Liquid-level measurement differential pressure method

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

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. 

Differential Pressure. The differential pressure method measures the pressure of a vertical column of the fluid as well as the height of the column to obtain the density. This method has the advantages of relatively simple equipment, small component size, and the possibility of application as a field-type instrument. But it also has several disadvantages. The method is dependent upon two separate measurements, pressure differential and fluid liquid level. Errors in the accuracy of either of these two separate measurements will affect the overall method accuracy. Because of the extreme low density of liquid hydrogen, for instance, the accuracy, sensitivity, and hysteresis of the differential pressure measurement can be adversely affected. 

The most common method of measuring liquid level is to measure the differential pressure between the bottom of the vessel compared to the free vapor space above the liquid. A second technique is the bubble, tube method, wherein the pneumatic back pressure exerted on a bubble tube which bubbles gas through the liquid, is directly proportional to the liquid level. This method is not recommended for reactor pool water level, since the oxygen inside of the bubbles can be activated by the neutron flux. 

The proposed optimization strategy will replace the traditional method of controlling the release of Oz. Today, the rate of 02 released is controlled to maintain the d/p between the electrolyte chambers in order to limit the force that the separation diaphragm has to withstand. When the pressure differential is detected and controlled by conventional d/p cells, the measurement cannot be sensitive or accurate therefore, the diaphragm has to be strong, and the electrolyzer (or fuel cell) must be bulky and heavy. In this optimized design (if a liquid electrolyte design is selected), differential level control (ALC-12) will be used, which can control minute differentials. 

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