LEVELMARK™ Model 661 Level Transmitter is a Non-Contact continuous level measurement system that determines level by measuring the time of flight of an ultrasonic signal.  The system consists of an electronic controller/transmitter and a piezoelectric based transducer that is mounted above the material being measured.  A burst of electrical energy is generated by the controller and delivered to the system’s transducer.  The transducer converts this electrical energy into an ultrasonic signal that is transmitted through the atmosphere to the surface of the material being measured.  When the signal reaches the material it is reflected back to the transducer where it is received as an echo and converted back into electrical energy.  The controller/transmitter measures the total time of flight (time of transmit to time of receipt as an echo).  The distance from the face of the transducer to the surface of the measured material is a product of the speed of sound and 1/2 the time of flight.  The controller can easily determine the level by subtracting the measured distance from the vessel height.