TABLE OF CONTENTS Pneumatic drives use compressed air [1] to store and transmit power or signals. Its properties are therefore significant for the behaviour of the drives and a good mathematical model is needed for reliable numerical analysis and simulation.
Clean, dry air is a mechanical mixture of approximately 78 % by volume nitrogen and 21 % oxygen. The remaining 1 % consists of minor quantities of some fourteen other gases. The composition of air remains substantially the same between sea level and an altitude of about 20 km, but its density varies with pressure and temperature. At standard technical reference conditions, with a pressure of 10 5 Pa, a temperature of 20 C and a relative humidity of 65 65 % the density of air is 1.185 kg/m 3. At this pressure and temperature, 1 kg of air has a therefore a volume of 0.844 m 3.
At standard temperature and pressure, the mean velocity of gas molecules is of the order of 500 m/s, with a mean free path between intermolecular collision of the order of 10 -7 to 10 -8m. The rate of collision is responsible for the pressure exerted by air.
The effect of a change in temperature is to modify the value of the mean velocity. The resultant pressure therefore varies with the temperature. Similarly, any change in volume or mass effects the pressure. Thus pressure, temperature and volume are interrelated.
For all following derivations the assumption is made that the...
