Abstract: An arrangement for determining the temperature gradient within a fluid mem, which is used to ascertain the magnitude of vertical water movement, utilizes a pair of thermistors connected in a differential Wheatstone bridge that senses the temperature at two locations. The DC bridge driving voltage has its polarity periodically reversed for equal time intervals by a logic signal. The bridge output signal is converted to a variable frequency and counted during each of these different intervals in opposite directions. This subtraction process, which yields the temperature difference at the two locations, also, among other things, eliminates the effects of drift from the temperature measurement. The system also contains provisions for minimizing any offset caused by changes in the bridge driving voltage, amplifier offset instabilities and variations in other circuit components.