At first it may be difficult to comprehend how pressure can be used to measure the level of a liquid. But if one realizes that a column of liquid that is 1 inch square at its base will cause a pressure at the bottom of the column that is proportional to the height of the column, one can see how pressure can be converted to height. What this means is that the deeper a liquid becomes, the greater the pressure becomes at the bottom of the tank or vessel that holds the liquid. The pressure can be measured and formulas are available to calculate the depth or, if you prefer, the height of the liquid. This pressure is called head pressure or hydrostatic head pressure. The exact pressure at the bottom of the tank will be determined by the depth of the liquid, the specific gravity (density) of the liquid, and the atmospheric pressure pushing down on the liquid. The effects of atmospheric pressure can be equalized by using a differential pressure sensor (Δ P sensor) as explained previously. Since one side of the Δ P sensor is open to atmospheric pressure, it will not matter what the atmospheric pressure is that pushes down on the liquid because the same pressure is pressing on the open side of the pressure differential sensor.
The concept of determining the level of liquid in a tank by calculating
its head pressure may best be explained by calculating the pressure that
a column of water exerts on the bottom of a tank when the level of water
in the tank is, say, 50 ft deep. From pressure conversion tables it's known that water exerts 0.434 psi per each foot of water column. This
means that if the water is 1 ft deep in the tank, the pressure at the
bottom of the tank would be 0.434 psi. The formula for determining the
total amount of head pressure is:
P = depth (ft) X pressure of 1 ft of water (psi)
Exercise 1:
Find the pressure at the bottom of a tank that is 50 feet deep.
Ptotal = D X Pat 1 ft
P = 50 x 0.434 P = 21.7 psi
Answer:
The formula for calculating the height of a column of water can be determined by changing the formula to solve for depth. It should be understood that the depth of the water would be equal to the height of the column of water.
D = Ptotal/(pressure of 1 ft of water)
Exercise 2:
Determine the level of water in a tank if the pressure at the bottom of a tank of water is 21.7 psi. (Do not account for atmospheric pressure because an Δ P sensor is being used to measure the pressure.)
Answer:
D = Ptotal/(pressure of 1 ft of water)
D = 21.7 psi/0.434 psi per ft
D = 50 ft
Figs. 1 and 2 show examples of differential pressure sensors used to measure liquid level in various applications. The pressure sensor in this type of instrument uses strain-gauge technology which is integrated into a flexible diaphragm.
This is a good example of where this type of sensor is used to measure the level of liquid in a tank, but one must engineer or troubleshoot the sensor as if it's a pressure transducer and strain gauge. This is typical of many sensors that use one type of technology to measure one parameter and convert it to another.
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Related Articles:
Above: ill. 1: A pressure differential sensor
that determines the level of liquid by using the pressure of a column
of liquid to calculate the level.
Above: Fig 2. (a) A Δ-P level sensor used to measure the level in
a buried tank. (b) An application where the delta-P sensor can be mounted
in the top of a tank or in the side of a tank to determine the level of
liquid in the tank.