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   Cleanroom Classification Formula from ISO Standard 14644-1   Cleanroom Classification Formula from ISO Standard 14644-1

ISO 14644-1 gives a method to classify cleanrooms The classification is based on the following equation:

where:

Cn  is the maximum permitted concentration (in particles/m3 of air) of airborne particles that are equal to, or larger, than the considered particle size.

Cn  is rounded to the nearest whole number, using no more than three significant figures.

N    is the ISO classification number, which shall not exceed the value of 9. Intermediate ISO classification numbers may be specified, with 0.1 the smallest permitted increment of N.

D    is the considered particle size in µm.

0.1  is a constant with a dimension of µm.

From the equation, the maximum permitted airborne concentration of particles, i.e. the class limit can be calculated for any given particle size. Shown in table 2 are the classes selected by ISO 14644-1 to illustrate class limits.

Table 2 Selected ISO 14644-1 airborne particulate cleanliness classes for cleanrooms and clean zones

ISO Classification number Maximum concentration limits (particles/m3 of air) for particles equal to and larger than the considered sizes shown below
  >=0.1µm >=0.2µm >=0.3µm >=0.5µm >=1µm >=5.0µm
ISO Class 1 10 2        
ISO Class 2 100 24 10 4    
ISO Class 3 1 000 237 102 35 8  
ISO Class 4 10 000 2 370 1 020 352 83  
ISO Class 5 100 000 23 700 10 200 3 520 832 29
ISO Class 6 1 000 000 237 000 102 000 35 200 8 320 293
ISO Class 7       352 000 83 200 2 930
ISO Class 8       3 520 000 832 000 29 300
ISO Class 9       35 200 000 8 320 000 293 000


It should be noted that there is a crossover to the Federal Standard 209 classes. If the particle concentration/m3 in the ISO standard is divided by 35.2 the count is converted to counts/ft3 . The equivalent Federal Standard 209 classification is then found at the 0.5 µm size, e.g. an ISO Class 5 is equivalent to Federal Standard 209 Class 100. A comparison in given in table 3.

Table 3. Comparison between selected equivalent classes of FS 209 and ISO 14644-1

ISO 14644-1Classes Class 3 Class 4 Class 5 Class 6 Class 7 Class 8
FS 209Classes Class1 Class 10 Class 100 Class 1000 Class 10 000 Class 6100 000

It should be appreciated that the airborne particle concentration of a given cleanroom is dependent on the particle generating activities going on in the room. If a room is empty, a very low particle concentration can be achieved, this closely reflecting the quality of air supplied. If the room has in it production equipment that is operating, there should be a greater particle concentration, but the greatest concentration occurs when the room is in full production. The classification of the room may therefore be carried out in these different occupancy states. The occupancy states defined in ISO 14644-1 are as follows:

As built: the condition where the installation is complete with all services connected and functioning, but with no production equipment, materials or personnel present.

At-rest: The condition where the installation is complete with equipment installed and operating in a manner agreed between the customer and supplier, but with no personnel present.

Operational: The condition where the installation is functioning in the specified manner, with the specified number of personnel present and working in the manner agreed upon.

The ISO 14644-1 standard also gives a method by which the classification of a cleanroom is ascertained. The method is for determining the number of sampling locations, the sampling volume and counting the number of airborne particles.