A new twist on level detection
By Staff -- Plant Engineering, 5/1/1998
I read with great interest your article on level indicators ("Selecting Liquid Level Detectors For Tanks," PE, February 1998, p 39, File 3599).My company has used or experimented with most, if not all, of the various types of level sensors mentioned in the article. Some are obviously better suited to certain applications than others; almost all are bothered by dirty or foamy liquids, requiring frequent cleaning.
For many years, we have used a type of level detection not covered in your article. We call them bubbler tubes. They probably fall closest to the category of pressure-type sensors. In this process, a very small stream of instrument air is forced down a dip-tube to within about 1 in. of the tank bottom. The flow rate is set by a small rotometer. The residual pressure in the bubbler tube is directly proportional to the fluid level. A pressure transducer converts the signal to a 4-20 mA output which can be calibrated to read the liquid depth, or to control valves, etc.
We have used this system for many years to monitor and control the levels in brine tanks that serve our desiccant refrigerant systems. The biggest advantage is that they work well in dirty solutions or in applications where foaming is present, and they are relatively simple and inexpensive. One disadvantage is that they are subject to deviation caused by changes in the solution specific gravity.
We are currently experimenting with ultrasonic devices, primarily for the purpose of eliminating the P/I transducer required to convert an electronic signal compatible with our PLC control systems. The ultrasonic sensor generates a 4-20 mA signal directly, thus eliminating one control device. Because foaming of the desiccant solution is an occasional problem, we mount the ultrasonic devices in stilling wells, but we have some concerns for the possibility of getting false signals from deposits that may form on the inside of the wells.
If ultrasonic devices do not prove reliable in some of our applications, we will go back to the old reliable bubbler tubes. -- Robert E. Sudduth, PE, Anheuser-Busch Companies, Inc., St. Louis, MO
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