CALL FOR BEST PRICE
+1-800-724-4209
How Do Ultrasonic Sensors Work? Range, Blind Zone and Best Uses
An ultrasonic sensor measures distance with sound. It sends a short burst of high-frequency sound, far above human hearing, and times how long the echo takes to return from the target. Because it uses sound rather than light, it does not care about the target's color, transparency or shine, which makes it ideal for clear bottles, liquids and level measurement.
Time of flight
Sound travels about 343 m/s in air at 20 °C. The sensor divides the round-trip time in half:
Distance = speed of sound × echo time ÷ 2
An echo returning after 1 millisecond means the target is about 17 cm away. The same transducer usually both sends and receives, alternating between the two.
Key terms
| Term | What it means |
|---|---|
| Sensing range | Minimum to maximum distance the sensor can measure, e.g. 40–300 mm |
| Blind zone | The area right in front of the face where targets cannot be detected, because the transducer is still ringing after sending. The 40 mm in a 40–300 mm sensor. |
| Beam angle | Sound spreads in a cone; nearby objects inside the cone can cause false echoes |
| Window / setpoints | Switching points you teach for a digital output |
| Analog output | Distance as 4–20 mA or 0–10 V across the taught range |
What affects accuracy
- Temperature: the speed of sound changes about 0.17% per °C. Many sensors compensate internally; large temperature swings or gradients still add error.
- Target angle: a flat target tilted away reflects the echo away from the sensor. Keep targets roughly perpendicular to the beam.
- Soft or uneven surfaces: foam, fabric, wool and some powders absorb sound and return weak echoes.
- Air movement: strong drafts, steam or turbulence can scatter the sound.
- Crosstalk: two ultrasonic sensors close together may hear each other; many models can be synchronized.
Best applications
- Liquid and bulk level in tanks and hoppers.
- Detecting clear glass, film and plastic bottles.
- Loop and roll diameter control on web and coil lines.
- Presence and height checks on mixed-color products.
- Dusty or dirty environments that would blind an optical sensor.
Ultrasonic vs. other sensors
| Sensor | Choose it when |
|---|---|
| Ultrasonic | Clear, shiny or colored targets; liquids; you need a distance value |
| Photoelectric | Small targets, fast response, longer range, a tight spot |
| Capacitive | Level through a non-metallic tank wall at very short range |
| Inductive | Metal targets within a few millimeters; the most robust and cheapest |
Setting up an ultrasonic sensor
- Mount it so the target is beyond the blind zone and inside the range, with the face perpendicular to the target.
- Keep other objects out of the beam cone, including tank walls and fittings.
- Teach the switch points or analog range with the real target, using the sensor's teach button or wire.
- Wire the output to the PLC; see 4–20 mA vs. 0–10 V for analog.
HTM ultrasonic sensors
HTM's M18 ultrasonic sensors measure 40–300 mm with a digital switching output plus either 4–20 mA or 0–10 V analog output. Browse the ultrasonic collection, call 1-800-724-4209, or request a quote.
Frequently asked questions
Can ultrasonic sensors detect water?
Yes. The surface of a liquid is a good reflector, which is why ultrasonic is popular for tank level.
Do ultrasonic sensors work in a vacuum?
No. They need air (or another gas) to carry sound.
Why does my sensor read nothing at close range?
The target is inside the blind zone. Move the sensor back.