PNEUMATIC TECHNOLOGY

The atmosphere of earth exerts a pressure on its surface. As a standard, measured at sea level, this pressure can be described as a force per unit of measurement. The metric unit of pressure is the Pascal (Pa).

1 Pa = 1 N/m" (Newton per square meter)

This unit is extremely small and so, to avoid huge numbers in practice, an agreement has been made to use the bar (approximately the force exerted by the atmosphere at sea level) as a unit of 100,000 Pa.

It corresponds with sufficient accuracy for practical purposes with the old metric unit kgf/cm2. More precise equivalents are 1 STD atm =14.696 psi =1.01325 bar =1.03323 kgf/cm2. In English units pressure is expressed in psi (almost never referred to as p.s.i. as one would expect), or pounds per square inch, also relating a force to an area.

1 MPa =10 bar or 145 psig

Fig. 3.4 the various systems of pressure indication

A pressure in the context of pneumatics is assumed as over-pressure i.e. above atmospheric pressure and is commonly referred to as gauge (also gage) pressure (GA or psig). A pressure can also be expressed as absolute pressure (ABS or psia) i.e. a pressure relative to a full vacuum. In vacuum technology a pressure below atmospheric i.e. under pressure is used. The various ways of indicating pressure are illustrated in fig 3.4, using a standard atmospheric pressure of 1013 m/bar as a reference. Note that this (the weight of the atmosphere) is not 1 bar, although for normal pneumatic calculations the difference can be ignored.

PRESSURE AND FLOW
The most important relationship for pneumatics is that between pressure and flow.
THEY ARE NOT THE SAME. DO NOT THINK THEY ARE INTERCHANGEABLE TERMS... e.g. a flow control is not a regulator (repeat as required until retained). It is the relationship between flow and pressure that we will now consider.
If there is no flow, the pressure in an entire system is the same at every point, but when there is flow from one point to another, the pressure in the latter will always be lower that at the first. This difference is called pressure drop. It depends on three values:
• initial pressure
• volume of flow
• flow resistance of the connection

The flow resistance for air has no unit; in electricity its equivalent is Ohm (Q). In pneumatics, the opposite of resistance is used, conductance. The ability to allow flow can be defined as the equivalent flow section S. The equivalent flow section S is expressed in mm2 and represents the area of an orifice in a thin plate (diaphragm) tha creates the same relationship between pressures and flow as the element defined by it. Other standards include the kv or C^ factor (a dimensionless number referring to a flow coefficient) — consider Cy as a conductance value. Valves have complicated orifice shapes, therefore the flow rate through the device is measured first (along with pressure drop, temperature, etc), and then the device may be assigned the corresponding equivalent flow section. An easy approximation would be that:
C^ of 1 = 18 S mm2, e.g. equivalent orifice of 18 mm2 equals the flow of a Cy 1.

This relationship is by definition the same as in electricity, where "voltage drop equals current times resistance" This can be transformed for pneumatics to "pressure drop equals flow divided by Flow Section", only, while the electric units are directly proportional, the relationship for air is very complex and never simply proportional. In electricity, a current of 1 A (one Ampere) creates over a resistor of 1 Ohm a voltage drop of 1 Volt. Regardless if this drop is from 100 to 99 or from 4 to 3 volts, the pressure drop over the same object and with the same standan volume flow varies with the initial pressure and also with the temperature.

For defining one of the four interrelated data, mentioned previously, from the other three, we require a diagram

Fig. 3.9 Diagram showing the relationship between pressure and flow for an orifice with an equivalent Flow Section of 1 mm2

The triangle in the lower right corner marks the range of "sonic flow speed". When the airflow reaches a speed close to the speed of sound* flow can no longer increase — whatever the difference of pressure between input and output might be. As you can see, all the curves drop vertically inside this triangle. This means that the flow no longer depends on the pressure drop, but only on the input pressure.

Use of the diagram:
The pressure scale at the left side indicates both input and output pressure. At the first vertical line on the left, which represents a zero flow, input and output pressures are the same. The various curves, for input pressures from 1 to 10 bar, indicate how the output pressure decreases with increasing flow.

* Sound is, after all, vibrating air molecules. Thus the "speed of sound" (sonic condition, Mach #) is the terminal velocity for air movement. For compressed air to flow there must be a pressure drop — and maximum flow occurs at a certain % pressure drop. There can be a greater pressure drop (up to 100%) but maximum flow (for whatever size orifice) occurs at 46% of pi.