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Measuring Hydraulic Fluid Cleanliness

Hydraulic cleanliness is an important aspect to be considered when specifying hydraulic systems and components. It is usual to request that the system is flushed to achieve a specific degree of cleanliness, e.g. NAS 6. Measuring hydraulic cleanliness is not straight forward, consequently various standards exist on this subject. See our page on comparing cleanliness classes.
 
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ISO 4406

The ISO Cleanliness Code, ISO 4406, 1987 is the perhaps the most widely used International standard for representing the contamination level of industrial fluid power systems. Under ISO 4406 cleanliness is classified by a two number code, e.g. 16/13, based on the number of particles greater than 5 µm and 15 µm respectively in a known volume of fluid. However some manufacturers have expanded the code to three numbers by the addition of a code number representing the number of particles greater than 2 µm, e.g. 18/16/13. Using the table below, we can see a cleanliness rating of 18/16/13 would mean that there were
1300 - 2500 particles greater than 2 micron in size
320 - 640 particles greater than 5 micron in size, and
40 - 80 particles greater than 15 microns in size.
 
The full table of ranges for ISO 4406 is shown below
 
Range NumberNo of Particles per ml
More ThanUp to and including
2480 000160 000
2340 00080 000
2220 00040 000
2110 00020 000
205 00010 000
192 5005 000
181 3002 500
176401 300
16320640
15160320
1480160
134080
122040
111020
10510
92.55
81.32.5
70.641.3
60.320.64

 

NAS 1638

The NAS 1638 cleanliness standard was originally developed for aerospace components in the US but is still widely used for industrial and aerospace fluid power applications. It is used widely in the UK North Sea industries. NAS 1638 is comprised of fluid cleanliness classes, each class defined in terms of maximum allowed particle counts for designated particle size ranges. See below.
 
ClassMaximum Number of Particles / 100 ml
5 - 1515 - 2525 - 5050 - 100>100
0012522410
025044820
1500891631
210001783261
3200035663112
44000712126224
580001425253458
61600028505069016
7320005700101218032
86400011400202536064
9128000228004050720128
102560004560081001440256
1151200091200162002880512
121024001824003240057601024


 

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