The transmitter sits on top of the tank and ultrasonically measures the level of oil in the tank. It then transmits the information to a receiver unit that's plugged into an electric socket in your house. The height of the oil is displayed on an LCD screen on the receiver. When the level of oil falls below 10%, a fuel pump symbol appears, telling the owner it's time to order more oil.
The Watchman Alarm oil tank alarm uses ultrasonic measurement to continually measure the oil tank level to help prevent fuel theft. A transmitter sets off an alarm when there is a sudden drop.