Abstract: A system including a method and means for trimming capacitive-type remotely-located fuel probes that eliminates the previously-used variable trim capacitor and substitutes a rugged, inexpensive, standard-value signature resistor connected between the AC input and ground to provide "calibration offset" data needed by the fuel measurement electronics to make a determination of actual probe capacitance. The system may be applied to any such probe having a blocking capacitor in series with the probe capacitor, which blocking capacitor insures accurate measurement of the signature resistor when the leakage resistance is significant. When applied to DC or AC FHC probes, the C.sub.fhc capacitor is replaced with a non-critical DC-blocking capacitor, that is typically at least 100 times greater in capacitance value than the C.sub.

Abstract: A combustible gas detection system in which remotely located combustible gas sensors of the analog type are monitored by a central controller. The sensor analog data is converted to digital data and transmitted to the controller. The controller tests the integrity and validity of data received from the sensor, identifies the nature of any faults therein and vocally signals the cause of such faults. The controller computes the concentration of combustible gas at the sensor and automatically determines sensor calibration constants from received sensor data.

Abstract: A combustible gas detection system in which analog signals produced by a remotely located combustible gas sensor are converted to digital signals at the sensor and the digital signals control a coherent frequency shift keying modulator which impresses alternating signal currents, through an optically coupled isolator, on a transmission line connecting the sensor with a control station. The transmission line is a two wire link of ordinary quality which carries power from the controller to the sensor as well as signals from the sensor to the controller.