Liquid level is a particularly important process variable for the maintenance of a stable plant operation. The operation of the Parshall flume needs the water surface elevation to be determined; for example, how are Ha and Hb determined? As may be deduced from Figure 3.6, they are measured by the float chambers labeled Ha and Hb, respectively. Liquid levels are measured by gages such as floats (as in the case of the Parshall flume), pressure cells or diaphragms, pneumatic tubes and other
devices that use capacitance probes and acoustic techniques. Figure 3.8 shows a float-gaging arrangement (a), a pressure cell (b), and a pneumatic tube sensing indicator (c).
As shown in the figure, float gaging is implemented using a float that rests on the surface of the liquid inside the float chamber. As water or wastewater enters the tank, the liquid level rises increasing the head. The increase in head causes the liquid to flow to the float chamber through the connecting pipe. The liquid level in the chamber then rises. This rise is sensed by the float which communicates with the liquid-level indicator. The indicator can be calibrated to read the liquid level in the tank directly.
In the pressure-cell measuring arrangement, a sensitive diaphragm is installed at the bottom of the tank. As the liquid enters the tank, the increase in head pushes against the diaphragm. The pressure is then communicated to the liquid-level indicator, which can be calibrated to read directly in terms of the level in the tank.
The pneumatic tubes shown in (c) relies on a continuous supply of air into the system. The air is purged into the bottom of the tank. As the liquid level in the tank rises, more pressure is needed to push the air into the bottom of the tank. Thus, the pressure required to push the air into the system is a measure of the liquid level in the tank. As shown in the figure, the indicator and recorder may be calibrated to read levels in the tank directly.
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