Water tank level sensors: ultrasonic, pressure and float, and how accurate each is
A water tank level sensor measures how deep the water is in one of three ways: an ultrasonic sensor in the lid times an echo off the water, a pressure sensor at the bottom weighs the water above it, and a float rides on the surface. A good ultrasonic sensor reads within about 1 inch on a tank 8 feet deep, and the temperature of the air in the tank can throw it off by more than that.
By Luke Patterson, co-founder of Trivertix. Last checked October 13, 2026.
The three kinds, side by side
All three answer the same question, how deep the water is. They differ in what they touch and in what can fool them.
| Ultrasonic | Pressure | Float | |
|---|---|---|---|
| How it measures | Sends a pulse of sound down from the lid and times the echo off the water. | Sits at the bottom and reads the weight of the water above it: about 0.433 psi a foot. | A float rises and falls with the water and trips a switch or moves a marker. |
| Touches the water | No | Yes, on a cable down through the lid | Yes |
| What throws it off | Cold or hot air in the tank, foam, and drops of condensation on its face. | Silt or algae over the sensor, and a vent tube in its cable that gets wet. | A float that sticks or fouls, and an arm or cable that wears. |
| What it reports | A level, as often as it is set to read. | A level, as often as it is set to read. | Often only full, empty or a few points between. |
Source, read October 7, 2026: Waterworks Mathematics by Alvord and Blasberg, section 1.7.1.
How a sensor compares with a float gauge, a sight tube and a pressure gaugeHow accurate each one is
Makers give accuracy as a percent of the range the sensor reads. One maker of all three kinds puts a typical ultrasonic sensor within 0.5 to 1 percent, and a typical submersible pressure sensor within 0.25 percent. A float that only switches at set points has no figure of this kind: it is right at its points and says nothing between them.
- Within 0.25 percent
- 0.24 inches on a tank 8 feet deep
- Within 0.5 percent
- 0.48 inches on a tank 8 feet deep
- Within 1 percent
- 0.96 inches on a tank 8 feet deep
The sum is the percent times the depth: 8 feet is 96 inches, and 1 percent of that is 0.96 inches. For a tank of another depth, scale it.
Those are figures for a sensor on a test bench. In a tank, the air, the lid and the mounting matter as much, and the next section is about the largest of them.
Source, read October 13, 2026: Icon Process Controls: Ultrasonic level sensor alternatives.
Why cold air fools an ultrasonic sensor
An ultrasonic sensor works out the distance from how long the echo takes, so it has to know how fast sound travels. That speed depends on the air: about 331.5 meters a second at freezing and 344 at room temperature.
A sensor that takes the room temperature speed on a freezing morning hears the echo late, by about 3.8 percent, and reads the water that much farther away. With 6 feet of air between the sensor and the water, that is about 2.7 inches: more than any bench figure above. On a hot afternoon under a dark lid the error runs the other way.
So ask a maker whether its sensor measures the air temperature and corrects for it. And read the level on the same sensor over a few days: a level that dips at dawn and comes back in the afternoon, with no water added, is the air and not the tank.
Source, read October 13, 2026: HyperPhysics, Georgia State University: Speed of Sound.
Resolution is not accuracy
Resolution is the smallest change in level a sensor reports. Accuracy is how close a reading is to the true level. A sensor can print the level to a tenth of an inch and still be an inch off, so a spec sheet that gives one figure has not told you the other.
For watching a tank, resolution tells you how soon you see a change: a slow leak or a float valve that has started to drip. Accuracy tells you how far to trust the number of gallons left. Set a low alert with room to spare for the second.
Will it fit the lid of a poly tank?
- Measure the opening across the inside, and compare it with the sensor's body or its mounting thread. Lids are sold in several sizes.
- Look down through the opening. An ultrasonic sensor needs a clear path straight down to the water, past any ladder, inlet pipe or float valve.
- Check the depth the sensor reads. Every ultrasonic sensor has a blind zone just under its face, so a tank filled to the lid can read wrong at the top.
- For a pressure sensor, see that its cable can reach the bottom and that the outlet will not pull silt over it.
Where Trivertix fits
Trivertix uses the first kind. Its sensor is ultrasonic, mounted at the top of the tank and reading down to the water, in tanks up to 16 feet deep. It reports a change as small as a quarter of an inch.
On a closed tank it sits in a 4-inch opening in the tank lid. An adapter fits openings from 4.5 to 5.5 inches. On an open stock tank it stands on a pole over the water. It reads about every 15 minutes and sends the level by LoRa radio to a receiver in the house.
It does not fit every tank. A tank deeper than it reads, a lid with an opening it does not fit, or foam or steam over the water are reasons to look at the other kinds. And for a tank you pass every day, a float gauge may be all you need.
The monitor on an open stock tankQuestions about tank level sensors
What is the difference between resolution and accuracy in a tank level sensor?
Resolution is the smallest change in level the sensor reports. Accuracy is how close a reading is to the true level. A sensor can report the level to the millimeter and still be an inch off, so a maker who gives one figure has not told you the other.
Does cold weather throw off an ultrasonic tank sensor?
It can. Sound takes about 3.8 percent longer to cover the same distance in air at freezing than at room temperature, so a sensor that does not correct for the air temperature reads the water about that much farther away. With 6 feet of air between the sensor and the water, that is about 2.7 inches.
Is a pressure sensor more accurate than an ultrasonic one?
Often, on paper. One maker of both puts a typical submersible pressure sensor within 0.25 percent of its range and a typical ultrasonic sensor within 0.5 to 1 percent. But a pressure sensor sits in the water on a cable, where it can clog or foul, and an ultrasonic one touches nothing. Which matters more depends on the tank.
What size opening does a tank level sensor need in the lid?
As large as the sensor's body or its mounting thread, with a clear path straight down to the water for an ultrasonic one. Measure the opening across the inside before you buy, and check that no ladder, inlet pipe or float valve sits under it.
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