Ultrasonic Sensor: How It Works & Applications
An ultrasonic sensor uses sound waves above the range of human hearing (typically 40 kHz to 200 kHz) to measure distance to a target. The sensor emits a burst, listens for the echo, and calculates distance from the time-of-flight. Ultrasonic sensors work on virtually any target material (metal, plastic, liquid, solid) and are the workhorse of level measurement, but performance varies wildly with environmental conditions.
How ultrasonic sensors work
- The sensor emits a short burst of high-frequency sound (typically 40-200 kHz).
- Sound travels through air at ~343 m/s, hits the target, and reflects back.
- The sensor listens for the echo, measures the time delay.
- Distance = (sound speed × time) / 2.
- Output is typically 4-20 mA analog or RS-485 digital — and on/off switching outputs at user-configurable trip points.
Key parameters
- Range — typically 30 mm (very short) to 10 m (long). Some specialty sensors reach 20-50 m.
- Beam angle — typically 8-15° cone. Wider beams cover more area but receive more false echoes.
- Accuracy — ±0.2-0.5% of full scale typical. Affected by temperature (sound speed varies with temperature) and humidity.
- Dead zone — minimum distance to detect (typically 30-200 mm). Within the dead zone, echo arrives before the sensor stops transmitting.
- Update rate — typically 5-20 Hz; longer-range sensors are slower.
- Temperature compensation — built-in or external. Sound speed changes ~0.6 m/s per °C, so 30°C swing = 5% range error without compensation.
Common applications
- Tank level measurement — most common application. Mount on top of tank, measure to liquid surface.
- Bin level for solids — grain, plastic pellets, powders. Be aware of angle-of-repose effects.
- Distance/positioning on conveyors — locate part position on a moving belt.
- Loop control / sag detection — web tension control via dancer-roll position sensing.
- Width and height measurement — pre-process metrology.
- Counting and presence detection — non-contact alternative to photoeyes for shiny or dark surfaces.
Limitations to know about
- Foam and dust absorb ultrasound — fail in dusty grain bins or foam-topped chemical tanks. Use radar instead.
- Steam and condensation distort the sound beam. Use radar or guided-wave radar in steam applications.
- Wind in outdoor installations bends the beam — false readings during storms.
- Soft targets (foam, fabric) reflect poorly. Vision sensors or capacitive better.
- Multiple targets in the beam confuse the sensor — first echo wins.
- Temperature variation across the measurement path — stratified air in a tall tank gives wrong distance.
Frequently asked questions
What is an ultrasonic sensor?
An ultrasonic sensor uses high-frequency sound waves (40-200 kHz, above human hearing range) to measure distance to a target by timing the echo return. The sensor emits a burst of sound, listens for the reflected echo, and calculates distance from the time-of-flight. Used widely for liquid level measurement, distance sensing, and position feedback in industrial automation.
What is the range of an ultrasonic sensor?
Typical industrial ultrasonic sensors range from 30 mm (very short) to 10 m (long), with most general-purpose sensors covering 0.2-2 m. Specialty long-range ultrasonic sensors reach 20-50 m for tank level measurement. There is always a "dead zone" near the sensor face (30-200 mm) where targets cannot be detected because the echo arrives before the sensor stops transmitting.
Can ultrasonic sensors measure liquid level?
Yes — liquid level measurement is the most common application. Mount the sensor at the top of the tank pointing down at the liquid surface; the sensor measures distance to the surface and the PLC calculates fill level (tank height minus measured distance). Works well for clean liquids in non-foamy non-steamy environments. For foam, steam, or dusty solids, use radar or guided-wave radar instead.
What is the difference between ultrasonic and radar level sensors?
Ultrasonic uses sound waves; radar uses electromagnetic (microwave) waves. Sound is affected by temperature, humidity, foam, dust, and steam — radar is not. Ultrasonic costs $200-1500; radar costs $1000-5000. Use ultrasonic for clean liquid/solid level in stable air conditions; use radar for steam-topped tanks, foamy chemicals, dusty grain bins, or hot environments where ultrasonic fails.
What affects ultrasonic sensor accuracy?
Temperature (sound speed varies ~0.6 m/s per °C, ~5% range error per 30°C without compensation), humidity, target surface (rough surfaces give weaker echoes), beam obstructions (pipes, structures inside the beam), wind (outdoor only), and stratified air conditions in tall tanks. Look for sensors with built-in temperature compensation, and use shorter-range sensors when accuracy matters more than range.