A somewhat high-tech solution to continuous fill level monitoring, radar fill level transmitters use radio wave emissions similar to a radar. Typically, these devices are mounted at or near the top of a tank filled with a liquid.
The transmitter sends out a radar signal into the liquid in the tank and receives a reflection of that signal in return. These transmitters then estimate the current fill level of the tank based on how long it takes for the transmitted signal to return.
The more time it takes to receive a return signal, the greater the distance between the sensor and the liquid, so the less liquid there is in the container.
These systems can use either wire-guided radar emissions, or free-emitting, non-contact sensors. In most cases, guided wire transmitters have better accuracy than non-contact systems.