Figure 6. — The influence 

 of temperature gradi- 

 ents between the 

 thermocouple reference 

 junctions and sensing 

 junction. At tem- 

 perature equilibrium 

 thermocouple output 

 is zero. 



Time, minutes 



Since at temperature equilibrium (no temperature gradients between the sample and 

 the psychrometer) the two reference junctions of the psychrometer are at the same 

 temperature as the sensing junction, the e.m.f. output of the thermocouple is zero. 

 When the thermocouple output is switched via the switchbox directly through the volt- 

 meter ("read" position), the voltmeter is adjusted to "zero" or some other convenient 

 position with the "zero-adjust" knob on the switchbox. Then the thermocouple is 

 immediately switched to the cooling position for 15 seconds, which cools the sensing 

 junction with a 5 ma. current. After 15 seconds, the thermocouple is immediately 

 switched back to the "read" position, and the e.m.f. deflection is read on the volt- 

 meter. The difference stated in uv between the e.m.f. prior to cooling and immediately 

 after cooling (maximum deflection) is the output for that particular water potential. 

 This subtraction of the e.m.f. before cooling from the e.m.f. after cooling corrects 

 for temperature differences between the sensing junction and reference junction, and 

 also cancels any parasitic e.m.f. 's resulting from temperature differences between the 

 various junctions in the thermocouple circuit. Furthermore, this subtraction reduces 

 the need for thermal grounding normally required at junctions and terminals in the 

 circuit during low voltage measurements (Rawlins and Dalton 1967) . 



The circuit diagram for the switchbox in Appendix 6 provides for the functions of 

 "read," "zero," "cool," "heat," and "voltage check." The "heat" position is the reverse 

 of the "cool" position and provides a slight heating current which can be used to 

 quickly dry residual water from the thermocouple junction if needed. The "voltage 

 check" position measures the internal voltage of the switchbox, and is used only with 

 a recorder and amplifier in the system. The circuit- shown in Appendix 6 has an internal 

 voltage output of about 10 yv which can be used to calibrate the scale on a linear re- 

 corder. All solder junctions in the switchbox should be made with thermal-free solder, 

 and binding posts should be gold-plated to reduce parasitic e.m.f. 's at the terminal 

 connections. 



14 



