAN 

 WIRE 



SILVER RING 



SOIL SAMPLE 

 END-CAP 



also found greater errors in estimates of water potential with the Richards and Ogata 

 (wet-loop) psychrometer . Either psychrometer will cause the true water potential to be 

 somewhat disturbed, but the wet- loop psychrometer adds vapor to the system resulting in 

 high estimates of water potential. The small amount of vapor withdrawn from the atmos- 

 phere with the Spanner thermocouple will have a negligible influence in most cases 

 (Zollinger and others 1966) . 



A number of modifications and advances in techniques have been suggested for both 

 types of psychrometers ; but even with these advances, the Spanner psychrometer has come 

 into more general use. The theory of the Spanner psychrometer and design criteria for 

 its construction, together with sources of error in its use, have been thoroughly in- 

 vestigated (Rawlins 1966; Peck 1968, 1969; Dalton and Rawlins 1968; and Barrs 1968). 

 Special techniques and precautions to follow during construction are discussed by 

 Merrill and others (1968), Campbell and others (1968), and Wiebe (1970). Some of these 

 points are discussed below. 



One of the most useful adaptations for laboratory analysis of leaf and soil water 

 potentials is the rapid sample changer suggested for use with Peltier psychrometers by 

 Campbell and others (1966) . I have used a modification of this sample changer for 

 several years, and have obtained very satisfactory determinations of water potential. 

 The sample changer consists of six chambers lined with small (0.75 cm. 3 ) Teflon cups in 

 which the leaf or soil samples are placed. The same Peltier thermocouple is used to 



7 



