Abstract: A liquid sensing apparatus for a fuel tank comprises a first optical sensor for producing an output having a first state that corresponds to the first optical sensor being exposed to air and a second state that corresponds to the first optical sensor being exposed to fuel; and a circuit for detecting the first and second optical sensor output states. The sensor circuit also has self test capability for testing both active and passive components. The sensor circuits can be accessed using a two wire only interface, and thermal stability can be achieved with the electronic circuit as shown.

Abstract: An externally mounted transducer assembly for an ultrasonic liquid level measuring device that utilizes a piezoelectric crystal for generating ultrasonic pulses to be transmitted through a tank or the line containing a volume of liquid. Electrical conductors are formed integrally with the fuel tank for connecting the transducer to other components of the system. The wall of the tank is provided with an arrangement for removably attaching the transducer housing thereto. The tank is provided with electrical contacts that connect to the conductors that are integral with the tank. Also, electrical contacts in the transducer housing are connected to the transducer for providing current thereto. When the transducer housing is attached to the tank, the transducer is thereby connected to the measuring system circuit.