ISO 8573-1 classifies compressed air purity by particles, water and oil. For water, classes 1 to 6 correspond to pressure dew points of minus 70, minus 40, minus 20, plus 3, plus 7 and plus 10 C or lower. Refrigerant dryers reach about plus 3 C; desiccant dryers reach minus 40 C or lower. A dew point analyser after the dryer catches dryer failure before moisture reaches the process.
- State the ISO 8573-1 water classes and their dew points
- Compare refrigerant and desiccant dryers
- Place a dew point analyser
- Explain the pressure dew point
ISO 8573-1: three contaminants
The standard classifies compressed air by solid particles, water and total oil, with a class number for each. A specification such as 1.2.1 gives the class for particles, water and oil. Class 0 means a stricter value agreed with the user.
Water classes
| Class | Pressure dew point, at or below |
|---|---|
| 1 | minus 70 C |
| 2 | minus 40 C |
| 3 | minus 20 C |
| 4 | plus 3 C |
| 5 | plus 7 C |
| 6 | plus 10 C |
Always a pressure dew point: the dew point at line pressure. The same air at atmospheric pressure has a much lower dew point: Dew point to ppm calculator for gases.
Dryers
- Refrigerant dryers cool air to about plus 3 C and drain the water; they suit general plant air (class 4).
- Desiccant dryers pass air through an adsorbent that holds water, reaching minus 40 C (class 2) or minus 70 C (class 1); they suit instrument air, pharma, electronics and outdoor lines in cold climates.
- Membrane dryers remove water by diffusion for small flows.
Where to measure
- After the dryer, to confirm the class and catch failure of a desiccant bed or a refrigerant dryer.
- At the point of use for critical processes, since pipework can add moisture.
- On instrument air, since a wet instrument line can freeze or corrode.
ACCURRA-S M015 reads dew point from minus 80 to plus 20 C with a ceramic sensor, per the catalogue, at line pressure. It is supplied without a pump and has no auto-calibration, so it suits a pressurised line: Compressed-air and instrument-air dew point (ISO 8573).
Good habits
Use stainless steel sample lines, short, with no dead legs: Sample lines: materials, length, diameter and lag. State whether a dew point is at line pressure. Check the dew point against the class limit on a regular basis, and alarm when it rises: Moisture measurement methods compared for dry gas.
- ISO 8573-1 water classes 1 to 6 run from minus 70 C to plus 10 C pressure dew point.
- Refrigerant dryers reach about plus 3 C; desiccant dryers minus 40 C or lower.
- Measure after the dryer and at critical points of use.
- Quote pressure dew point at line pressure.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What pressure dew point does water class 2 require?
Minus 40 C or lower.
Why use a dew point analyser after the dryer?
To catch dryer failure before moisture reaches the process.
Why must dew point be stated at a pressure?
At lower pressure the same air has a much lower dew point; the numbers differ by many degrees.
Key terms
Frequently asked questions
What dew point is instrument air?
Often class 2, minus 40 C pressure dew point or lower, depending on the application and climate.
What is the difference between refrigerant and desiccant dryers?
Refrigerant dryers cool to about 3 C; desiccant dryers adsorb water to reach minus 40 C or lower.
Which analyser measures compressed air dew point?
ACCURRA-S M015, minus 80 to plus 20 C.
Does ISO 8573-1 cover CO and CO2?
Gaseous contaminants are handled in other parts of the standard; medical and breathing air have their own specifications.
