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Level measurement is the determination of the linear vertical distance between a reference point or datum plane and the surface of a liquid or the top of a pile of divided solids.
Liquid level measurement
Satisfactory measurements are possible only when the liquid is undisturbed by turbulence or wave action. When a liquid is too turbulent for the average level to be read, a baffle or stilling chamber is inserted in the tank or vessel to provide a satisfactory surface.
Stick, hook, and tape gages are used in open vessels where the surface of the liquid can readily be observed. The stick gage is a suitably divided vertical rod, or stick, anchored in the vessel so that the magnitude of the rise and fall of the liquid level may be observed directly. The hook gage provides a needle point, which is adjusted to produce a very tiny pimple in the liquid surface at the level reading, thereby minimizing the meniscus error. The tape gage reads the correct elevation when the point of a bob just touches the liquid surface.
Many forms of gage glass are available for the measurement of liquid level. Liquid in a tank or vessel is connected to the gage glass by a suitable fitting, and when the tank is under pressure the upper end of the glass must be connected to the tank vapor space. Thus the liquid rises to substantially the same height in the glass as in the tank, and this height is measured by suitable scale.
Various types of float mechanism are also used for liquid level measurement. The float, tape, and pulley gage provides an excellent method of measuring large changes in level with accuracy. It has the advantage that the scale can be placed for convenient reading at any point within a reasonable distance of the tank or vessel.
The change in buoyancy of a solid as its immersion in a liquid is varied is used to measure liquid level. This principle is used only when the densities of the liquid and vapor are substantially constant. Temperature changes will produce errors of greater magnitude than with the float mechanisms.
Hydrostatic head may also be used to measure liquid level. The pressure exerted by a column of liquid varies directly with its density as well as with its height, and thus this method of measurement requires that density be substantially constant. Densities of liquids vary with temperature; errors are therefore introduced with temperature changes, or the measuring element must be temperature compensated.
Electrode or probe systems are used in various forms for level indication and control. The number of electrodes and their design depend upon the characteristics of the liquid and the application. Fundamentally, a circuit through a relay coil is closed (or opened) when the liquid contacts a probe.
Capacitance-measuring devices can be used to measure levels of both dielectric (insulating) and conducting liquids. If the liquid being measured is a dielectric, one or two probes or rods, extending nearly to the bottom of the tank, are supported in an insulating mounting. The probes may be bare or covered with insulation. If the liquid is a conductor of electricity, only one probe is necessary but it must be covered with an insulating coating.
Nuclear level gages are used for difficult applications. Basically, all of the units involve a source of gamma (y) radiation and a detector separated by the vessel or a portion of the vessel in which a liquid level varies. As the level rises, the detector receives less y-radiation and thus the level is measured.
The sonic level detector is based on the time increment between the emission of a sound wave pulse and its reflection from liquid surface. The sound wave pulse is generated electronically, and its time in transit is measured very accurately by electronic means. If the speed of sound in the liquid or vapor is known accurately, the liquid level is known.
Solids level measurement:
Solids level detectors are used to locate the top of a pile of divided solids in large vessels or processing equipment. The instruments are designed for the different solids handled, and the installation must be carefully made to ensure proper measurements. Because solids funnel, cone, and vary in average density with the particle size, shape, distribution, moisture content, and other factors, these detectors provide only an approximate indication of the volume present or the top of the pile. Solids level detectors are classified as continuous or fixed point. Continuous detectors provide a continuous measurement of the level over the range for which they were designed. Their output is an analog representation of the level of the solids. Fixed-point detectors indicate when a specific level has been reached and are used mainly for actuating alarm signals. By installing a number of these, however, at different points, the combined response can be made to approach that of a continuous detector.
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