I would like to know if quantitative ferrography (DRF) only measures ferrous particles.

Hello Duarte

One of the hypotheses that both analytical ferrography and direct reading ferrography is based on dictates that the magnetic field will trap both ferrous and "paramagnetic" particles. A paramagnetic particle is a non-ferrous particle that has been in contact with a ferrous surface under pressure, and has thereby "picked up" submicronic particles. These embedded submicronic particles make the nonferrous particle responsive to the magnetic field, and as such will cause both the analytical ferrograph to trap the particle on the slide, and the DR Ferrograph to suspend the particle in its glass tube, enabling this particle to contribute to the particle index.

The unfortunate part about this hypothesis is that it is impossible to come up with an objective test that would empirically prove it to be true. Any mechanism that could be devised to create "paramagnetic" particles would require a ferrous plate or ball, along with some mechanism to cause nonferrous particles to be pressured against this ferrous object - however, the very act of pressuring particulate against a ferrous object would neccesitate the generation of ferrous particles worn off the ferrous object. Therefore, you wouldn't know if you were measuring particles made paramagnetic, or simply the ferrous particles created by the test procedure. QED, it is not possible to create paramagnetic particles in isolation from ferrous particles, therefore it is not possible to create a test mechanism that would prove without a doubt that a DR ferrograph (DR) is able to detect particles that are not strictly ferrous.

On the other hand, I spent a couple of years of my life looking down a microscope at analytical ferrograms (AF), and on numerous occasions I detected brass, bronze and even hard contaminants that were obviously aligned to the orientation of the magnetic field. So, even though I couldn't prove it, I always assumed that the DR would suspend and measure particles in a similar fashion to the AF. However, in the absence of a conclusive test, I always had to take this as a workable, but ultimately unprovable hypothesis.

Regards

Steve

I have asked just about the same question here:

http://maintenanceforums.com/eve/forums/a/tpc/f/3191034...341024471#8341024471

and here:

http://www.eng-tips.com/viewthread.cfm?qid=128034

Very interesting comments by Steve. I had never even considered how paramagnetism might play a role.

With mention of paramagnetism, I can’t resist going down a small tangent. Most people are familiar with ferromagnetism. But almost all materials are weakly magnetic, either paramagnetic (weakly attracts a magnet) or diamagnetic (weakly repels a magnet).

I have prepared a demonstration which vividly shows the diamagnetic properites of water.... you pass a very powerful neodymium magnet under a bowl of water and you can easily see the distortion in reflections on the surface as the magnet passes by. Take a look....
http://www.youtube.com/watch?v=2FvWtEdY4sE

This message has been edited. Last edited by: electricpete,

As interesting background - check out MetalScan by GasTops. I am not a vendor, we installed one of these online devices on one of our gas turbines.

It is basically a tube section (2-3 inches long) installed in a lube oil drain line (same ID as the orginal lube drain so no resistance). The tube section contains 3 coils - 2 with the same polarity, the other with reverse polarity. When a metallic particle passes through, each coil generates a pulse - the size of thepulse indicates the size of the particle and the direction of the pulse indicates whether or not it is ferrrous or non-ferrous. The 3 coils allow good discrimination.

The output is fed to a PC where buckets are defined (sized) so you can see a bar graph plot showing the distribution of the different particles sizes along with trends showing rate of deterioration.