# Basics of Pressure Measurement

Since level can be determined by pressure, or head, many pressure measuring devices are used for indicating level.

Hydrostatic Pressure

A liquid at rest in a vessel exerts a pressure on the walls of the vessel. At any given point the pressure on the wall of the vessel is proportional to the vertical distance between that point and the surface of the liquid, and varies with the height of the liquid. The relationship between the weight produced by the vertical height of a column of water and the pressure exerted on the supporting surfaces of the vessel can be used to determine level. The relationship between pressure and level makes it possible to convert hydrostatic measurements directly to level in feet or inches. In the following equations, “WC” stands for water column and is usually omitted from equations as understood in discussions of hydrostatic pressure.

1 lb./in.2 = 2.31 feet water

= 27.7 inches water (WC)

1 psi = 2.31 feet

= 27.7 inches

If level is to be determined and indicated by measuring pressure, the specific gravity of the liquid must be known. The specific gravity of water is 1.00. If the liquid has a lower specific gravity, the pressure exerted by the column of liquid will be less than that exerted by a column of water of the same height. For liquids with a specific gravity greater than 1.00, the pressure exerted by the column of liquid will be greater. To compensate for the difference in specific gravity, the following equation is used:

h = (p (2.31 ft.)) / G

where:

h = height in feet

p = pressure

G = specific gravity

Diaphragm Box

The diaphragm box is submerged in the process liquid and connected to a pressure gage by a gage line. The hydrostatic head produced by the level of the liquid in the tank exerts pressure on the bottom of the diaphragm causing it to flex upward. This action compresses the gas in the box and the gage line. The pressure is applied to a gage or other pressure element that is part of an indicator assembly calibrated to indicate liquid level units.

Air-Trap Sensors

As the liquid level rises, the hydrostatic head forces liquid up into an air trap sensor, or inverted bell. As the level of the liquid rises, it compresses the air trapped in the bell and the gage line until an equilibrium between the air pressure and the pressure exerted by the hydrostatic head is reached.

Air Bubble or Surge Tube

Known by various names, including an air bubble, a surge tube, an air purge and a dip tube, this type of system uses a continuous air supply that is connected to a tube that extends into the tank to a point that represents the minimum level line. An air regulator controls the air flow. It increases air flow to the tube until all liquid is forced from the tube. At this pressure and flow rate, the air begins to bubble out of the bottom of the tube. This indicates that the air pressure forcing the liquid out of the tube is equal to the hydrostatic head produced by the height of the process liquid being forced into the tube. The air pressure acting against the hydrostatic head provides the pressure indication to the gage.

This is most useful for applications such as underground tanks and water wells. However, as with other hydrostatic pressure systems, the major limitation of these systems is that they are generally limited to open-tank applications.

Closed-Tank Applications

In open tanks, measurements are referenced to atmospheric pressure. At atmospheric pressure, the pressure on the surface of the liquid is equal to the pressure on the reference side of the pressure element in the measuring instrument. When atmospheric pressure changes, the change is equal on both the surface of the liquid and the reference side of the measuring element. To compensate for the effects on level measurement caused by such pressure variations in closed-tank applications, a differential pressure (d/p) cell is often used to measure and indicate level. The d/p cell only responds to differences in pressure applied to two measuring taps. One pressure tap is the measuring point on the tank, which is usually below the minimum level point for the liquid. The other tap is usually located near the top of the tank. The tap in the liquid region of the tank is referred to as the high-side; the other tap, located above the level of the liquid, is referred to as the low-side. System pressure is sensed by both the high and low sides. In addition to system pressure, the high side also senses the pressure exerted by the height of the liquid. Since both sides are exposed to the same system pressure, the effects of system pressure are canceled and the differential pressure cell only indicates liquid level.

An instrument can be calibrated to compensate for the additional static pressure created by the condensed liquid. This compensation or adjustment is called zero elevation. Other means are also available to eliminate inaccuracies due to wet leg problems. For instance, in what is referred to as a wet-leg installation, the low pressure leg is deliberately filled with liquid. Another method involves the use of a device called a pressure repeater or one-to-one relay. The repeater is installed at the top of the tank and linked by pipe to an air relay. The pressure in the tank actuates the air relay, which is connected to an air supply. When the pressure in the tank increases, the relay increases the air pressure on the low-pressure leg. The relay regulates the air pressure so that it is equal to that of the tank pressure. When the pressure in the tank decreases, the relay vents air from the low pressure leg to maintain the equilibrium. Zero suppression, is the correction adjustment required to compensate for error caused by the mounting position of the instrument with respect to the level measurement reference.