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

3.2 – Thermal and Vapor EquilibrationThermal equilibration can be easily determined by comparing the reading on INPUT SHORT and on READ. Thedifference between these two readings represents the difference between the junction temperature and thetemperature at the head of the psychrometer chamber where the chromel and constantan wires fasten to theconnecting pins. The offset should be less than 3 µvolts for meaningful measurements. Higher offsets areevidence of excessive thermal gradients (Section 7).Vapor equilibration can occur very rapidly for a sample chamber containing a solution (often less than oneminute). For a ceramic cup in a calibration solution, equilibration may take one or two hours. For ceramic cupsburied in soil, it is necessary that the disturbed soil come to equilibrium with the surrounding soil. It is advisableto allow 24 hours after installing soil psychrometers before reading the water potential.The equilibration time for in situ measurement of water potential of leaves with the L-51 or L-51A LeafPsychrometers varies with the type of leaf being measured and the conditions of measurement. Some leavesequilibrate in 15 minutes or less but others may require several hours.Repeatable water potential readings indicate that equilibration has occurred. Care must be taken that the periodbetween measurements is long enough to detect small changes, especially where long equilibration times areinvolved.14