01 July 2003
Right on radar
Level radar races to finish line.
By Ellen Fussell
"Right now radar is the sexy level technology because it's new—state of the art," said Jerry Boisvert, product manager at Siemens Energy and Automation, Process Instrumentation Division in Grand Prairie, Texas.
Boisvert said radar is immune to a lot of problems ultrasonic technologies have with vapors in the air space of a tank. Liquid levels can deal with vapors in the air space, whereas with ultrasonics, "as you're transmitting a sound wave, vapors tend to stratify in layers," he said. Those vapors change the wave of sound. "But radar doesn't care what the air medium is, it just blasts through it. It doesn't rely on air molecules like ultrasonics."
Ultrasonics haven't experienced too much change over the years, except the microprocessing capabilities have improved in transducer materials, said Karl Reid, product line manager at Scientific Technologies, Inc., in Logan, Utah. "The algorithms inside the microcontrollers give us better stability and allow us to do a better job of filtering and detecting a target," Reid said.
Avoiding obstacles through ultrasonic technology is one of Reid's goals. "We make products that detect distance and obstacles—how far away an object is from a transducer, for example," Reid said. Automatic guided vehicle (AGV) equipment—vehicles without a person on board that move product—are prevalent at plants like Coca-Cola. Being able to slow down and stop the vehicle without losing a pile of cans on top is where obstacle avoidance comes in. "We supply them with a system that detects objects and applies brakes so it will slow down smoothly instead of at an abrupt stop," Reid said.
"An integrated circuit—a processor like a minicomputer—allows us to pull the transducer at a specified frequency and evaluates the return echo to filter out unwanted signals and to track the correct level or correct signal," he said.
In ultrasonic level technology, ultrasonic sound transmits anywhere above 20 kilohertz—so it will transmit the sound and wait for a return echo to come back from that sound, Reid said. "By understanding the time of flight, the time it takes for sound to travel (1,100 feet per second), we can determine distance to target," he said. "Based on that distance we can do different types of outputs for different applications. For example, 4–20 mA output in a level control application will give you level in a tank." In level control applications there's a sensor mounted in a tank. "We build sensors that mount in the top of a tank, they don't touch the liquids, but the transducer has to go inside."
Pulse radar component prices are much more affordable than they used to be, Reid said. Anyone looking into level control would see big advantages in high temperature—where temperature gradient is a problem. It's a problem for ultrasonic sound because sound will bounce off the thermal layers. "Some of those thermal layers have enough mass that they'll return an echo; the sound will reflect because of the density of the heat, which is a problem because you only want to look at the level," he said. And pulse radar sensors are not affected by temperature because of the magnetic wave. "So it will come back and reflect through the high temperature."
Pulse radar sees use with foams—in breweries with foam on top of a level tank "where they don't want something that protrudes into the liquid to be able to monitor the level—because of sanitary requirements," Reid said. The goal is to measure level and not be concerned with the foam. With ultrasonics, an electromagnetic wave will go through the foam, he said. While radar has been around for years, pulse radar is much less expensive.
Reid added that the environmental challenges are to operate in temperatures from -40°C to +60°C in harsh environments and to design parameters around those types of applications. There's also the challenge of complying with operating standards in hazardous environments—in a Class 1 division (explosion-proof area)—in developing sensors to meet the criteria to operate in those areas. "The sensor we're developing now is ultrasonic used in Class 1, Division 1, areas, and we manufacture the sensor so it'll meet intrinsically safe requirements," Reid said. "It has limited power to a maximum of 20 mA. The challenge is the power management of the sensor—to be able to store power and transmit when we have enough and maintain a 4–20 mA current loop."
HOW HAS RADAR CHANGED?
Four-wire units were the norm five years ago, Boisvert said. This made working with interfering obstacles like agitator blades more difficult than today. What has changed with radar over the past few years is the advancement of two-wire designs. "Five years ago radar units were very expensive, and two-wire designs were expensive and tough to manufacturer," Boisvert said. "Now the cost of radar and microwave components is coming down. Designing two-wire radar is not as difficult." With two wires, just a twisted pair of wires goes to the unit, which means costs on installation are lower.
Custody transfer radar was popular with big oil companies that housed storage tanks of gasoline and jet fuel and looked for high accuracy—custody transfer accuracy. "They're looking for 1 millimeter accuracy, and those radar units are very expensive," Boisvert said. They can range anywhere from $5,000 to $9,000. "The electronics are very sophisticated and the components are heightened," he said. Custody transfer is popular in the process market—where reactors and pressure and temperature changes mean tough applications.
Then there's the storage chemical market, Boisvert said. "Chemical plants that have three, four, or ten storage tanks of different materials are the companies that like two-wire radar designs because of their low cost."
Auto false echo suppression is another burgeoning advancement. "We can 'learn out' any obstructions in the vessel," Boisvert said. Some companies call it bin mapping. Although bin mapping occurs in the solids market, in the liquids market, "it allows you to learn the tank—learn any obstructions in there," Boisvert said. "You can stop those obstructions from causing false readings on your level measurement with radar. Our ultrasonic units have that as well on our transmitters," he said.
WHY NOT USE RADAR?
Although Boisvert said radar was the latest and greatest technology in level technologies, it isn't always the best answer. You wouldn't want to use it on point level control—interface level (two immiscible liquids or materials or nonmixing materials). could have water and water or oil and water. Customers want to track that interface level, where the oil and water meet," he said. A noncontact radar device cannot see or track an interface. A contact radar can do that—with guided-wave radar.
Noncontact radar is not used for liquid-liquid interfaces. Some ultrasonic transmitters are less expensive. "So for a simple application for measuring water in a vessel, you could use an ultrasonic device and spend $700 or $800 versus noncontact radar that costs $1,400," he said. Ultrasonic is ultimately cheaper for simple applications—air-liquid (such as in water-tank level).
Most of the radar devices out right now are simple-to-configure, noncontacting de-vices. They're also easier to mount. "Once you flange it in, it'll work for years and years without wear and tear," Boisvert said. Contacting devices that touch the material are capacitance, differential pressure, and mechanical float displacers. "You have to worry about corrosion compatibility and buildup on the contacting device."
FUTURE OF RADAR
Radar is going to be the technology for the next five to seven years, Boisvert said. "Things will get better; costs will go down," he said. Lower-cost designs and designs you can get remote readings from through satellite dishes are just a few of the advancements.
With features such as blanking (determining dead zones at a certain point in a vessel), response time, beam angles, and smaller antenna configurations, radar technologies will all get better. "It'll get to a point where you can manufacture a radar antenna with a three-fourth inch national pipe thread process connection—very scaled down," Boisvert said. "There's so much focus on radar as a level technology. Everyone is sinking research and development dollars into making it better."
The companies that cross the finish line first will be those that can get smaller antennas, lower costs to the customer, improved blanking by shortening and decreasing beam angles, and better techniques of signal processing, which improves accuracy and range extension. "We're looking ahead at a radar design that can shoot 300 feet," Boisvert said. The record now is 150 feet.
Reid's team is also looking to the future—trying to achieve reliable operation on oil drilling rigs. "The water reclaim system, where they shoot water into the drilling hole and it comes up with mud, goes through a series of tanks where they can separate and use it again," Reid said. Hazardous environments on the drilling rig and the drilling rigs in Canada and Alaska are in a wide range of extreme temperatures. "They need to be able to maintain reliable readings in an explosion-proof environment," Reid said. "We're trying to perfect that; right now we're working on how to get better results and faster reaction times." IT