## Air Pressure

### From DT Online

**Pressure** is measured as the Force per Unit Area. In the case of **Atmospheric Pressure** at sea level for example, the standard unit of measurement is **1 ****Bar** which is equivalent to 100,000 **Newtons** of force on each square metre of surface area or, in **SI Units**, 1 Bar = 100KNm-2 *(kilonewtons per square metre)*. Alternatively, 1 Bar = 100KP, since a Nm^{2} = a pascal *(named after the French inventor Blaise Pascal)* or, in

**Imperial Units**, 14.5 pounds per square inch

*(PSI)*.

Atmospheric Pressure is an * absolute pressure* measurement because a perfect vacuum is taken as the zero value

If atmospheric pressure is used as a zero-reference, the pressure measurement is called * gauge pressure* and is used for

**pressure measurement**of closed systems, such as a tyre pressure or within a pneumatic system.

It follows that * absolute pressure* is equal to

*plus*

**gauge pressure****Atmospheric Pressure**.

- Force = Pressure x Area

Force produced by cylinder*(N)* = Pressure*(Nmm ^{2})* x Area of Piston

*(mm*

^{2})*But:*Area = π x Radius^{2}= π x Diameter^{2}÷ 4

Therefore:

- Force = Pressure x 3.14 x Diameter
^{2}÷ 4

- Force = Pressure x 3.14 x Diameter

In practice, losses due to friction, escaping air, etc., mean that the full amount of force calculated will not be available at the output of the piston, or * outstroke* and on the

*of a double-acting cylinder, the surface area is reduced because of the piston rod. This means that the compressed air does not have as big an area to push against and so it does not produce as big a force. The resulting surface area, or*

**instroke***is found subtracting the area of the piston rod from the surface area of the piston - i.e.*

**effective area***Effective area = piston area – piston rod area*