Contamination:



 

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Contamination Level Correlation

Fluid contamination can be described by a number of techniques:

  • The Gravimetric Method: The contaminant level is expressed as the mass of contaminant
    per unit volume of fluid.
  • Parts Per Million: The degree of contamination is based on mass or volume per million
    units (e.g. gms/106 gms).

The above techniques describe bulk or total contamination but give little information regarding size of contaminant. For example, unless the size and density of the contaminating particles is known, no conclusions may be drawn relative to numbers of particles.

Other techniques look at numbers of particles, describing contamination in terms of its size and concentration. These may be an interval concentration, for example the number of contaminant particles (per unit volume) between 5 and 15 µ in size. Additionally, contamination may be expressed as a cumulative concentration. In this case contamination levels are described by the total number of particles per unit volume above a given size. For example, the number of particles above 25 µ in size per 100 mL.

Most commonly used cleanliness specifications are based on numbers of particles rather than gravimetric techniques. However, particle distributions which were determined to be representative of service distributions (e.g. NAS 1638 distributions) correlate reasonably with those obtained gravimetrically with AC test dust. The table here (from An Encyclopedia of Fluid Contamination Control by E.C. Fitch) provides a correlation of some different cleanliness specifications.

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NAS* 1638 Number of Particles per 100 mL Micron Range ISO 4406
Class 5-15 µ 15-25 µ 25-50 µ 50-100 µ >100 µ Class
00 125 22 4 1 8/5
0 250 44 8 2 9/6
1 500 89 16 3 1 10/7
2 1K 178 32 6 1 11/8
3 2K 356 63 11 2 12/9
4 4K 712 126 22 4 13/10
5 8K 1425 253 45 8 14/11
6 16K 2.8K 506 90 16 15/12
7 32K 5.7K 1012 180 32 16/13
8 64K 11.4K 2.0K 360 64 17/14
9 128K 22.8K 4.1K 720 128 18/15
10 256K 45.6K 8.1K 1440 256 19/16
11 512K 91.2K 16.2K 2.8K 512 20/17
12 1M 182K 32.4K 5.8K 1024 21/18

• SAE standard AS 4059 also applies. This lists fluid particulate contamination cumulatively for 5 ranges for contamination classes from 000 to 12.

•  >2 µ    •  >5 µ    •  >15 µ    •  >25 µ    •  >50 µ

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ISO 4406 Code Particles
Per mL >10 µ
ACFTD Gravimetric,
Level mg/L
MIL-STD 1246 Level NAS 1638 Class
26/23 140 000 1000
25/23 85 000 1000
23/20 14 000 100 700
21/18 4 500 12
20/18 2 400 500
20/17 2 300 11
20/16 1 400 10
19/16 1 200 10
18/15 580 9
17/14 280 300 8
16/13 140 1 7
15/12 70 6
14/12 40 200
14/11 35 5
13/10 14 .1 4
12/9 9 3
18/8 5 2
10/8 3 100
10/7 2.3 1
10/6 1.4 .01
9/6 1.2 0
8/5 0.6 00
7/5 0.3 50
6/3 0.14 .001
5/2 0.04 25
2/.8 0.01 10

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Particle Size Comparison

SIZES OF FAMILIAR OBJECTS
SUBSTANCE MICRONS INCHES
Grain of Table Salt 100 0.0039
Human Hair 70 0.0016
Lower Limit of Visibility 40 0.0016
White Blood Cells 25 0.0010
Talcum Powder 10 0.0004
Red Blood Cells 8 0.0003
Bacteria (Average) 2 0.00008

 

SCREEN SIZES
U.S. SIEVE NO. OPENING IN INCHES OPENING IN MICRONS
50 0.0117 297
60 0.0090 228
70 0.0083 210
100 0.0059 149
140 0.0041 105
200 0.0029 74
270 0.0021 53
325 0.0017 44
Paper 0.00039 10
Paper 0.00019 5

Clogging

As a safety screen accumulates particles, the pressure drop will slowly increase until the screen is almost fully clogged. Then the pressure drop increases dramatically. For example, consider a clean safety screen in a 3000 psi hydraulic system. At its normal flow rate the screen pressure drop is 6 psi. That same screen will see a pressure drop of only 150 psi when 80% clogged. However, at 95% clogged, the differential pressure jumps to 2500 psi. This phenomenon is represented by the following formula:

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