Ultrasonic Tank Level Monitor
Waterproof LoRaWAN ultrasonic units supplied to customers in multiple countries to monitor liquid height in tanks. Maximum range ~6 metres, battery life typically exceeding one year.
Ultrasonic level monitors measure distance to a liquid surface or some other target by calculating the time it takes for the sound to travel to the target and back.
My sensor units transmit range data over long distance wirelessly using LoRaWAN. Though that could easily be modified to use WiFi or even cellular.
Maximum ultrasonic detection range is typically 4 to 6 metres with my waterproof sensor designs, depending on the type of sensor module used. Longer ranges are possible using non-waterproof (but otherwise environmentally rugged) sensor modules.
Sensor units also have what's known as a 'dead zone', which is the minimum distance detectable. The minimum for my unit is around 3cm, again depending on the type of sensor module implemented. Batteries typically last well over a year.