What is the Meaning of the PQ Index?


12 FEB 2019 Pedro Hernandes

"PQ” stands for Particle Quantification (not to be confused with Particle Counting which is a completely different test). The PQ Index is a relative measurement of the total Ferrous (Iron) metal content of your oil regardless of the size or shape of the debris by means of detection by a magnetic field. The higher the PQ index reading in a sample the higher the total concentration of ferrous material generally associated with wear.

So why does ALS measure iron using two tests (PQ Index and Wear Metals - Iron)? This is done because these two tests provide us with different information about your sample. The spectroscopic wear metal measurement of Iron only detects the iron that is suspended in your oil with a particle size of 10 microns (micrometers) or less. The larger the size of the iron particle, the less total mass that is detected. This test gives us a very accurate concentration measurement, in ppm (parts per million), of iron particles that may be contributing to wear or the product of wear. The PQ Index gives a measurement of the total relative concentration of ferrous
material. This test will pick up the presence of larger iron particles, such as a gear tooth or slivers, which would not be completely detected by a spectroscopic iron measurement.

PQ Index data can also help determine overall size concentration of iron particles. Low iron test results and a high PQI would indicate overall particles are large (>20 micron). High iron test results and a low PQI would indicate overall particles are small (<10 micron). If iron and PQI results are at relatively the same concentration then it usually indicates overall size range of particles is small. Seeing a test report with low iron test results and a high PQI, indicating overall large particles, can also predict accelerated wear or impending catastrophic wear, especially if there is a sudden upswing in the trend analysis.

Advantages of PQ Index analysis

The PQ Index is an excellent trending tool and can also be used to screen for samples that may require further, more detailed, investigation. The amount of ferrous particles generated varies from component to component, depending on equipment design. As an example, a compressor will have lower iron concentration in the oil, thus a lower limit PQ Index reading than a gearbox. Please be aware that benchmark values for the PQ Index test are being continually reassessed for suitability and may change from time to time.