TABLE OF CONTENTS This chapter discusses the measurement of the level of liquids and free-flowing solids. There are many widely varying methods for the measurement of liquid level. Level measurement is an important part of process control. Level sensing can be single point, continuous, direct, or indirect. Continuous level monitoring measures the level of the liquid on an uninterrupted basis. In this case, the level of the material will be constantly monitored, and hence the volume can be continuously monitored, if the cross-sectional area of the container is known.
Examples of direct and indirect level measurements are using a float technique, or measuring pressure and calculating the liquid level. Accurate level measurement techniques have been developed. New measurement techniques are constantly being introduced and old ones improved [1]. Level measuring devices should have easy access for inspection, maintenance, and replacement. Free-flowing solids include dry powders, crystals, rice, grain, and so forth.
[1]Harrelson, D., and J. Rowe, "Multivariable Transmitters, A New Approach for Liquid Level," ISA Expo. 2004.
Level sensing devices can be divided into four categories: (1) direct sensing, in which the actual level is monitored; (2) indirect sensing, in which a property of the liquid, such as pressure, is sensed to determine the liquid level; (3) single point measurement, in which it is only necessary to detect the presence or absence of a liquid at a specific level; and (4) free-flowing solid level sensing [2].
A number of techniques...
