Oil analysis: how to read thedata?

As a minimum an oil analysis should include:
1.      Particle count

2.      Water content in ppm

3.      Viscosity

4.      Acidity level (TAN)

·           If the oil additive content is of interest, a Spectral Analysis should be included.

·           A 0,8 micron Millipore membrane for sludge detection.

Maximum values:
1.      Particle count : the 1st rule is that the oil cleanliness should always be adjusted to the cleanliness requirements of the
most sensitive component of the system. For example, if the hydraulic system is using servo-vales, a NAS class 6 or better
is of upmost importance. In general hydraulic systems should never exceed a NAS 7 class.

2.      Water: water content is expressed in PPM (parts per million). 1 PPM = 0,0001%. As a rule of thumb water
concentration should not exceed 300 PPM.

3.      Viscosity: the viscosity is expressed in cSt. For hydraulic oil typical oil viscosity are 32 cSt, 46 cSt and 68 cSt
mg/KOH gr. For lube oil typical oil viscosity are 220 cSt and 320 cSt. The oil viscosity may vary within a range of 25% up or down the intial viscosity. Meaning that a 46 cSt oil
will still be within an acceptable range if the measured viscosity is between 33 and 59 cSt.

4.      Acidity level – Total Acid Number (TAN): the acidity level or TAN is expressed in mg/KOH gr. Every oil has a maximum TAN value. Ask your oil supplier about your oil
maximum TAN level. Typically for hydraulic the maximum level is 1,0 mg/KOH gr. New fresh oil will start around 0,2 mg/KOH gr.


Particle counting standards:
1. ISO 4406 Standard:

The ISO 4406/2000 classification of particle contents was introduced to facilitate comparisons in particle counting.

Sudden breakdown in an oil system is often caused by large particles (>14 micron) in the oil while slower, progressive faults, e.g. wear and tear, are caused by the smaller particles (4-6 micron).

This is one of the explanations why the particle reference sizes were set to 4 micron, 6 micron and 14 micron in ISO 4406/2000.

A typical sample contains in every 100 ml of oil:

·       450,000 particles >4 micron

·       120,000 particles >6 micron

·       14,000 particles >14 micron

Introduced in the ISO classification table (on the right), this oil sample has a contamination class of 19/17/14.


2. NAS 1638 standard:

NAS 1628 is a American standard thatreferences the contamination load based on a breakdown of the different particle sizes for specific particle sizes: 5 to 15 micron, 15 to 25 micron, 25 to 50 micron, 50 to 100 micron, >100 micron. 


The difference is that the NAS gives a detailed breakdown of the different particle sizes lager than 5 micron.


Evaluation of the particle count:

The obtained ISO and NAS code is an indication of the cleanliness of the oil in the system and can be verified in the contamination charts shown above.

Contamination guide for hydraulic and lube oil systems:

·           ISO 14/12/10 - NAS 4: Very clean oil, best for all oil systems.

·           ISO 16/14/11 - NAS 5: Clean oil, an absolute necessity for servo & high pressure hydraulics.

·           ISO 17/15/12 - NAS 6: Light contaminated oil, standard hydraulic and lube oil systems.

·           ISO 19/17/14 - NAS 8: New oil, for medium to low pressure systems.

·           ISO 22/20/17 - NAS 12: Very contaminated oil, not suitable for oil systems

For every oil-filled system, a cleanliness goal should be specified. This is the basic requirement to insure reliability at the lowest possible cost.
A millipore membrane shows oil degradation if a 0,8 micron cellulose membrane is utilized.

Frequency of analysis:

In the implementation phase of a condition monitoring system, analyses must be made frequently – at least every six months – in order to establish a knowledge data base.
Every oil system should have a log where analysis results are registered.
The logbook must also contain information about oil type, oil changes, break-downs, targeted ISO class code and oil analysis results.



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