Particle Quantifying (PQ) and Direct Reading Ferrography (DR) both offer valuable data in measuring ferrous density and identifying potential abnormal wear conditions. To determine which test is the best choice for your application, consider the advantages each may have in providing you the information you need at a reasonable cost.
ICP metals analysis is one of the most comprehensive and inexpensive routine tests included in an oil analysis program for analyzing wear metal trends, oil additive levels and contamination in lubricated systems with the potential for abnormal ferrous wear. However, ICP is limited to detecting particles <8 microns in size, leaving larger ferrous particles – and possibly an abnormal wear condition – undetected.
Particle Quantifying, or PQ, detects large ferrous wear in used oils. The test is quick, requires little prep time and, when used in conjunction with ICP (Inductively Coupled Plasma) metals analysis, PQ becomes a very cost-effective way to screen every sample for large wear concentrations. Further testing by DR would then provide more information on those samples with potential problems.
Particle Quantifying measures ferrous density using “The Hall Effect” to determine the ferromagnetic particle concentration of the sample. The measurement is determined by comparing a reference magnet voltage to the measurement magnet. When placed over the measurement magnet, the ferrous material in the sample disrupts the magnetic field between the two magnets causing an induction effect and producing the PQ index, a unit of measurement that correlates well with DR ferro large. The higher the concentration of ferromagnetic wear debris present, the higher the Hall voltage, which results in a higher index value.
Direct Reading Ferrography measures the amount of ferrous or ferromagnetic wear debris in an oil sample with results given in terms of particle size - DS for particles <5 microns in size and DL for particles >5 microns.DR runs the sample through a small precipitator tube over a high-powered magnet. The larger ferromagnetic particles, up to 200 microns, will be attracted to the magnet first, near the beginning of the tube, and then the smaller particles that typically have a less of a magnetic pull will accumulate and settle over the exit end. Light is then transmitted through the sample and photo detectors pick up the amount of light passing through the oil to determine DL and DS values. These two values are not only stand-alone data but also allow for the calculation of Wear Particle Concentration (WPC) and Percent Large Particles (PLP).
While PQ is similar to DR in that it provides an index for measuring ferrous density, PQ provides no separation of sizes. Results are given as a single index value versus the two values provided by DR results. However, even though the PQ is not sensitive to particle size, when used along with ICP results, various determinations can be made. If both the PQ and ICP values increase, it is likely that many small particles are being generated. However, if PQ increases and there is no change or a decrease in ICP ferrous wear, this suggests large particles are being generated which may indicate an abnormal level of wear.
The advantage of DR ferrography is the additional wear particle data that may be calculated from the DR results. However, DR takes longer than PQ and requires samples be diluted at specific ratios by the lab technician, which can increase the potential for human error. Also, there are the additional costs of the solvent itself and disposal. In contrast, PQ is a quick, inexpensive test that is performed in the same container the sample arrived in yet not as much data can be calculated from the results as with DR.
DR and PQ testing both offer valuable ferrous density data for determining potential abnormal wear conditions and can be added to a routine oil analysis program at a relatively low cost when using PQ to determine which samples need the additional information DR can provide.