Instrument Air Quality Standard (ANSI/ISA-S7.0.01-1996)

Instrument air quality standard establishes four elements of instrument air quality for use in pneumatic instruments
1. Pressure Dew point
The pressure dew point as measured at the dew point outlet shall be atleast 10 deg C (18 deg F) below the minimum temperature to which any apart of the instrument air system is exposed.The pressure dew point shall not exceed 4 deg C (39 deg F) at line pressure.A monitored alarm is preferred . However if a monitored alarm is not available , then per shift monitoring is recommended. ( for explanation of "Dew point" see Appendix A below)
2. Particle size
A maximum 40 micrometer particle size in the instrument air system is acceptable for the majority of pneumatic devices.Pneumatic devices that require instrument air with less than 40 micrometer particle size shall have additional filtration to meet the particulate size limit for the device.
Subsequent to any maintenence or modification of the air system , maximum particle size in the instrument air system should be verified to be less than 40 micrometers.
3. Lubricant Content
The lubricant content should be as close to zero as possible, and under no circumstances shall it exceed 1 ppm w/w or v/v.Any lubricant in the compressed air system shall be evaluated for compatibility with end-use pneumatic devices. For example , the use of automatic oilers is strongly discouraged.
4. Contaminants
Instrument air should be free of corrosive contaminants and hazardous gases , which could be drawn in the instrument air supply .The air supply intake should be monitored for contaminants. If contaminants exists in the compressor intake area , the intake should be moved to a different elevation or location free from contamination.Some sources of contamination are
a. painting
b. chemical cleaning
c. engine exhaust

Appendix A
ISA-S7.0.01 establishes a maximum pressure dew point to protect the instrument air systems from the presence of moisture.
Compression and cooling stages in an instrument air system causes condensation.Compression increases the partial pressure of the water vapour present.If the water vapour partial pressure is increased to the saturation water vapour pressure , condensation occurs.Cooling reduces the saturation water vapour pressure , a temperature dependent variable .If the saturation water vapour pressure is reduced to the partial pressure of the water vapour present , water or ice will result.Therefore, moisture removal is a major consideration of instrument air treatment systems.
The most common methods of moisture removal are compression cooling , absorption, chemical methods, mechanical seperation, and combination of these methods.

Note : This is only a small extract of ISA standard ANSI/ISA-S7.0.01-1996.For complete information please refer the ISA document mentioned.