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Typical Contamination Levels

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The tables and numbers in this chapter represent typical system levels and classes of contamination as established by various organizations. Do not consider them “acceptable” levels unless they were designated as acceptable for your testing requirements. This data is helpful as a guide in establishing a reliability program if you do not have actual performance data available yet.

Hydraulic Fluids

This table lists the Navy Standard for Hydraulic Fluids Used for Aircraft Hydraulic Systems (NAV AIR 10-1A-17):

Size  Range0123456
5–10 µm 2,700 4,600 9,700 24,000 32,000 87,000 128,000
10–25 µm 670 1,340 2,680 5,360 10,700 21,400 42,000
25–50 µm 93 210 380 780 1,510 3,130 6,500
50–100 µm 16 28 56 110 225 430 1,000
>100 µm 1 3 5 11 21 41 92

ISO 4406 Hydraulic Fluid Cleanliness Codes

ISO 4406 is a coding system designed to translate the data obtained from particle counting of hydraulic fluid samples into classes of cleanliness. It supersedes previous coding systems for hydraulic components, such as NAS 1638.

This table shows how to use particle counts obtained from fluid samples to obtain the ISO 4406 rating.

Merck:/Freestyle/BI-Bioscience/Filters-Particle-Monitoring/A030 Images/particle-count-table2.jpg

Aircraft Fuels

ActivityTotal Sediment
Commercial (average) 0.2 mg/L
International Air Transport Association (IATA) 1.0 mg/USG (max)
Military Standards 4.0 mg/USG (max)

United States Military Land-Based Hydraulics

This table lists the maximum allowable particles per 100 mL in stated particle size ranges for MILH-5606, MIL-H-27601 A, and MIL-H-81019A hydraulic fluids:

Particle Size RangeMax. Allowable Particles
5–15 µm 2500
16–25 µm 1000
26–50 µm 250
51–100 µm 25
>100 µm None

CFC-Free Cleaning Solvent

MIL-PRF-5606H gives the following limits for petroleum-based hydraulic fluids in aircraft, missiles and ordnance:

Particle Size RangeMax. Allowable Particles
5–15 µm 10,000
15–25 µm 1,000
25–50 µm 150
50–100 µm 25
>100 µm 5

Rocket Propulsion & Service Fuels*

Because of the more generous metering and pumping clearances for Missile Propellants and Service Gases, relatively high contamination levels are tolerated. The major (particulate) risk is in clogging of pump inlet screens. Fibers that will initiate clogging and silting, therefore, are specially controlled and held typically to 400 µm maximum size.

FluidA.F. Use Limits
LO1 (Liquid Oxygen) 2.5 mg/L
LN1 (Liquid Nitrogen) 2.5 mg/L
RP-1 (Rocket Engine Fuel, Grade RP-1) 1.5 mg/L
GO1 (Gaseous Oxygen) 0.01 mg/L
GN1 (Gaseous Nitrogen) 0.01 mg/L
He (Helium) 0.01 mg/L
*per AFBS 61-3 (revised)

Ambient Air

This table lists the U.S. Air Force Tech. Order 00-25-203, maximum allowable particles (≥5 µm diameter) per per cubic foot:

ConditionAt RestOperational
Air Force Std. Clean Room 70 680
Air Force Clean Work Station 0 7

Cleanroom Garments

The ASTM Decontamination Process Garment Classification Levels are:

ClassContamination level per square foot of fabricParticle type
A Less than 1000 ≥5 µm particles
Maximum 10 Fibers
B Less than 5000 ≥5 µm particles
Maximum 25 Fibers
C Less than 10,000 ≥5 µm particles
Maximum 50 Fibers
D Less than 15,000 ≥5 µm particles
Maximum 125 Fibers
E Less than 25,000 ≥5 µm particles
Maximum 175 Fibers

Obviously broken fibers and lint-bearing seams on outer surfaces of garments, wiping cloths, caps, hoods, booties, and fabrics are cause for rework or rejection. Decontamination-processed clean room fabrics are free from persistent objectionable odors.