PRESSURE

## PRESSURE

Pressure is defined as the force applied over a unit area, and can be expressed in the following formula:

Where P = Pressure measured in Pascal (Pa)

F = Force measured in Newton (N)

A = Area measured in m2

We can see from the equation above, that the greater the area over which a force acts, the less is the pressure and vice versa.

Example: If a 4000 Newton block is placed on a table surface measuring 0.5m2, the pressure could be calculated as:

### P = F / A

4000 / 0.5 = 8000 Pa = 8kPa

#### Types of Pressure Measurements

There are four types of pressure measurement:

1. Atmospheric Pressure: Although we do not feel it, we are subjected to the pressure of earth’s atmosphere. We use sea level as a common reference point, where the air exerts a pressure of 14.7 psi.
1. Gauge Pressure: Gauge pressure uses atmospheric pressure as the starting point for all measurements. If, for example, a gauge showing 50 psi, this means 50 psi more than atmospheric pressure.

Most pressures measured in industry are referred to as gauge pressure. Gauge pressure is shortened as psig.

1. Absolute Pressure: Absolute pressure scales use AbsoluteZero as its starting point. Absolute pressure is the complete absence of pressure including atmospheric pressure. Absolute pressure is shortened as psia.

Note: An unconnected absolute pressure gauge would read 14.7 psia at sea level.

Absolute pressure is equal to gauge pressure plus atmospheric pressure.

###### Absolute pressure = gauge pressure + atmospheric pressure

Absolute pressure = gauge pressure + 14.7 psi

In other words, to find absolute pressure, add 14.7 psi to a pressure gauge reading.

1. Vacuum Pressure: This pressure is measured below atmospheric pressure. It is gauge pressure that is less than atmospheric.

Vacuum pressure is expressed in inches (or mm) of mercury.

Hg is the symbol for mercury.

Note: At sea level, atmospheric pressure = 29.92 in Hg or 760mm Hg

For example, if a vacuum pressure is 10 in Hg, this means that the pressure has been reduced below atmospheric pressure by 10 in Hg. Since atmospheric pressure = 29.92, a vacuum of 10 in Hg would be equivalent to a pressure of 10.0 in Hg.

##### Methods of Pressure Measurements

There are several methods used to measure pressure in industry.

1. Manometer
2. Bourdon tube
1. Manometer: The basic manometer consists of a uniform glass tube, bent in the shape of a letterU. It is partly filled with a liquid (usually mercury) and both ends are left open to atmosphere as shown below.

###### Basic Manometer

The simple U-tube manometer is very cheap and quite accurate and is used to compare a pressure with atmospheric pressure. In figure (A) below both end of the manometer are open to the atmosphere and so the height of the mercury in both sides of the manometer are the same.

In figure (B), one end is connected to 20 psi input, and the other side is left open. Since the input pressure is more than the atmospheric pressure, the mercury level has risen to the side of the open end.

In figure (C), the input pressure is less than atmospheric. Therefore the atmospheric pressure raises the mercury level in the 8-psi side.

Manometer can also measure the difference between to working pressures. If one pressure is connected to one end of the manometer and the other pressure is connected to the other side, the difference in the height of the mercury in the two sides is the differential pressure.

Differential pressure: is the difference between two different pressures.

1. Bourdon Tube: The bourdon tube is a flat oval shaped tube that is bent to form part of a circle. The bourdon tube is a pressure-sensing device. It changes its shape (stretches out) when pressure is applied to it. The movement of the tube is almost proportional with the applied pressure. The total movement of the free end is very small, about 3/8 of an inch. The figure below shows the Bourdon tube as part of a pressure gauge. The tip of the tube is connected to a gear mechanism, lever and a pointer. When pressure is applied to the gauge, the Bourdon tube moves. This movement causes the gear mechanism to change the position of the pointer on the scale.

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