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Frequently Asked Questions (FAQs)

Concerning ISO 4406:1999 Hydraulic Fluid Power Solid Contaminations Code

• The way particle size is specified – the new standard moves the measurement reference in order to correct for inaccurate calibration assumptions in the previous standard.
• Particle size was previously only measured in 2 dimensions, while the new standards account for particle size in 3 dimensions. The calibration standard for automatic (optical) particle counting equipment had to be modified to accommodate this change. A new calibration standard, ISO 11171, Automatic Particle Counter calibration procedures, replaced the old ISO 4402 standard. ISO 11171 specifies ISO Medium Test Dust (ISO MTD) as the Standard Reference Material (SRM), replacing the previously used material, AC Fine Test Dust (ACFTD). The ISO MTD material is National Institute of Standards and Technology (NIST) traceable, a characteristic ACFTD never enjoyed. The net result of these changes is a calibration procedure that provides more consistent and verifiable particle counting results in different labs around the world.
  

What hasn't changed as a result of the new standard?

• The amount and size of particulate contamination in your system.
• The functioning of your filtration and contaminant removal systems.
• The importance of proactive contamination measurement and control in extending the life of your machines.
  

How do the new and old measurement references compare?

• The size classification of the particles has changed. This is illustrated by the following table:

[Note: The particles didn't actually change size; the reference we use to measure them was more accurately defined, resulting in the new numbers.]

Do these changes affect how I specify my cleanliness targets?

• No, your targets remain the same, unless you were using the old 2-character code (greater than or equal to 5 µ and greater than or equal to 15 µ). In that case, you would use the old codes for the greater than or equal to 6 µ and greater than or equal to 14 µ ranges, and select an additional code for the greater than or equal to 4 µ range, typically 1 or 2 scale numbers higher than the greater than or equal to 6 µ scale number. Of course, if you decide to set tighter targets, you are certainly free to do so.

Do all oil analysis labs use the new standards?

• No, this change is still “filtering” its way through the industry. However, the consensus is that those labs that are focused on high standards for quality and consistency of results are moving the fastest to embrace the new standards. [Editor’s Note: Bently Tribology Services ((BTS)), the lubricant condition analysis lab with which Bently Nevada has a cooperative agreement, has adopted this new ISO standard.]