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Continuous Level Measurement

Many industrial processes require the accurate measurement of fluid or solid (powder, granule, etc.) height within a vessel. Some process vessels hold a stratified combination of fluids, naturally separated into different layers by virtue of differing densities, where the height of the interface point between liquid layers is of interest.

A wide variety of technologies exist to measure the level of substances in a vessel, each exploiting a different principle of physics. This chapter explores the major level-measurement technologies in current use.

 

Level gauges (sightglasses) - The level gauge, or sightglass is to liquid level measurement as manometers are to pressure measurement: a very simple and effective technology for direct visual indication of process level. In its simplest form, a level gauge is nothing more than a clear tube through which process liquid may be seen. Click here to read more...

Float - Perhaps the simplest form of solid or liquid level measurement is with a float: a device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest, and it must not corrode or otherwise react with the substance. Click here to read more...

Hydrostatic pressure - A vertical column of fluid exerts a pressure due to the column’s weight. The relationship between column height and fluid pressure at the bottom of the column is constant for any particular fluid (density) regardless of vessel width or shape. Click here to read more...

Displacement - Displacer level instruments exploit Archimedes’ Principle to detect liquid level by continuously measuring the weight of a rod immersed in the process liquid. As liquid level increases, the displacer rod experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a proportional output signal. Click here to read more...

Echo - A completely different way of measuring liquid level in vessels is to bounce a traveling wave off the surface of the liquid – typically from a location at the top of the vessel – using the time-of-flight for the waves as an indicator of distance, and therefore an indicator of liquid height inside the vessel. Echo-based level instruments enjoy the distinct advantage of immunity to changes in liquid density, a factor crucial to the accurate calibration of hydrostatic and displacement level instruments. In this regard, they are quite comparable with float-based level measurement systems. Click here to read more...

Weight - Weight-based level instruments sense process level in a vessel by directly measuring the weight of the vessel. If the vessel’s empty weight (tare weight) is known, process weight becomes a simple calculation of total weight minus tare weight. Obviously, weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement. Click here to read more...

Capacitive - Capacitive level instruments measure electrical capacitance of a conductive rod inserted vertically into a process vessel. As process level increases, capacitance increases between the rod and the vessel walls, causing the instrument to output a greater signal. Click here to read more...

Radiation - Certain types of nuclear radiation easily penetrates the walls of industrial vessels, but is attenuated by traveling through the bulk of material stored within those vessels. By placing a radioactive source on one side of the vessel and measuring the radiation making it through to the other side of the vessel, an approximate indication of level within that vessel may be obtained. Click here to read more...

Level sensor accessories - Disturbances in the liquid tend to complicate liquid level measurement. These disturbances may result from liquid introduced into a vessel above the liquid level (splashing into the liquid’s surface), the rotation of agitator paddles, and/or turbulent flows from mixing pumps. Click here to read more...

 

References

“Autolevel” Application Note AN 01C22A01-01E, Yokogawa Electric Corporation, 2006.

“Boiler Drum Level Transmitter Calibration”, application data sheet 00800-0100-3055, Rosemount, Inc., Chanhassen, MN, 2001.

Brumbi, Detlef, Fundamentals of Radar Technology for Level Gauging, 4th Edition, Krohne Messtechnik GmbH & Co. KG, Duisburg, Germany, 2003.

“Bubble Tube Installations For Liquid Level, Density, and Interface Measurements”, document MI 020-328, The Foxboro Company, Foxboro, MA, 1988.

“DOE Fundamentals Handbook, Instrumentation and Control, Volume 2 of 2”, document DOEHDBK-1013/2-92, U.S. Department of Energy, Washington, D.C., 1992.

Fribance, Austin E., Industrial Instrumentation Fundamentals, McGraw-Hill Book Company, New York, NY, 1962.

Kallen, Howard P., Handbook of Instrumentation and Controls, McGraw-Hill Book Company, Inc., New York, NY, 1961.

“Level Measurement Technology: Radar”, document 00816-0100-3209, revision AA, Rosemount, Inc., Chanhassen, MN, 1999.

Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Measurement and Analysis Volume I, Fourth Edition, CRC Press, New York, NY, 2003.

MacBeth, Michael, IAEA CANDU Instrumentation & Control Course, SNERDI, Shanghai, 1998. “Model 1151 Alphaline Pressure Transmitters”, product manual 00809-0100-4360, revision AA, Rosemount, Inc., Chanhassen, MN, 1997.

“Replacing Displacers with Guided Wave Radar”, technical note 3300 2 02 CA, Rosemount, Inc., Chanhassen, MN, 2003.

“The Art of Tank Gauging For Safety And Precision”, IN 4416.650, revision 6, Enraf B.V., The Netherlands.

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