"What type of relationship can you associate with iron particles when performing a spectrochemical analysis versus a direct-read ferrography?
I believe spectrochemical determines the number of particles less than 8 to 10 microns, and the direct-read ferrography determines the number of iron particles greater than or less than 5 microns.
I would think that if I have a certain value of iron particles from the spectrochemical, I would not see a larger number in the direct read. However, I quite often do. Why is this?"
You are correct that the spectrometric elemental analysis (usually inductively coupled plasma or ICP) has a size limitation. The upper size limit of a particle that the ICP can detect is normally quoted at about 5 to 8 microns. To detect (ferrous) particles greater than this, you must use other techniques, of which ferrous density is one. Direct-reading (DR) ferrography is one means of determining the ferrous density of the sample.
In DR ferrography, the ratio of the large particle reading (DL) to the small particle reading (DS) is a function of the predominant wear mode. This comparison of the DL and DS is often represented as the percent of large particles (PLP). In a corrosion situation, where the majority of the particles are extremely small, the PLP would be low. If surface fatigue is the predominant wear mode, then the PLP could be expected to be high, as this wear mode is characterized by large particle generation.
Assuming the amount of ferrous debris in the sample is the same for the two situations described above, the spectrometric iron would be fairly high for the corrosion situation and low for the fatigue situation. This is as a result of the size limitation of the spectrometer.
Comparing the trends of elemental iron and ferrous density, in particular their changes relative to each other, can be a useful tool in used oil analysis for determining the size ratio of the particles in the oil. Of course, if you note excessive increases in elemental iron and/or ferrous density, then exception tests should be carried out. These might include patch microscopy or analytical ferrography.
Analytical ferrography allows wear particles to be observed by the analyst via microscopic analysis. In this evaluation, active machine wear as well as multiple different modes of wear can be determined. This method has an outstanding sensitivity for larger particles.