I came across a great write up on guided wave radar level measurement technology by Emerson’s Anna Olander who is based in the Rosemount Tank Radar facility in Sweden. I’ll highlight some of the key points I took away from the write up.
The history of radar used in level measurement goes back to the mid-1970s when Saab (whose TankRadar line was acquired by Emerson in 2001) saw a great need for far more reliable and accurate level measuring systems for use in the cargo tanks, compared to existing systems. This recognition resulted in the development of radar level gauge for liquids tanks. The advantage over earlier level measurement technologies was that radar waves are not affected by the atmosphere above the product in the tank, the only part located within the tank is the antenna, and there are no moving parts resulting in greater reliability. The accuracy was also greater because liquid density changes, turbulence, and vibrations did not impact the measurements.
Mechanical level measurement technologies such as displacers have been a mainstay in level measurement for decades. They operate based on the buoyancy of the displacer in the fluid. Fluid density is a key factor in sizing the displacer and determines the stability of the signal, with variations in density affecting accuracy. Build-up on the displacer element also affects accuracy.
In a Guided Wave Radar installation, the GWR is mounted on top of the tank or chamber with a probe extending to the full depth of the vessel. Based on the tank configuration and application requirement there are a number of different probes including co-axial, rigid single, flexible single, rigid twin, and flexible twin probes. There are also probes for aggressive environments and extreme temperatures. These extreme applications can range from -196 degC to +400 degC or 0 to 5000 psig/345 bar.
This technology works on a low energy pulse of microwaves being sent down the probe. When the pulse reaches the media surface, a reflection is sent back to the transmitter. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back and an on-board microprocessor accurately calculates the distance to the media surface using time-of-flight principles.
GWR is recommended over non-contacting radar in most chamber applications because it is unaffected by the size and shape of the chamber and is better at managing shorter measuring ranges. Measurements can be made below the top liquid or foam surface, as the probe penetrates the liquid. It can also measure the interface level in separators and provide true bulk surface level measurement in boiling, turbulent or frothy liquids.
Anna references some common applications for GWR level measurement including gas/oil and oil/water separators, distillation columns, liquefied gases and Freon’s, power and industrial utility water level to name a few. She sums up the advantages as being more accurate, reliable and trouble-free measurement of liquid surface and interface levels. Measurement is unaffected by changing density, temperature and pressure, as well as condensing vapors or liquid drops attached to the probe, and provides reliable measurement of turbulent, boiling liquid surfaces and low reflective applications such as liquefied gases.