FINA Lubricants - Ferrography - Forensic Scan

Direct Reading Ferrography (DR)

Glossary description: Literally translated as “iron writing”; it is a technique utilizing a precision magnetic gradient to systematically strip iron-laden and other susceptible particles from a lube for study.

How is the test performed? A small amount of sample is diluted with solvent and flowed through a small capillary tube that runs through a magnetic field. Iron-based particles are systematically (largest to smallest) “pinned” by the magnetic field as the sample flows through the capillary tube. Two optical sensors are respectively set at the entrance and slightly downstream of the capillary tube to measure the density of the (iron) particles collected at each of the two points. The instrument utilized is a Direct Reading Ferrogram DR III.

Results range: (L or S): 0.0 – 198 (higher involves dilution).

Repeatability: +/- 5% or 0.3 reporting unit, whichever is greater, thus: 150 (142.5 – 157.5) or 0.8 (0.5 – 1.1).

Technical overview: DR Ferrography does not yield information about the particle morphology (shape), nor does it do other than size particles into two broad categories. It does, however, possess the advantage of not being highly affected by sample opacity or water contamination and is, as such, a fairly repeatable test, given proper sampling procedure. It can be applied, therefore, in circumstances where other methods are rendered ineffective. It is, for example, a quite reasonable alternative to particle counting.

Result/interpretation: Two scalar reading, “L” and “S”, are obtained:

“L” or Large particles (approx. >5 microns) L + S = WPC (Wear particle concentration)

“S” or Small particles (approx. <5 microns) L (L – S) = WSI (Wear severity index)

Various systems show different “typical” levels. It is very important to watch trend development as opposed to numerical “limits”.

It is unwise to inspect a unit solely on the basis of “abnormal” DR Ferrograph readings without:

1. Results from additional tests that support such a decision.

2. Resubmitting a sample for verification.

Alternatively, consider having an Analytical Ferrogram developed for microscopic study. Levels alone are not always the determining factor: a reading of 75.3 (L) / 23.00 (S) {a ratio of 3.3} might be considerably more of a concern than, say, one of 140 / 130 {a ratio of 1.1}. A change in ratio may be very meaningful in certain instances.

Suggested application: In the broadest sense, almost any system is amenable to ferrography, but the most useful application is with non-filtered rotary systems (most differentials, manual transmissions, speed increasers/reducers, isolated bearings/shafts); the most amenable filtered systems: gas turbines, rotary screw compressors, slow/medium speed reciprocating engines and compressors (e.g., natural gas and marine installations) diesel, and gasoline engines.

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