Liquid-level measurement

Measurement by Liquid Level. The flow rate of Hquids flowing in open channels is often measured by the use of weirs (see Liquid-LEVEL measurement). The most common type is the rectangular weir shown in Figure 22e. The flow rate across such a weir varies approximately with the quantity. Other shapes of weirs are also employed. Standard civil engineering handbooks describe the precautions necessary for constmcting and interpreting data from weirs. 

All advantages of the liquid level measurements can also be found by the on-line-weighing. Since mass differences are more easy measurable also in cases of little changes, this technique can be used successfully in laboratory applications. Some technical problems in the realization in case of an industrial application seem however solvable. 

Liquid helium Liquid hydrogen Liquid-inj ection-molded Liquid-mj ec tion moldmg Liquid inks Liquid invert Liquid level measurement... 

Rotameter m flow measurement [LIQUID LEVEL MEASUREMENT] (V ol 15)... 

A y-ray source (e.g. 137Cs, Co, 226Ra) is frequently used for liquid level measurement and for liquid and solid level detection. The absorption of the y radiation varies with the thickness x and nature of the absorbing material between the source and the detector according to the Beer-Lambert law, viz ... 

Liquid level measuring devices are classified into two groups (a) direct method, and (b) inferred method. An example of the direct method is the dipstick in your car which measures the height of the oil in the oil pan. An example of the inferred method is a pressure gauge at the bottom of a tank which measures the hydrostatic head pressure from the height of the liquid. 

The differential pressure (AP) detector method of liquid level measurement uses a AP detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the AP detector. Figure 9 illustrates a typical differential pressure detector attached to an open tank. 

Measurements of liquid density are closely related to quantity and liquid-level measurements since both are often required simultaneously to establish the mass contents of a tank, and the same physical principle may often be used for either measurement, since liquid-level detectors sense the steep density gradient at the liquid-vapor interface. Thus, the methods of density determination include the following techniques direct weighing, differential pressure, capacitance, optical, acoustic, and nuclear radiation attenuation. In general, the various liquid level principles apply to density measurement techniques as well. 

The case of nonhomogeneous fluids is quite different. LNG is often a mixture of five or more components whose composition and, hence, density vary with time and place. Accordingly, temperature and pressure measurements alone will not suffice. A dynamic, direct measurement is required, embodying one or more of the liquid-level principles used in liquid-level measurements. 

Remark 5.2. Industrial implementations of the distributed layer of this control structure would depend on sensor availability. Thus, the holdup of the reactor and the gas phase in the condenser would be stabilized by controlling the pressure in these vessels, while the liquid-level measurements would be used to estimate and control the liquid holdup. A potential process and instrumentation diagram (P ID) for this process is presented in Figure 5.9. 

Liquid level measurement may be accomphshed in several ways. The classical way is the use of a liquid level sensor, either based on conductivity or on electrical capacity. They can be supplied by all fermenter companies. Both are affected by foam. 

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