Guide · by mithillessh-garg
Plant air instrument air and breathing air: knowing which you need
Compressed air is classified by ISO 8573 on 3 contaminants: particulates, water and oil, each given its own class number. Plant air tolerates a great deal, instrument air needs a low dew point and effectively no oil, and breathing air is a different product entirely with its own testing regime.
- Classification standard
- ISO 8573
- Contaminants classified
- Particulates
- Instrument air requirement
- Low pressure dew point and effectively oil-free
- Breathing air
- A separate product with its own testing regime
- Common failure
- Dew point above the coldest point in the line
Air is not one product
“Compressed air” describes a utility the way “water” does. The air driving an impact wrench and the air operating a control valve on a process line are not interchangeable, and the air a person breathes through a mask is a different product altogether.
ISO 8573 is how that gets specified rather than argued about.
Three contaminants, three numbers
ISO 8573 classifies compressed air on three contaminants separately, each with its own class number:
| What it covers | Why it matters | |
|---|---|---|
| Particulates | Solid contamination, by size and count | Wears valves and seals, blocks orifices |
| Water | Expressed as pressure dew point | Corrosion, freezing, product contamination |
| Oil | Aerosol, liquid and vapour | Ruins instruments, contaminates product |
A specification is therefore a triplet, not a single grade. Quoting one figure tells you almost nothing, because a system can be excellent on oil and poor on moisture at the same time — and usually is.
What each application needs
Plant air. Tools, clamps, blowguns, general pneumatics. Tolerant of moisture, oil carry-over and particulates. This is what most compressor houses produce and it is entirely adequate for the duty.
Instrument air. Control valves, positioners, analysers, actuators. These are precision devices that a trace of oil gums and liquid water destroys. Needs a low pressure dew point and effectively no oil. Upgrading plant air to instrument air is usually a matter of drying and filtration, and it is not optional if you want the instruments to hold calibration.
Breathing air. A separate product, with its own specification and testing regime covering carbon monoxide, carbon dioxide, oil and odour, and its own supply arrangements. Plant air is never acceptable for breathing — compressor lubricant carry-over alone rules it out, before you consider what the intake is drawing in.
The mistake almost everyone makes
Dew point measured at the wrong place.
Pressure dew point is the temperature at which water condenses out of the compressed air. If that dew point is above the coldest temperature the line ever reaches, liquid water appears inside the pipe.
The coldest point is rarely the compressor house. It is:
- the run that goes outside across a yard
- the cold morning in January that nobody sized for
- the point of expansion at a valve or tool, where the air cools as it does work
Size the drying to the coldest point in the system, in the coldest month, and the water problem goes away. Size it to the compressor room in August and it will not.
Where compressed gas cylinders fit
A compressor is the right answer for volume. Cylinder-filled compressed air answers a different question: a defined, certified quality in a controlled quantity — for calibration, for portable supply, or as a backup where a compressor failure would stop production.
Most sites need both, and the ones that have both are usually using each correctly without having thought about why.
Air as feedstock
Compressed air is also the raw material for on-site gas generation. A PSA nitrogen plant or PSA oxygen plant separates air into its components using a molecular sieve — which means the air quality feeding it is part of the plant’s specification, not a detail. Oil and moisture reaching the sieve shorten its life considerably, and sieve replacement is not a small item.
Sources
Plant air — frequently asked questions
What do the ISO 8573 class numbers mean?
They are three separate numbers, one each for particulates, water and oil, and they are quoted together — so a specification looks like a triplet rather than a single grade. A lower number is cleaner. Quoting one number alone is meaningless, because a system can be excellent on oil and poor on moisture at the same time.
What is the difference between plant air and instrument air?
Plant air drives tools, clamps, blowguns and general pneumatics, and tolerates moisture, oil carry-over and particulates. Instrument air operates control valves, positioners and analysers, which are precision devices ruined by oil and by liquid water — so it needs a much lower dew point and effectively no oil at all.
Why does dew point matter so much?
Because dew point is the temperature at which water condenses out of the air. If the pressure dew point is above the coldest temperature the line ever reaches — a run outside, a cold morning, expansion at a valve — liquid water appears inside the pipe. Then you get corrosion, contaminated product, frozen valves in winter and instruments reading nonsense.
Can I use plant air for breathing?
No, under any circumstances. Breathing air is a separate product with its own specification and testing regime, covering carbon monoxide, carbon dioxide, oil and odour, and it requires a supply system built and maintained for that purpose. Compressor lubricant carry-over alone makes ordinary plant air unfit to breathe.
Is oil-free compressed air really oil-free?
An oil-free compressor removes the lubricant from the compression chamber, which is the main source. It does not deal with hydrocarbons already present in the air being drawn in, which is why intake location matters and why oil-vapour filtration is still specified where the class demands it. Read the specification rather than the marketing term.
Should I use cylinders or a compressor?
A compressor for volume duty, cylinders where you need a defined, certified air quality in a controlled quantity — calibration, portable supply, or as a backup where a compressor failure would stop production. They solve different problems and most sites with both are using each correctly.
Enquiries
Request a quotation
State the gas or equipment, the quantity or capacity, and your location. KIGL will confirm the grade, the cylinder or plant size, and whether the location can be served from Noonmati, or through our agent network.