The Importance of LNF Particle Counts in Oil Condition Monitoring for Fluid Power Applications

A vital facet of oil condition monitoring for fluid power applications is contamination control. For this purpose, clean oil cleanliness levels, particularly for larger particles, need to be reliably reported. In any ingress contamination distribution, the occurrence level of large abrasive contamination is typically less than that of smaller particles.

When the cleanliness levels are very low (ISO Codes 9, 10 and 11), repeatability of the measurement is a tough ask, especially for applications such as filter manufacturers, engine test cell operators, service labs, and power generation plants, where ultra fine filtration has been installed. These customers are keen to see the operation of Spectro Scientific’s LNF product line at these super clean levels.

Applications Project

A recent applications project was undertaken to assist a diesel engine development team keen on using an LNF for its test cell. The project demonstrated the LNF’s capability for low particulate contamination levels. Large abrasive contamination becomes a concern at very low cleanliness levels for the LNF imaging systems with inferior repeatability data. This is mainly because of the inherent statistical sampling accuracy of any direct imaging device. The detection capability or calibration of the instrument plays no role here.

The larger particles present in a distribution area are a clear indication of a more serious problem, and hence, are very essential. In the case of heavily filtered industrial instruments, it is very likely that large particles occur due to machine wear rather than ingress of external contamination, such as dirt or sand.

Table 1 illustrates the construction of the ISO class table for the lower ranges using the Renard series. The table with the Renard series translated to a cleanliness code can be difficult for the end user to interpret. Additionally, the raw particle count data should not be completely ignored. Even a single particle increase could cause a transfer from one ISO class to the next, in the same way as a 400% increase in particles.

Table 1. ISO code table, Low levels Class 9 to 13

MORE THAN (P/ML) UP TO AND INCLUDING (P/ML) ISO CODE
40 80 13
20 40 12
10 20 11
5 10 10
2.5 5 9

The basic rule of thumb is that the “possible” number of particles increases two times for every ISO code increase. This coding system is simple, and enables easier tracking of cleanliness targets than just trending raw particle count numbers only. However, there are some constraints.

Variations of +/- 1 ISO code level are not major, and are mostly an artifact of how close the count is to the cut off level between sizes. However, these differences do not increase the system’s actual contamination level.

The standard for instrument ISO code accuracy accepted by the industry is illustrated in the form of +/- ½ ISO code when the particle counts are centered in a code range, and +/- a single ISO code throughout the whole cleanliness classification system.

For instance, if the user’s average range of results is 7.5 p/ml, then the deviation should fall between 5 and 10 particles. This is described as +/- ½ ISO code precision. At very low contamination levels, this can be judged as a statistical precision issue that can be corrected by increasing the sample volume at the rate of additional analysis time.

The Engine OEM has provided a series of transmission fluid samples that were analyzed on an LNF with each sample measured multiple times. The sample volume, running at 2.0 ml (1.5 minutes) and 4.0 ml (3.0 minutes), respectively, increase statistical precision as the amount of particles present was small. The results of this analysis are displayed in Table 2.

The data in Table 2 illustrates that a +/- ½ ISO code accuracy can be realized at the class 10 and 9 levels by doubling the sample volume through the analysis time from 1.0 ml to 2.0 ml, and then from 2.0 ml to 4.0 ml, respectively.

Table 2. ISO code statistical accuracy at 2.0 and 4.0 ml sample volumes.

Measurement # 1 2 3 4 5 6 7 8 9 Average Stdev RSD %
PARTICLES / ml (2.0 ml SAMPLE VOLUME ~ 1.5 MINUTES)
Sample 1 9.79 6.29 7.79 9.79 6.29 5.79 9.79 6.979 6.29 7.62 1.71 22.49
ISO Code 10 10 10 10 10 10 10 10 10 ISO 10
Sample 2 5.35 3.35 2.85 5.35 3.35 2.35 5.85 3.35 3.35 3.91 1.26 32.29
ISO Code 10 9 9 10 9 8 10 9 9 ISO 9
PARTICLES / ml (4.0 ml SAMPLE VOLUME ~ 3.0 MINUTES)
Sample 3 8.75 5.25 7.25 6.75 8.25 8 9.25 5.25 8.75 7.50 1.49 19.86
ISO Code 10 10 10 10 10 10 10 10 10 ISO 10
Sample 4 4.19 2.69 3.94 2.69 3.69 4.69 4.94 2.94 3.94 3.75 0.83 22.08
ISO Code 9 9 9 9 9 9 9 9 9 ISO 9

Table 3 shows the adjusted values of LaserNet Fines volumes based on a typical cleanliness application and its related ISO code. The required sample volumes will differ from 0.3 ml to 4.0 ml in accordance with the criticality of the asset and the related cleanest ISO code class.

Table 3. Recommended sampling volume based on application to achieve ½ ISO code repeatability.

SO Code Application Components Sensitivity Recommended LaserNet Fines Sampling volume (ml)
23/21/17

Low pressure systems, large clearances

Ram Pumps

Low

0.3

20/18/15

Typical cleanliness of new hydraulic oil from manufacturer low pressure heavy industrial systems.

Long life not critical

Flow control valves cylinders

Average

0.5

19/17/14

General machinery, medium pressure and capacity
Mobile systems

Gear pumps motors

Important

0.5

18/16/13

Fuel Charter cleanliness standard: filling staon nozzle General machine requirements

Valve and piston pumps pressure control valves

Very important

1.0

17/15/12

Highly sophiscated systems and hydrostactic transmissions

Proportional valves

Critical

0.5

16/14/11 (10)

High performance servo and high pressure long life systems.

Critical roller bearings and test stands

Industrial servo valves

Critical

2.0

15/13/09

Silt sensive control systems: high reliability laboratory and/or aerospace

High performance servo valves

Super critical

4.0

Conclusion

The results presented in this article show the versatility of the LNF products for use in a myriad of applications, where contamination control is a problem. They are ideal for customers requiring reliable and reproducible data.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.

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