Open channel flow radar measurements keep working, even when the temperature drops

Every wastewater treatment plant in the United States measures influent and effluent – flow going in and out of the plant – and reports these measurements to a state or federal regulatory agency. Many facilities make these measurements using a level measurement sensor in a flume or weir.

The City of Mankato WRRF had been using a flume with an ultrasonic sensor. Cold temperatures and wintry weather, however, was affecting the accuracy and reliability of their measurement. With relatively warm water flowing below the sensor and the cold air around it, frost constantly formed on the sensor face. A small amount of frost on the face of the ultrasonic sensor caused measurement errors, and during long cold stretches, excessive frost would cause sensor failures.

Mankato’s WRRF Superintendent, Josh Gad, contacted his local VEGA contact to discuss the difficulties he’d been experiencing with the ultrasonic devices on his influent flume. VEGA offered a free trial of a VEGAPULS C 21 radar level sensor to install right next to the ultrasonic sensor for comparison.

During the trial period, the wastewater treatment plant kept the insulated housing over both sensors. The ultrasonic sensor still experienced the occasional failures, but the radar unit continued to work flawlessly. After the trial period, Superintendent Josh Gad opted to remove two old ultrasonic sensors measuring on parallel flumes and replace them with VEGAPULS C 21 radar sensors. The ultrasonic and radar sensors were similarly priced, so it was an easy decision to make the switch to a more reliable technology.

A true test in the cold

With the ultrasonic sensors removed, they removed the insulated box and space heater. This made it easier to access the sensors, but it also exposed the two new radar sensors to the elements. The upcoming Minnesota winter would be a true test for the reliability and accuracy of these sensors.

Radar signals are immune to changing temperatures and the VEGAPULS C 21 can withstand temperatures as cold as -40°F (-40°C). In February 2021 in Mankato, temperatures didn’t warm above 0°F (-18°C) for a period of 130 hours (5 ½ days). Nighttime temperatures dropped to -25°F (-32°C) several nights in a row. The radar sensor continued to operate without fail through it all.

Throughout the cold stretch, a thick layer of frost formed on the face of the radar sensor, but it continued to output a strong, reliable measurement. The VEGAPULS C 21 is an 80 GHz radar sensor equipped with intelligent software, both of which work together to read through the frost and reliably make the measurement operators need.

After years of regular maintenance and workarounds with ultrasonic transmitters, the Mankato Water Resource Recovery Facility had an easy, reliable solution they could count on year-round in their all-important open flow channel measurements. Shortly after the aforementioned cold stretch, superintendent Josh Gad said, “This thing is great! It was -24°F (-31°C) outside, and these things didn’t miss a beat!”