Ferrous Wear Concentration versus PQ Index
Since elemental spectroscopy is generally accepted as having lower accuracy measuring wear particles larger than approximately 10 microns, most labs employ some form of simple, large particle analysis specific to ferrous content. Ferrous content is chosen not only because it tends to represent the most common metal in machines, but also because it can be quantified quickly and inexpensively using magnetometer technology. However, not all methods for measuring abnormal ferrous content are created equally.
The Particle Quantifier (PQ) instrument employs two magnetic coils, one measuring and one reference, in balance. If a sample containing iron is place on the measuring coil, the magnetic field is strengthened and upsets the balance with the reference coil, resulting in induction. The induction is measured and reported as a dimensionless index, based on calibrating the instrument with PQ=0 and PQ=750 reference samples.
The fact the result is dimensionless is not the most significant drawback to this test, a more concerning factor is how the sample is physically handled. Common analyzers rely on the sample staying within its original sample bottle, but need the bottle to be inverted and placed on top of the analyzer. Depending on the temperature and subsequent viscosity of the sample, and the size of the ferrous particles, not all sizes of particles may settle quickly enough towards the measuring coil to be measured accurately. While this test purports to measure all sizes of particles, it should be clear that it will be less effective on smaller particles in thicker fluids, especially when the sample bottle is relatively full.
Ferrous Wear Concentration requires the sample be introduced into the center of a magnetic field; this is accomplished by drawing a homogenized sample into a syringe and inserting the syringe into the instrument. This method of delivery negates any particle size or viscosity effects, ensuring a more representative measurement is provided. Also, these results are reported as a concentration in part per million (ppm), which tends to be easier to understand and correlate with other parameters.
No matter the test, trending is always the most useful method for identifying impending issues, however one additional correlation may be made between ferrous content and iron by elemental spectroscopy. If the iron by elemental spectroscopy is elevated, but ferrous wear concentration remains low, it may be surmised that the wear particles are small (<10 microns) and therefore from normal wear modes. When results from both tests are elevated, then the wear mode is likely transitioning from normal to abnormal; and if iron by elemental spectroscopy is low or consistent, but ferrous wear concentration is elevated or increasing, then the wear particles are likely large (>10 microns) and considered to be due to abnormal or severe wear modes.