Method and apparatus for automatic measurement of evapotranspiration

摘要

An automatic atmometer includes an evaporating head that is coupled to a metering vial through a three-way valve controlled by fluid sensors disposed proximate inlet and open ends of the metering vial. The three- way valve is ordinarily in a quiescient state that connects the evaporating head to the metering vial. As water evaporates from the evaporating head water is drawn by suction out of the metering vial. When a sensor proximate the inlet of the metering vial indicates that the vial is empty the three-way valve is activated. This refills the metering vial from a fluid reservoir above the metering vial. As soon as one or more sensors proximate the open end of the metering vial indicate that the vial is full the three-way valve is returned to its quiescient condition. An output signal activates a counter or other logging device to record the number of times the metering vial has been emptied and refilled. Fluid presence at various portions of the metering vial is sensed by capacitive structures each producing a varying amount of capacitance according to the presence or absence of the fluid. The amount of an RF signal coupled through each sensor is measured and compared to a reference voltage. The reference voltage is itself produced from a similar capacitive structure so that the measured voltage and the reference voltage have the same temperature coefficient.