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# 07___Level - Introduction Sensing liquid levels fall into...

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Introduction 1 Sensing liquid levels fall into two categories; firstly, single point sensing and secondly, continuous level monitoring. In the case of singlepoint sensing the actual level of the material is detected when it reaches a predetermined level, so that the appropriate action can be taken to prevent overflowing or to refill the container. 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 calculated if the cross-sectional area of the container is known. Level measurements can be direct or indirect; examples of these are using a float technique or measuring pressure and calculating the liquid level .

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Basic equations h g p = . . ρ 2 Pressure is often used as an indirect method of measuring liquid levels. Pressure increases as the depth increases in a fluid. The pressure is given by; p : change in pressure ρ : density h : change in depth Capacitive probes can be used in nonconductive liquids and free flowing solids for level measurement. Many materials, when placed between the plates of a capacitor, increase the capacitance by a factor μ called the dielectric constant of the material. For instance, air has a dielectric constant of 1 and water 80.
Basic equations α α μ C r d C C d + = 3 Two capacitor plates partially immersed in a nonconductive liquid is shown in the figure. The capacitance ( C d ) is given by; C α : capacitance with no liquid μ : dielectric constant of the liquid between the plates r : height of the plates d : depth or level of the liquid between the plates

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07___Level - Introduction Sensing liquid levels fall into...

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