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Understanding ISO Cleanliness Codes

The ISO cleanliness codes are derived from International Standard ISO 4406:2017. 

ISO codes show 3 sets of separated numbers. These numbers refer to ranges depicting the number of particles 'larger than' 4 micron, 6 micron and 14 micron per 1mL respectively. Obviously, as 6 micron and 14 micron particles are both larger than 4 micron, those particles are all also present in the first number. The second number only shows particles larger than 6 micron. The last number only shows particles larger than 14 micron.

The table below shows the number of particles in 100mL of fluid that is specified against each number in the range. Based on the example shown, there are literally millions of small particles present in just 100mL of fuel or oil at a cleanliness level of ISO 22/21/18.

ISO 4406 Contamination Codes

Range of number of particles per 100 milliliters 

Code More Than Up to & Including
24 8,000,000 16,000,000
23 4,000,000 8,000,000
22 2,000,000 4,000,000
21 1,000,000 2,000,000
20 500,000 1,000,000
19 250,000 500,000
18 130,000 250,000
17 64,000 130,000
16 32,000 64,000
15 16,000 32,000
14 8,000 16,000
13 4,000 8,000
12 2,000 4,000
11 1,000 2,000
10 500 1,000
9 250 500
8 130 250
7 64 130
6 32 64
5 16 32
4 8 16
3 4 8
2 2 4
1 1 2

Did You Know?

A 120 lpm / 32 gpm pump working 8 hours a day using fluid that is ISO 18/16/13 will pass 9 kgs / 20 lbs of dirt in 200 days.

Common Industry ISO Cleanliness Ratings

ISO 22/21/18 - Common cleanliness level of fluid
ISO 18/16/13 - Target rating for heavy gear/engine oils
ISO 16/14/11 - Target rating for hydraulic/transmission oils
ISO 14/13/11 - Target rating for diesel fuel

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