Because the SI permits some choice of alternative units which could be confusiing, the Compressed Air Industry has decided to standardize on certain units as shown below:
| Description | Units | Notes |
|---|
| Linear | metres (m) | above 1000 mm |
| | millimetres (mm) | below 1000 mm |
| Volume (see Note 1) | m3 | above 1000 litres |
| litres | L=decimetres (dm3) | below 1000 litres |
| Receiver capacity | m3 | above 1000 litres |
| | L | below 1000 litres |
| Velocity (gas or piston speed) | metres/second | |
| Rate of air flow (see Note 2) | m3/s or m3/h | large compressors |
| | L/s or m3/h | smaller compressors |
| | dm3/s | pneumatic control equipment |
| Speed of rotation | rpm | r/s is the consistent unit but rpm is well established in practice |
| Lifiting capacity | kg or t | t = tonne = 1000kg being more practical than the theoretically correct unit the Newton (N) |
| Pressure (see Note 3) | bar | 1 bar = 100 kPa |
| Torque | N.m | |
| Work/energy/quantity of heat | Joule (J) | |
| Power | W or kW | The use of bhp is obsolete and is replaced by the wording 'brake power' |
| Specific power consumption | kW s/m3 = J/dm3 | |
| Gauge temperature | degrees C | 1°C = K - 273.16 |
| Kinematic viscosity | cSt | 1 cSt = 10-6 m2/s |
Notes:
1. The compressor and air tool manufacturers prefer litres; pneumatc equipment manufacturers prefer dm3.
2. Normally quoted as free air at specified conditions. In the case of air compresors the conditions are usually those prevailing at the compressor inlet. For pneumatic tools and control equipment the conditions are those of a standard reference atmosphere (ANR). Use of the letters A.N.R. (Atmosphére Normale de Référence) after the rate of flow indicates that the flow is free air at standard reference atmosphere conditions.
3. Unless qualified otherwise, pressures in bar are assumed to be gauge pressures. It should be noted that all British and European regulations concerned with pressure and safety use the bar and not the kPa or Mpa as the unit of pressure.
4. For further information on SI units see ISO 1000, BS 5555 and CETOP RP 71. Also the BSI publication PD 5686: 1978.
