The purpose of oil sampling is to achieve the highest level of machine performance and reliability by checking the condition and the contamination level of the oil.

The NAS 1638 cleanliness standard was developed for aerospace components in the US and is still widely used for industrial and aerospace fluid power applications. It may be noted that NAS 1638 has now been made inactive for new designs. For information on a complete range of Paperback & Kindle eBooks on Pneumatics.

Based on the oil analysis results benchmarks can be established and machines with critical levels will be identified.

Where to take an oil sample?

Derive the oil from a preferably upwards pointing pipe with continuous flow to produce a representative sample. Sampling points fitted on the lower perimeter of a pipe tend to allow depositing of particles in the sampling valve.
A sample taken between the pump and the filter housing of an off-line filter is normally the worst contaminated part of the oil system, and therefore the most representative for the total systems contamination load..

The conversion from NAS 1638 cleanliness class specifications to AS4059 class specifications is defined. The comparison of the NAS 1638 classes to AS4059 classes and levels is provided and are defined and the differences explained (Appendix B). NAS 1638 classes based on weight of particles are not applicable to these classes and are not included. The contamination classes selected are based on the widely accepted older NAS 1638 cleanliness classes. The table below shows the number of particles/100 mL fluid based on differential particle count, assuming use of an automated particle counter such as the Hiac.

If no off-line filter system is installed a vacuum type sampling pump is a valid option. In such case the sample should be drawn 10 cm off the lowest part of the tank.

To take an oil sample, the following is required:

• a 200 ml particle free glass bottle
• a five litre open oil container
• a cloth

Please read the following instructions carefully before taking the oil sample.

1. Place the oil container beneath the sampling valve.
2. Open and close the valve five times and leave it open.
3. Flush the pipe by draining one litre into the container.
4. Open the sample bottle but avoid contamination to ingress.
5. Place the bottle under the oil flow without touching the sampling valve.
6. Fill the bottle approximately 80% full.
7. Place the cap on the bottle immediately after taking the sample.
8. Shake the bottle, empty the oil in the container and repeat filling.
9. Close the sampling valve.
10. All samples must be clearly marked with reference number, machine reference number, machine make and model, place of sampling, date and oil type/make.

Samples should only be collected with the machine operating at normal working temperature.

Nas Cleanliness Chart

When sampling for particle counting the method is very important.
Remember that you can never make a sample any better (cleaner) than the oil in the system, but it is easy to make it worse

Oil analysis: how to read the data?

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 1^st 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 that references the contamination load based on a breakdown of the different particle sizesfor specific particle sizes: 5 to 15 micron, 15 to 25 micron, 25 to 50 micron, 50 to 100 micron, >100 micron. Click to download the NAS 1638 pdf file .

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.

SAE AS4059 is the standard that regulates fuel cleanliness in the US and Canada. It is the latest revision of the now defunct NAS 1638 code, which was developed in 1964 to define classes of particulate contamination in aircraft components and hydraulic fluids, but has been invalid since 2001 and has since been replaced by SAE AS4059.

Pages 18-25 of our Fuel Conditioning Handbook explore international fuel standards and regulation in more detail. Click here to request your free copy

The SAE AS4059 standard is a code set by the Society of Automotive Engineers (SAE International) to measure the solid particulate content of fuel samples and hydraulic fluid.

Nas 1638 Pdf Online

SAE International is a U.S.-based, globally active professional association and standards developing organisation for engineering professionals in various industries.

Nas 1638 Class 8

The below table shows how solid particulate contamination in fuel can be classified according to the SAE AS4059 code. The figures refer to the solid particle count per millilitre of stored fuel.

Nas Classification

Nas 1638 Pdf Documents

*This is the exact equivalent to the ISO code 18/16/13, which is the particulate cleanliness level required in Europe for fuel to be recognised as ‘clean’.