Today´s article covers the terms “dead zone” and “blocking distance” for level transmitters using the time-of-flight measuring principle.  Let’s start with a quick rundown of this principle.

When selecting level transmitters, we come across different terms from different vendors. For example, what some vendors call free space radar level transmitters are called non-contact radar level transmitters by others.

What is time of flight?

When a device sends radar pulses or ultrasonic waves, those signals reflect off the product surface and return to the receiver. In this way, the wave’s time of flight (ToF) shows the distance between the device and the product surface and then calculates the level by factoring in the tank height.

Going further with this principle, you’ll find three categories of ToF device:

  1. Guided wave radar
  2. Non-contact radar e.g: Wireless radar level sensor Senz2
  3. Ultrasonic level transmitter e.g: Ecometer ultrasonic level sensor from E-sensorix

To select the right level transmitter, we have to consider the Dead Zone or Blocking Distance of the instrument that meets the accuracy specifications, while configuring the instrument range of the above-mentioned level transmitters.

What is dead zone?

A level transmitter can send or receive a signal, but it can’t do both at the same time. It has to send, and only then receive. So, the dead zone defines the short distance between the device and the product surface where the signal returns too fast for the device to receive.

We call the dead zone from the upper edge of the tank the “upper dead zone,” naturally. So you can figure that we call the distance from the bottom of the tank the “lower dead zone.”

What is blocking distance?

The blocking distance involves a similar concept, a device too close to the surface to measure properly. Measuring devices become less accurate within the blocking distance. For accurate level measurement, you have to take this distance into account. At the top of the tank you’ll have the “upper blocking distance” and at the bottom the “lower blocking distance.”

Image of blocking distance
Blocking Distance, Image courtesy of Endress+Hauser


Both these terms have enough overlap to make most people consider them the same. We should not get confused with terms like dead zone, blocking distance or transition zone.

However, we just need to keep in mind that we cannot make a reliable measurement in this area. All these ranges vary by the type of sensor, kind of manufacturer and  DC value of the Instrument.

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To know more about products using such terminologies, you can get in touch with our engineers!

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