Tape floats (& servos). With tapes floats and servos, a plumb bob (or yo-yo) is a weight lowered by a motor until it "lands" on the surface of the material, changing tension in the wire, which stops the motor. The length of wire used indicates the space of material encountered. Measurement is independent of density, moisture and granular size.
Tape floats are excellent choices for the continuous measurement of thin fluids. They have limited use with thick fluids. Tape floats can ignore foams. A servo plumb bob is suitable for solids and interfaces, but mechanical hang can be a problem.
Thermal dispersion devices. With this technology, a self-heated thermistor (negative-temperature-coefficient resistor) changes resistance as the heat dispersion changes with the change in exposure to the fluid (as the level changes in the vessel, or the fluid reaches the sensor).
Thermal dispersion devices work very well with thin fluids, and are fair choices for thick fluids. They ignore foams, but can measure them. Foam detection is limited by thermal conductivity, and interface detection is limited by differential thermal conductivity.
Time domain reflectometry (TDR) devices. This guided electromagnetic wave method uses a rigid or flexible probe (or sometimes two probes). The electromagnetic field is focused into a tight beam by traveling along rod(s) or a cable. This focused beam makes the technology better suited to granular service without the need for high levels of microwave associated with FMCW radar.
TDR is an excellent choice for the continuous measurement of thin fluids and for conducting thick fluids, aqueous slurries, powders and solid chunks. It has limited application for sticky solids and locating interfaces. TDR ignores organic foams and can measure aqueous foams. Long nozzles are a problem, and the range and accuracy on insulating material is greater with a high dielectric constant. The technology can have significant "dead zones," depending on mounting.
Ultrasonic (non-contact) devices. Ultrasonic gap switches have one crystal electrically resonating at a certain frequency that generates sound waves. The sound waves travel across air gap toward the second crystal. As material fills the gap, the sound has a better medium to travel, causing a second crystal to resonate with the first. The continuous version sends a very high-frequency sound wave from a transmitter to the contents of the vessel and measures the length of time it takes for that signal to return to the transducer.
Ultrasonic devices perform excellently with thin fluids and ignore foam. Continuous measurement versions are fair choices for thick fluids, but switches are limited in that application. With continuous measurement versions, dust and dew in the vapor space hurt performance, and the range is limited by foam and angled or aerated solids. With switches, air bubbles and solid particles in the liquid will produce a "low" signal.
Vibrating switches. With this technology, a tuning fork vibrates tines or paddles at a specific frequency, which is dampened when the material reaches it.
Vibrating switches are excellent choices for powders and chunky solids, and fair choices for thick fluids. They are of limited use with thin fluids. Vibrating switches ignore foam. Excessive material buildup can prevent operation, and the devices are sensitive to mechanical shock.
A variety of technologies are available to meet most users' level measurement applications. However, the topic is far more complex than the brief generalizations here. Users also must consider factors such as locating interfaces between materials, sensor combinations and mounting positions.
Look for a solution, not a just technology. And remember: A newer technology does not necessarily mean a better technology for your application.
The table on page 39 attempts a summary of currently available technologies and also offers some suggestions regarding use. However, no good decision is based on a "magic formula." To choose correctly, talk to your level instrumentation provider about your specific application. CP
Herb works for AMETEK Drexelbrook in Horsham, Pa. He is the author of textbooks and courses for ISA, and also has written numerous instrumentation-related articles. Contact him at Sam.Herb@Drexelbrook.com. Some of the material in this article was made available through the help of John Roede, Don Koeneman and Bill Sholette, all of Drexelbrook.