A level measuring system employing the differential pressure method with a strai

Question

A level measuring system employing the differential pressure method with a strain gauge pressure sensor is used to measure the level of liquid of which range is 0 to 1.0 m in an open tank as shown in Figure 5. The level transmitter output should be a current signal in the range of 4 mA to 20 mA. Explain the operating principle of the level measuring system and describe how to install it in the tank. You may want to use formulae and simple sketches to illustrate your answers. If the reading of the level transmitter is 16 mA. what is the level in metre? What is the differential pressure that the level transmitter measures? It is assumed that the liquid density is 950 kg/m^3 and the acceleration of gravity is 9.81 m/s^2.

Explanation / Answer

The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically, we have:

P = SH
where
P = Pressure (Pa)
S = Weight density of the liquid (N/m3) = g
H = Height of liquid column (m)
= Density (kg/m3)
g = acceleration due to gravity (9.81 m/s2)

but in case of open tank

Phigh = Patm + SH
Plow = Patm
Differential pressure P = Phigh - Plow = SH
The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level

For each 0.0625 m of the the tank there is 1 mA current output of the transmisster hence at 20mA there would be 1m (Full tank).

(b)
the level on meter would be 0.75m in case of 16mA at output

Differential pressure can be calculated using above formula (P=S.H)

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