Level Measurement · Glossary

What is Blocking Distance?

dead zone below the sensorLevel Measurement

Blocking distance is the zone just below an ultrasonic or radar sensor where it cannot measure, because it is still ringing from the pulse.

Blocking Distance Explained

After sending a pulse the transducer needs a short time to settle. Echoes from within this distance are lost, so the level must never rise into it.

Set the maximum level below the blocking distance, or raise the sensor with a nozzle extension. Radar blocking distances are generally shorter than ultrasonic ones.

Blocking Distance: Typical Values

Ultrasonic	typically a few tenths of a metre, larger for long range sensors
Radar	usually smaller; see the maker data

Calculators for Blocking Distance

CalculatorUltrasonic Level CalculatorFree online calculator with formula and worked example.CalculatorRadar Level CalculatorFree online calculator with formula and worked example.CalculatorTank Ullage CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Blocking Distance?

Blocking distance is the zone just below an ultrasonic or radar sensor where it cannot measure, because it is still ringing from the pulse.

What happens if level enters the blocking distance?

The sensor may lock on to a multiple echo and show a low level while the tank is overflowing. Protect with an independent high level switch.

Is blocking distance the same as blanking distance?

Yes, the terms are used interchangeably by many makers.

Learn more on the blog

Ultrasonic level transmitter working

A practical guide on instrumentationblog.in with more detail on blocking distance and how it is applied on real plants.

Read Full Article

More Level Measurement Terms

Level termDisplacer Level TransmitterA displacer level transmitter measures level from the buoyancy force on a heavy cylinder partly submerged…Level termFloat Level SwitchA float level switch uses a buoyant float that rises and falls with the liquid to operate a contact at…Level termVibrating Fork Level SwitchA vibrating fork level switch detects liquid when the frequency of its vibrating tines drops as they are…Level termMagnetic Level GaugeA magnetic level gauge shows level with coloured flaps turned by a magnetic float inside a chamber beside…Level termCapacitance Level ProbeA capacitance level probe measures level from the change in capacitance as liquid replaces air between…Level termLoad Cell Tank WeighingLoad cell tank weighing measures the mass of a vessel contents from load cells under its supports.

All level measurement terms · Full glossary A to Z