Dead zone or blocking distance of level transmitters


In order to have an appropriate level control, what we need is a correct level measurement. For this,  there are several options available in the market.

When selecting level transmitters, we come across different terms from different vendors. For example, some vendors sell “free space radar” devices, while others sell “non-contact radar” devices. There might also be a case where one looks at both options!

In this context, today´s article covers the terms “dead zone” and “blocking distance” for level transmitters. Both these terminologies use the time-of-flight method.  So, to understand things better, let’s start with a quick rundown on this principle. A read on…

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.

To know more about this, you can read our article on time of flight.

So 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 can consider 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 the dead zone?

A 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 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 the 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


Both these terms have enough of overlapping in them that makes most people consider them the same. We should not get confused with the 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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