1510	CHAPTER 20. CONTINUOUS LEVEL MEASUREMENT

20.10Review of fundamental principles

Shown here is a partial listing of principles applied in the subject matter of this chapter, given for the purpose of expanding the reader’s view of this chapter’s concepts and of their general interrelationships with concepts elsewhere in the book. Your abilities as a problem-solver and as a life-long learner will be greatly enhanced by mastering the applications of these principles to a wide variety of topics, the more varied the better.

•Definition of pressure: P = FA (pressure is the amount of force applied over a specified area by a fluid.

•Pascal’s principle: changes in fluid pressure are transmitted evenly throughout an enclosed fluid volume. Relevant to pressure measurement, as fluid pressure in all parts of an enclosed system will experience the same changes in pressure.

•Hydrostatic pressure: fluids having substantial weight generate pressure proportional to their density and to their vertical height (P = γh and P = ρgh). Relevant to pressure o sets generated in vertical spans of impulse or capillary tubing, causing a pressure instrument to register more or less pressure than that at the process vessel connection.

•Archimedes’ principle: the buoyant force experienced by an object submerged in liquid is equal to the weight of the fluid that object displaces, which is equal to the volume displaced multiplied by the weight density of the fluid (Fbuoyant = γV ). Relevant to displacer-type instruments, which work by sensing the buoyant force exerted on an object as liquid rises around it.

•Time, velocity, and distance: x = vt, describing the relationship between velocity (v), time of travel (t), and distance traveled (x). Relevant to all types of “echo” level instruments, where travel time of a wave is used to measure distance.

•Transmission lines: short-duration (pulsed) electrical signals travel along a cable at nearly the speed of light, reflecting o of any discontinuity along the cable. Relevant to guidedwave radar level-sensing instruments, where the waveguide serves the same purpose as a transmission line, and the fluid-fluid boundary constitutes a discontinuity (sudden change in electrical permittivity) causing a reflected signal to arise.

• Ideal Gas Law: P V = nRT , used to calculate corrections to gas permittivity. Relevant to the “gas phase e ect” of radar level instruments.

•Capacitance: C = ǫAd , capacitance being proportional to the area of two overlapping conductors (A), the permittivity of the insulating (dielectric) substance between them (ǫ), and the distance (d) separating the conductors. Relevant to capacitive level sensing, where changes in liquid level alter the e ective area, permittivity, and/or distance.

20.10. REVIEW OF FUNDAMENTAL PRINCIPLES

1511

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 DOE- HDBK-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. et al., 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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