The short answer
Ultrasonic Level Measurement
An ultrasonic level sensor sends a pulse of sound from above the liquid, times the echo from the surface, and converts the time to a distance using the speed of sound, which it corrects for temperature. Level is the vessel height minus that distance. Nothing touches the liquid, which is its advantage; the sound has to cross whatever is above the liquid, which is its limit. Foam absorbs it, vapor and temperature layers bend it, obstructions reflect it, and the sensor cannot see anything closer than its blanking distance.
Key points
- The sensor measures distance to the surface; the level is the reference height minus the distance, so the reference height is part of the setup.
- The speed of sound changes with air temperature, and the sensor compensates from its own temperature sensor, which is not the temperature of the air in the vessel.
- Foam, heavy vapor, and a turbulent surface weaken or scatter the echo; radar tolerates all three.
- The blanking zone near the sensor is a dead band, and the beam widens with distance, so mounting position decides what the sensor sees.
- Loss of echo is a fault the controller must treat as a bad signal, not as a level.
How it works
The transducer at the top of the vessel is both speaker and microphone. It emits a short burst of sound, in the tens of kilohertz, listens for the echo from the liquid surface, and measures the round-trip time. Distance is half the time multiplied by the speed of sound, about 1,125 feet per second in air at room temperature. Level is the distance from the sensor face to the vessel floor, entered at setup, minus the measured distance.
The speed of sound in air rises with temperature, by roughly 0.2 percent per degree Celsius, and a sensor that ignored that would read a level that changed with the weather. Every ultrasonic sensor carries a temperature sensor and corrects for it. The correction is for the temperature at the sensor, and in a vessel where the air near the surface is warmer or cooler than at the top, or where sunlight heats the sensor housing, the correction is off by the difference. That is a small error on a short range and a real one on a tall tank in the sun.
Blanking and beam
After emitting a pulse the transducer rings for a moment and cannot hear, so an echo from a surface too close arrives while it is deaf. That distance, the blanking or dead zone, is typically a foot or two and is stated on the datasheet. The sensor must be mounted at least that far above the highest level it needs to measure. The pulse also spreads in a cone, several degrees wide, and anything inside the cone, a ladder, an inlet pipe, a wall at an angle, returns an echo the sensor may take for the surface. Mounting position is chosen so that the cone sees liquid and nothing else all the way down.
What defeats it
| Condition | Effect | Remedy |
|---|---|---|
| Foam on the surface | Absorbs the pulse; weak or no echo | Radar, or a stilling well where the foam is excluded |
| Heavy vapor or mist | Attenuates and scatters the sound | Radar, or a sensor with more power and a shorter range |
| Temperature stratification | Speed of sound varies along the path; reading offset | Shade the sensor; radar on tall tanks |
| Turbulence or agitation | Scattered echo, noisy reading | Stilling well; filtering; radar |
| Condensation on the face | Attenuates the pulse | A self-cleaning or heated face; mounting angle for drainage |
| Obstructions in the cone | False echoes at a fixed distance | Relocate, or map the false echo out in the sensor configuration |
| Sloped or conical surfaces | Echo deflected away | Aim at flat liquid only |
Loss of echo
When the sensor gets no usable echo it declares loss of echo, and what it outputs then is configurable: hold the last value, go to a fault current, or go to a preset. Holding the last value is the dangerous choice, because the control system sees a plausible level that stopped moving. The sensor should be set to signal a fault, and the controller should validate the signal and fall back to floats, as the signal validation and wet well level pages describe. A loss of echo that happens every time it rains, in a wet well, is foam from high inflow, and it is telling you the application wants radar.
Setup and checks
- 1
Enter the reference distance.
The distance from the sensor face to the vessel floor, or to whatever zero level means at that site, measured with a tape. This is the number that is most often entered wrong.
- 2
Set the range and the blanking.
The maximum level the sensor must read must be at least the blanking distance below the face. If it is not, move the sensor.
- 3
Map false echoes.
With the vessel empty or low, run the sensor’s echo mapping so that it learns fixed reflections from ladders and pipes and ignores them.
- 4
Verify at two levels.
Compare the reading against a tape or a known level near the bottom and near the top. A constant offset is the reference distance; a proportional error is the speed of sound or the temperature.
- 5
Configure the fault behavior.
Loss of echo to a fault current, and the controller validating it.
Where it fits
Ultrasonic sensors are inexpensive, easy to mount, and accurate to a fraction of a percent on a clean surface in still air. Clean water tanks, chemical day tanks with a calm surface, open channels with a flume for flow measurement, and dry bulk hoppers are good applications. Wet wells with foam and grease, digesters with heavy vapor, and tall outdoor tanks in the sun are the applications where radar has replaced it, and the radar level page covers why.
Frequently asked questions
- What is the blanking distance on an ultrasonic level sensor?
- The zone directly in front of the sensor, usually a foot or two, in which it cannot measure because it is still ringing from its own pulse. The maximum level must stay below the face by at least that distance.
- Why does my ultrasonic sensor read wrong on hot days?
- The speed of sound rises with temperature and the sensor compensates from its own temperature, which in the sun is not the temperature of the air in the vessel. Shade the sensor, or use radar, which does not depend on the speed of sound in air.
- Ultrasonic or radar for a wet well?
- Radar. Foam, turbulence, and vapor in a wet well are the three things ultrasonic handles worst. Ultrasonic remains a good choice for clean, calm liquids and for open-channel flow with a flume.
- What should the sensor output on loss of echo?
- A fault current that the controller recognizes as a bad signal, so that control falls back to floats or a defined safe action. Never a held last value, which looks like a level that stopped changing.
Related topics
- Radar Level MeasurementNon-contact and guided wave radar, why it handles a wet well better than most alternatives, and the installation details that decide whether it works.
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- Hydrostatic Level MeasurementMeasuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
- Open Channel Flow MeasurementFlumes and weirs with a level sensor: how the head-discharge relationship works, Parshall flume and weir equations, where to put the sensor, submergence, and the errors that make a permit-reporting flow meter wrong.
- Signal ValidationCatching a failed analog input in logic before it runs a pump on a dead transmitter: range checks, module status bits, frozen-value detection, rate checks, and what the program does with a bad value.
- Differential Pressure LevelMeasuring level in a closed or pressurized vessel with a DP transmitter: high and low side connections, dry legs and wet legs, zero suppression and elevation, the range calculation with a worked example, remote seals, and the density dependence.
Direct contact
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