Abstract: The invention relates to a test unit for testing the operation and accuracy of a microcomputer controlled aircraft fuel gaging system or processor which involves a characterized liquid gaging system sensor and sensor apparatus for determining a depth of liquid in a tank at a particular location in the tank. The fuel gaging system sensor is the wetted length type capacitance probe. The test unit hereof performs tests on processors prior to their being installed in aircraft and operates to provide a real time simulation of capacitive probes which in effect replaces the aircraft tank and probes thereof in testing the processors. A single reference capacitor simulates by multiplexing the capacitances of the set of tank probes, which in operation are scanned in succession by the processor, to verify the proper operation of the processor.