Today’s star is the guided wave radar, a kind of radar level measurement device that constitutes a category of its own. This is the only kind of radar that doesn’t count as non-contact level measurement, due to contact with the measured product. Why? How? Read on to find out.

Guided wave radar vs non-contact radar level transmitter

If your process can take non-contact technology, you should consider going that route first, since non-contact radar level transmitters are cheaper and easier to set up. However, specific process conditions, such as low dielectric constants or high turbulence, can benefit from or even require a guided wave radar.

non-contact radar level transmitter vs guided wave radar diagram

I read an article written by a senior product manager who concluded by saying no perfect level measurement solution exists. That’s true. You have plenty of options on the market, and all the vendors will try to convince you they have the best solutions for your application.

Ultimately, the best solution for your application is the one you can afford and that keeps your process running smoothly. But that’s another discussion. Let’s go deeper into the guided wave radar universe.

The guided wave radar’s working principle

The guided wave radar is a special device because it uses the Time-of-Flight measuring principle but, unlike other devices using this principle, it is not a non-contact level transmitter. You can go back to our time of flight article if you need to brush up on the concept. We also have an article on frequency ranges that you should read. You should understand these topics as they relate to level measurement and time of flight devices.

Back to this topic! A guided wave radar installs at the top of the tank, with the probe of the radar extending into the tank to make contact with the product.

The device transmits a microwave signal down the probe to the product surface, which reflects the signal back. Then the transmitter can calculate that distance using the following formula:

Distance = (speed of light x time delay) / 2

So the radar measures the distance, but you want to know the level of the product, right? No worries. When you set up the transmitter, you include the minimum and maximum height of the application. Then the transmitter can calculate the level too! Good little gizmo!

Probe selection for a guided wave radar

Some probes have specific characteristics which can limit the guided wave radar, like length and mounting restrictions. You’ll also need to make sure the probe won’t come in contact with metallic objects or the walls of the tank. And if you have high turbulence, then you need to anchor the base of the sensor.

Guided wave radars generally use one of three types of probe – coaxial, twin element, and single element. Which will work best for you? Usually vendors provide tables or charts to help you select the right probe for your application, but let me give you a quick overview.

guided wave radar
Courtesy of Smar

Coaxial probes cover a wide range of applications, including those with low dielectric constants in their products. For a long measurement range, the twin element probe is a good choice, and you can get a flexible or rigid one. In sticky or viscous products, you should probably use the single element probe.

Interface measurement

One great application of the guided wave radar is for measuring the interface of two products, such as oil and water. Of course, for this application to work, you need a different dielectric constant in each product.

When the signal reaches the surface with the lower dielectric constant, a portion of the signal returns to the transmitter. When the signal reaches the product with the higher dielectric constant, more of the signal returns. If you set it up properly for these conditions, your transmitter can calculate the level of each product.

guided wave radar
Courtesy of ISA

Recommended guided wave radars on the market

You have plenty of options on the market, and each vendor has specific requirements to implement the device in your process. Most devices allow for high pressure and wide temperature ranges to fit more processes. But always remember that you must install the device correctly to get accurate readings.

Also, remember maintenance. Because the transmitter comes in contact with the product, it can develop problems with coating, buildup, clogging, and more. However, if you keep an eye on the diagnostics from your guided wave radar, then you can avoid unscheduled downtime.


If non-contact technology won’t fit your process, then consider a guided wave radar. As always, you need to be accurate when selecting a guided wave radar and think in the long term, about stability and maintenance down the line. So study your data and choose carefully!

To know more about radar level transmitter, please ask our engineers!

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