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

3.3 – Effect of Zero Drift on ReadingGiven the condition of water condensed on the junction at the dew point temperature, the direct reading of theinstrument in dew point mode will continue indefinitely. However, the accuracy of the reading will be directlyaffected by any thermally induced zero offset that occurs in the thermocouple or intermediary connecting points,or in the instrument itself. Zero drift in the instrument will normally be negligible, but other effects may becomesubstantial. Therefore, sustained continuous readings over several minutes in duration should not be relied uponfor accuracy unless extreme care has been taken to allow complete temperature equilibration and unless ambienttemperature variations and thermal gradients have been eliminated.3.4 – Ancillary Instrument FunctionsIn addition to the operational functions used to measure water potential as described in Section 2, the HR-33T alsofeatures three additional functions that are incorporated in the RANGE switch and the FUNCTION switch:HEAT+BATT-BATTsupplies heating current to thethermocouple to dry the junction whennecessary.measures internal battery voltage(full scale = 30 volts)3.5 – A v Versus TemperatureIt has been found, both theoretically and empirically, that A v changes by a factor of 0.7 µvolts/°C with WescorPsychrometers. A v for temperatures other than the temperature at which A v was measured is given by the formula:A v,T1 = 0.7(T 1 – T 0 ) + A v,T0whereA v,T1 is the value of A v at the new temperature (T 1 ), andA v,T0 is the value of A v at the temperature of measurement (T 0 ).An example of such a correction is as follows:Given:A v at 25/C = 55 µvoltsT = 12/CA v at 12/C = 0.7(12 – 25) + 55 = 45.9 µvoltsFor maximum accuracy, A v should be readjusted to the correct value whenever operating at temperatures differentfrom T 0 .15