Stephen Hall discusses the golden rules for design
Many situations call for measuring level without having the instrument contact the fluid. This article discusses non-contact devices, particularly radar instruments, but also touching on ultrasonic technology.
Radar instruments emit a microwave signal toward a target – in our case the surface of liquid in a tank – and measure the time for the signal to bounce off the surface and return to the instrument. Since the signal travels at the speed of light the instrument can compute distance from the measured time. After configuring the system to an empty tank the instrument calculates the level by subtracting the measured distance from the empty-tank distance.
Table 1 is a checklist of information that defines requirements for a radar level sensor and transmitter. A “free space” radar has an antenna that beams the signal toward the target. “Guided” radar adds a physical path that the signal propagates along. The purchaser should gather as much of this information as possible before soliciting proposals from vendors. Some items, especially in the “instrument selection” section, may be furnished by the vendor.
The strength of the reflected signal is highly dependent on the dielectric constant of the fluid. Water (ε = 80) is much easier to measure than hydrocarbons (ε <25). The sensors can be tuned to measure either, however.
Two operating principles are used: time of flight (TOF) and frequency modulated continuous wave (FMCW). Modern instruments have overcome deficiencies that once plagued the FMCW technology. Select the device on the basis of its accuracy, beam angle, and antenna diameter rather than on the type of radar signalling.
The absolute accuracy of a radar detector is improved with a higher signal frequency which is generally 6, 26, or 80 GHz. The accuracy is typically 6-10 mm at 6 GHz and <1 mm at 80 GHz. But the service accuracy is also affected by the nature of the surface of the fluid in the tank. The beam angle decreases with increased frequency. If the surface of the fluid is choppy or wavy, a wider angle might do a better job of smoothing out the signal. This is somewhat analogous to an auto-focus camera that can be set to focus on a tight spot or on the average of the wider scene; the spot will be correctly focused while another subject in the photo is blurry. However, algorithms in modern electronics can mitigate this effect and allow for the use of high frequency signals even with very turbulent surfaces.
Beam angle is an important consideration when deciding where to locate the transmitter on the tank head. You want the beam to avoid hitting the tank wall because that will disrupt the reflected signal and decrease the strength and robustness of the measurement. Radar sensors are fitted with an antenna; the diameter and shape of the antenna, in addition to the signal frequency, determine the beam angle. Many radar units can be programmed to “map out” signals that are reflected from internals in the tank, including the wall, but it is still better practice to keep them out of the beam (see Figure 1).
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Chief Process Engineer at Genesis AEC, a US design and construction service provider in the life science industry
He authored Rules of Thumb for Chemical Engineers, 6th Edition (Elsevier, 2018).
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