Measuring level with a differential pressure transmitter is an indirect but old, tried-and-true solution. Radar level transmitters, on the other hand, are modern, specialized level measurement devices. The former is more common but many people are considering switching to the latter. This article will help you decide if you should go for a radar level transmitter or a differential pressure based level measurement solution.

Radar level transmitter vs Differential pressure transmitter

Some process applications have conditions where a pressure transmitter will work better than a radar. However, we need to keep in mind that most advantages and limitations apply only to certain applications. And in many cases, the cost of switching to a new level measurement solution outweighs the gains.

Usually, the companies that promote radar point to the fact that pressure
transmitters don’t measure directly. And that’s true! A pressure transmitter derives its level measurement from the relation of height and density.

Widespread use in many applications, differential pressure transmitters can be used to determine fluid levels by determining the difference in head pressure between the low-pressure port and the high-pressure port in its usual configuration.

The difference in pressure becomes an output signal which is calibrated to indicate a fluid level. So, if your product has density changes, a pressure transmitter won’t provide good level measurements. That is unless you accurately factor in the variations.

Radar or pressure transmitter
Level transmitter – Copyright Endress+Hauser

Also, when you have a pressure transmitter using capillary, the temperature can affect the measurement. Plus, incorrect installation can decrease accuracy.

Have a look at the video below for a more visual explanation on how level is measured with differential pressure transmitter:

On the other hand, radar level transmitters measure the level directly without density influence. But, it has a much more complex setup. It also depends on the dielectric constant to work.

Radar is a non-contact method that entails bouncing an electromagnetic pulse off a fluid surface and measuring the time required for the pulse to return to the sensor. The quicker the pulse returns, the higher the fluid level.

The contact-free measurement has advantages in that media characteristics are not as restrictive. Radar works best in metal vessels though. The vessel may restrict certain media from using this technology.

Highly corrosive media, for example, would not be stored in a steel vessel. Radar may not be the best choice for this reason. Radar sensing technology requires some installation time. The software must be installed so you can calibrate the equipment. Calibration eliminates false echoes from inside the vessel.

These restrictions and calibrations imply another issue with radar: portability. If your application requires measuring many tanks or a more dense fluid below a floating level, radar may not be your best choice.

Radar sensors can be used when the process materials are flammable or dirty and when the composition or temperature of the vapor space varies. For example, a beer brewery vat, where the characteristics of the air in the vessel are going to change as the yeast works to release more CO2 forming a head or foam layer. In this application, any other type of sensor is not going to work.

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