Ultrasonic Sensors and Blanking Distance

In order to better understand blanking distance, it’s important to grasp the two keys to how ultrasonic sensors work: vibration, and time.

Operationally, ultrasonic level sensors produce and detect vibrations. They pulse to produce sound—much like a speaker—and they listen for the returning sound. When the sound wave returns, it produces a small vibration on the face of the sensor. This second vibration is what the sensor associates with the target surface.

The time the sensor waits for the return vibration is used to calculate distance. The sensor can then determine tank levels, volumes, flow rate, and more depending on the model and the features. An ultrasonic level sensor equates time with distance.

Because the sensor uses the same transducer to both produce and listen for the sound wave, it can become confused without a little logic. When the transducer first pulses, it naturally causes residual vibration for a split second. The sensor must be programmed to ignore signals for as long as it takes the residual vibration to stop. This programmed time equals a certain distance. Therefore, the waiting period is called a blanking distance.

Without a programmed blanking distance, the ultrasonic level sensor would immediately signal a false surface right up against the transducer face. The blanking distance is part of the sensor’s logic, and is how it distinguishes between pulse vibrations and return vibrations.

The length of the blanking distance is tied directly to the transducer’s frequency and the pulse’s power. A small, low-range transducer produces a smaller pulse that won’t vibrate as long. Smaller ultrasonic level sensors enjoy shorter blanking distances, as little as 4 inches. Large ultrasonic level sensors can have blanking distances as long as 1½ feet or more.

There are some advantages to having a blanking distance, especially a programmable one. Increasing the length of the blanking distance can block out unwanted return signals from obstacles, such as a ladder in a tank.

In the mean time, do you have any questions or comments about blanking distances? Reach out to our application engineers. They’re always happy to help.