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One of the challenges we face is keeping up to date and understanding ISO and other official documents. This blog discusses the 2010 version of ISO-8573. To specify the purity classification of compressed air the official nomenclature, as shown in the title above, deals with three kinds of contaminants, Classification A=particles: Classification B=humidity/liquid water and Classification C=oil.

Particles are classified as either the length of greatest distance between two external boundaries, d or as their concentration in mg/m3 Cp. There are six classes [0, 1, 2, 3, 4 and 5] based on d and three classes [6, 7 and X] based on Cp. Class 0 is the most stringent and is specified by the user or equipment manufacturer. Two common classifications are Class 3 and Class 5.

Table 1 Compressed air purity for particles
Class Maximum number of particles per cubic meter as a function of particle size, d
0.1 µm < d < 0.5 µm 0.5 µm < d < 1.0 µm 1.0 µm < d < 5.0 µm
3 Not specified < 90,000 <1,000
5 Not specified Not specified <100,000

For example, if a given compressed air system is classified only for particles and meets Class 3 its purity is expressed as ISO 8573-1:2010 [3:-:-], where – = no classification for neither humidity/water nor oil.

Contamination of compressed air by humidity or water is expressed in terms of Pressure dewpoint °C and concentration Cw in g/m3, respectively. There are seven classes [0, 1, 2, 3, 4, 5 and 6] based on Pressure dewpoint °C and four classes [7, 8, 9 and X] based on water concentration Cw in g/m3. Table 2 shows examples for compressed air containing humidity or water contamination.

Table 2 – Compressed air purity classes for Humidity and liquid water
Class Pressure dewpoint °C
1 < -70
3 < -20
6 < 10
Class Concentration Liquid Water Cw in g/m3
7 Cw < 0.5
8 0.5 < Cw < 5.0
X Cw < 10

For example, for a compressed air system classified for particles and humidity /water with results corresponding to Class 3 for Particles and Class X for Water the nomenclature would be: ISO 8573-1:2010 [3:X(15):-] meaning the concentration of water was 15 g/m3.

Total Oil contamination assessment involves the measurement of oil present as a liquid, aerosol or vapor as mg/m3. There are five classes [0, 1, 2, 3, 4 and X]. These classes are shown in Table 3.

Table 3 – Compressed air purity classes for total oil
Class Concentration of total oil liquid, aerosol or vapor as mg/m3
0 As specified by the equipment user or supplier and more stringent than class 1
1 < 0.01
2 < 0.1
3 < 1
4 < 5
X >5

For classes 3, 4 and X oil present as vapor is negligible and does not need to be measured. For example, let us consider this system as oil class 4. If we keep the previous designations for the classes pertaining to the previous purity levels pertaining to particle and humidity/water then the nomenclature would be: ISO 8573-1:2010 [3:X(15):4].


Daniel Prince, Ph.D.


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