instruments: ultrasonic, which uses sound-based measurement, and radar, which uses high-frequency electromagnetic waves to determine distance. Both ultrasonic and radar level measurement devices can also be used to measure flow rates in all types of open channel flow applications through the addition of a flume or weir.
Up until recently, ultrasonic was the unchallenged king of level and flow measurement in the water and wastewater sectors. But things are changing.
“The benefits of ultrasonic are that it is cheap, has low power consumption, is well developed and understood, and is basically the incumbent,” explained Oliver Grievson, a flow compliance and regulatory efficiency manager at Anglian Water Services. “But it is subject to interferences when it’s not monitored closely. This is where the potential for radar comes in.”
So which technology is best? Water Online examines how they compare in price, effectiveness and versatility, setup and use, and familiarity.
Price
Radar technology isn’t new — it has been on the market for nearly 20 years. But up until a few years ago, it was significantly more expensive than ultrasonic, causing most to stick with ultrasonic.
“Radar started dropping in price about three to four years back,” said Richard Lowrie of Krohne, an instrumentation manufacturer that sells both ultrasonic and radar instruments. “Radar has been streamlined; a lot of the things that were unnecessary have been trimmed for simpler application and to reduce the price.”
The average ultrasonic level measurement instrument costs $500 to $2,000. Radar level measurement instruments used to cost up to $4,000 but now have dropped down to $1,000 to1,200 on average, putting the two instruments at a nearly level playing field in terms of cost.
“People should still look at ultrasonic because it is still less expensive; a high-end ultrasonic will cost the same as a low-end radar,” Lowrie explained. “But I think the radar will keep dropping in price because people want the technology.”
Effectiveness And Versatility
One factor in determining which instrument is best is the intended application. Nicholas Paradiso, a consulting engineer with CDM Smith, recommends radar level measurement instruments to his clients that work in wastewater because oil, grease, and other coatings can sometimes interfere with the accuracy of ultrasonic measurement instruments.
“Any application where you aren’t dealing with 100% liquid, we would want to use radar over ultrasonic, because radar can see through solids better,” Paradiso explained. “But this is only really relevant in wastewater and not water treatment, unless you are dealing with a chemical tank.”
Dust, foam, and cobwebs can also present problems for ultrasonic level measuring instruments, and measurement accuracy can be affected by wind, temperature, and snow in outdoor tanks. Radar doesn't have these limitations, said Anglian’s Grievson.
“If the instrument isn't cleaned, which is a common problem, or there is foam, direct sunlight, or other difficult conditions, this is where radar has a benefit,” he said.
Operations that use silos, large wells, or oversized tanks may also benefit from radar, which has a longer range. Typically, an ultrasonic instrument can get an accurate return signal from a maximum of 60 feet away, while a radar instrument can get a return at up to 300 feet. However, this large range isn’t always necessary.
“In the water and wastewater industry, a lot of times ultrasonic’s range is suitable,” said Krohne’s Lowrie. “Normally you are dealing with 30- to 40-foot tanks at a wastewater facility. It could be considered overkill to use radar in some applications.”
Setup And Use
Complexity is also a consideration when it comes to selecting between ultrasonic and radar.
Ultrasonic meters have a shorter deadband — a minimum distance they have to be above the liquid surface — which can be limiting. But setting up a radar instrument isn’t as simple as setting up an ultrasonic instrument.
“With radar, when mounting the actual instrument, I’ve noticed that there are very specific distances you have to have it away from other equipment,” CDM Smith’s Paradiso pointed out. “You may also need to provide a metal plate above it, or the radar waves won’t reflect back properly.”
Lowrie feels ultrasonic level meters are also slightly easier to use.
“It is just simpler technology,” he says. “But I think radar has a wider range of applications.”
Familiarity
Despite the benefits of radar, Paradiso believes ultrasonic will stay in the lead for a while. This is because level measurement devices last 10 to 15 years, and aren’t likely to be replaced often. While Paradiso does believe radar devices are the future, he doesn’t recommend switching unnecessarily.
“If it isn’t broken and is still working, don’t mess with it,” Paradiso suggested. “There are other things that could be done to the plant aside from just going around and replacing all of your instruments.”
There is also some hesitation in the industry to try radar measurement devices because they are not as well known.
“Most of the industry is familiar with ultrasonic — it isn’t black magic as radar technology is thought of by some to be,” said Lowrie.
Grievson feels more education is necessary on radar use in both level and flow measurement before it can surpass ultrasonic. In the United Kingdom, where Grievson is located, few companies have promoted radar level measurement devices for flow measurement, as many ultrasonic level measurement companies have. This step will help the technology flourish.