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MEASUREMENT AND INSTRUMENTATION

FLOAT

107

PERMANENT MAGNET

HERMETICALLY SEALED MAGNETIC REED SWITCH

FIGURE 1 How a typical float switch works.

FIGURE 2 Typical float switches.

Level Detection and Measurement of Liquids by Use of Floats Level detection of liquids is often done with a float-type level switch. The float moves on a mechanical arm or sliding pole and activates a switch when the level moves it upward. Sometimes the floats themselves contain a small magnet that changes the state of a reed switch when the liquid level moving up moves it into position (Figs. 1 and 2). The simplest form of a float system for level measurement consists of a float, a small cable, two pulleys, and a weight that is suspended on the outside of the open tank. A scale is mounted on the outside of the tank, and the level of the tank contents is indicated by the position of the weight along the scale. In view of the requirements regarding mounting in sealed tanks, reading, and accuracy, the current industrial systems that use the float method are good examples of mechanical and electronic engineering, making them the most accurate level measuring systems for applications in very large storage tanks. The advantages of this method are that it is relatively simple, suitable for various products, and very accurate. The disadvantages are that it requires a certain amount of mechanical equipment, especially in pressure vessels. Level Detection and Measurement of Liquids by Use of Displacers The displacement method is based on the difference between the weight of the displacement body and the upward force exerted by the medium on this body. The upward force depends on the volume of the displacement body, the relative density of the medium, and the level of the medium. For given volumes and relative densities, the upward force will depend on only the level of the medium. Obviously the displacement body must be heavier than the medium to be displaced.

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Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

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