How Frequency Modulation Can Improve Effectiveness of Radar Level Measurement

 If you’re an analog kind of person and still listen to a plain old radio, chances are it’s FM because it sounds better. Maybe you still have a favorite AM station but it’s probably news or sports. But FM is used for all sorts of things radio, including measuring level in tanks and vessels with radar.

Ingemar Serneby explains why FM is growing in popularity for level applications in his article in the August, 2019 issue of Flow Control, How Frequency Modulation Can Improve Effectiveness of Radar Level Measurement. All radar level technologies bounce a signal off the surface of the tank’s contents, but how that turns into a distance measurement works differently with guided-wave radar (GWR) and non-contact radar (NCR).

Given that the distance from a radar instrument to the surface of a tank’s contents could be a little more than a foot or two, the amount of time required for the pulse to travel is virtually instantaneous. Getting an accurate measurement of something so quick requires some clever engineering. NCR uses one of two methods: pulse or frequency-modulated continuous wave (FMCW). GWR uses pulse, but the more focused signal provided by the wave guide creates different operating characteristics than NCR.

Ingemar concentrates on NCR for most of the article and examines the difference between pulse and FMCW. The reason to choose one over the other has some similarities to how you might listen to the radio. The article goes into more detail as to how they work differently and the effects on measurements.

The difference between these two approaches is analogous to AM versus FM radio broadcasting. Pulse systems are more like AM in that the signal is more susceptible to interference from sources such as internal tank structures, foam, high vapor concentrations and turbulence. Measurement accuracy can also be influenced by any drift of the pulse frequency and even temperature inside the tank. FMCW captures its process variable information in the frequency domain, which supports more accurate signal conversion. Moreover, FMCW has higher receiving sensitivity and uses higher-strength signals than pulse systems, helping it to perform better in difficult situations where there may be turbulence and foam.

There is no one-size-fits-all solution for radar level measurement, which is why Emerson has a large radar product line encompassing GWR and NCR, with both FMCW and pulse system products. Every application has its trade-offs, and the more options for solving a problem the better. That said, Ingemar shows how evolving technology is changing the picture. The downsides of FMCW transmitters are becoming less pronounced.

As for FMCW, the working assumption all along was that they were, by definition, big, bulky four-wire devices and would consume, at least by instrumentation standards, a lot of power. That situation has changed. Like many electronic devices, FMCW instruments have become far smaller and more efficient with improved power utilization. Therefore, two-wire FMCW NCR instruments have emerged on the market and are growing in popularity.

One unit which is disrupting conventional thinking is Emerson’s RosemountTm 5408 Level Transmitter. Using two-wire FMCW technology, the Rosemount 5408 deploys a continuous echo to maximize radar signal strength and produce a more robust and reliable measurement. Delivering ease-of-use at every touch point, it uses an intuitive software interface to guide the operator through installation, commissioning, proof-testing and maintenance. It may not be the first choice for every application, but it is growing quickly in popularity.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level Group and other specialty areas for suggestions and answers.