Interactive explainer
How a tank sensor knows your water level
There is no magic in it. Drag the level up and down, switch between the three common sensor types, and watch exactly how each one turns your water into a number on your phone.
Ultrasonic: measured from the top
A sensor in the roof of the tank sends a pulse of sound straight down and times how long the echo takes to come back. Right now the surface is close, so the distance is short, which means the tank is full. As the water drops, the distance grows and the reading falls.
This is a simplified picture to show the idea, not an engineering diagram. Real installs handle temperature, tank shape, foam and turbulence, and convert the raw reading into litres using your tank's dimensions. The point stands: every one of these sensors is just measuring one simple thing and turning it into a level you can see from anywhere.
Three ways to read a tank, one number on your phone
Whichever sensor you use, the job is the same: turn the water in a tank you cannot see into a percentage and a litre figure you can check from the kitchen or the far paddock. What changes is the trick each sensor uses to measure it.
Ultrasonic and radar sit in the top and measure the distance down to the surface. Full tank, short distance. Empty tank, long distance. They never touch the water, so there is nothing to foul or corrode, which is why they are the easy all-rounder for rainwater and stock tanks. Pressure sensors sit at the bottom and feel the weight of the water above them, so more water means more pressure. They are very accurate and shine in tall, buried or awkward tanks. Float devices ride the surface, and while a simple float switch is fine for kicking a pump on and off, it only knows one point, not the whole level, which is why it is not really monitoring.
The sensor is only half the story on a rural block. The reading still has to travel from a tank that may be hundreds of metres from the house, over a link that keeps working when the mobile signal does not, to a dashboard that shows the trend and sends an alert before you run dry. That is the part most guides skip, and it is covered in the tank monitoring guide. If you just want to know which sensor to fit, the water tank level monitoring page walks through it.
Where this comes from
The primary sources behind the above, so you can check any of it yourself rather than take our word for it:
- Level sensors, the engineering background on ultrasonic, radar, pressure and float measurement.
- Water tanks, on the tank types and shapes a sensor has to cope with.
- Telemetry, on getting a reading from where it is measured to where it is read.
- LoRaWAN, the long-range radio commonly used to carry those readings on a rural block.
Common questions
How does a tank level sensor actually measure the water?
Most sensors measure one of two things and turn it into a level. A top-mounted sensor measures the distance down to the surface of the water, so a shorter distance means a fuller tank. A bottom-mounted sensor measures the pressure of the water sitting above it, so more water means more pressure. Either reading is converted, using the height of your tank, into a percentage full and a volume in litres.
What is the difference between ultrasonic and pressure tank sensors?
An ultrasonic sensor sits in the top of the tank and sends a pulse of sound down to the water, timing the echo to work out the distance to the surface. It never touches the water. A pressure sensor sits at or near the bottom and measures the weight of the water column above it. Ultrasonic is the easy all-rounder for most tanks, while pressure is a good choice for very tall, buried, or awkwardly shaped tanks where a clear line down to the surface is hard to get.
Do tank level sensors touch the water?
It depends on the type. Ultrasonic and radar sensors are non-contact: they read from the top of the tank and never touch the water, so they do not foul or corrode and are easy to service. Pressure sensors are submersible and do sit in the water, which makes them very accurate but means a cable run down into the tank and the occasional clean. For rainwater and stock water, non-contact is usually the simpler long-term choice.
Are float switches good enough for tank monitoring?
A float switch only tells you that the tank has crossed one point, usually full or empty. That is fine for switching a pump, but it is not monitoring, because by the time an empty float trips you are already out of water with no warning. Proper monitoring uses a continuous sensor that reports the actual level all the way up and down, so you can see the trend and act before you run dry.
Can a tank sensor work on any tank shape or material?
Broadly yes, with a bit of setup. Non-contact sensors work in poly, steel and concrete tanks because they read the surface, not the wall. For odd or tapered shapes, the sensor still measures level accurately, and the level is converted to litres using the tank's dimensions so the volume figure matches reality. Very tall or narrow tanks, and buried tanks, are where a pressure sensor sometimes earns its place.
How accurate is tank level monitoring?
Accurate enough to run a property on, typically within a few percent of actual level, which is far more than you need to know whether to order a delivery or start the bore. What matters more than the last percentage point is that the reading is continuous and reliable, and that it reaches your phone in time to act. A perfectly accurate reading you never see is worth nothing.