HR-33T Dew Point Microvoltmeter Instruction/Service Manual ...

Repeated or prolonged cooling of the junction can gradually build up moisture in thepsychrometer, even without use of the HEAT function, and this water is only slowly dissipatedthroughout the soil. Thermal gradients may also cause condensation within the chamber.[e]Excessive Temperature GradientsThese can be detected in single junction psychrometers as a difference in readings where theFUNCTION switch is alternated between READ and INPUT SHORT. Differences above 1 µvoltindicate undesirable temperature gradients, and above 10 µvolts often preclude reliable determinations.Solution: Place the psychrometer perpendicular to the temperature gradient, and wait until itstemperature is uniform with the offset near zero to make readings. Routine checks of zero offset arehighly recommended.[f]Contaminated Chamber or ThermocoupleThe thermocouple or the chamber in which it is mounted may become contaminated, making accuratewater potential measurements impossible. One method of detecting a contaminated thermocouple isto compare the psychrometric plateau with the plateau produced by a clean psychrometer for the samewater potential and the same cooling time. Figure 7 shows typical curves for a clean and for acontaminated thermocouple. The fall-off from the plateau begins earlier and is more gradual for acontaminated psychrometer.32