Total Ferrous Measurement

In machinery lubrication oils, the measurement and trending of abnormal ferrous wear debris is arguably the most important indicator of Machinery HealthTM and a key predictor of catastrophic failure.

A well-established practice is monitoring abnormal ferrous content for industrial oil analysis. Common techniques include the time-resolved magnetometer or dielectric (5200 minilab) based designs like the Midas instrument and the Kittiwake Analex PQ series.

With some instruments, there is no quantified result of ferrous content in mg/ml or ppm, instead a dimensionless index, known as the “PQ” index, is reported. The PQ instrument reports values on a scale from PQ=0 (no ferrous) up to a maximum value, usually PQ=750. A PQ reading of < 25 is considered ‘normal’ wear.

The Spectro LaserNetFines Q230 Particle Counter, Wear Classifier, and Ferrous Monitor instrument includes an innovative, patent pending magnetometer design to meet the need for abnormal wear measurement and trending by ferrous measurement.

The dual magnetometer design not only provides a total ferrous measurement (mg/ml), but also a ferrous particle size distribution and particle count for Fe>25, 38, 50, and 100 µm.

This article discusses Q230’s total ferrous measurement capability and does not cover its wear classifier, or the ferrous particle count and particle size distribution capability. It, however, discusses measurement accuracy and repeatability.

Spectro Ferrous Monitor

The Q230 Ferrous Monitor is an advanced two-channel instrument that works by sensing the change in inductance of a coil whenever a small amount of ferrous material is introduced to a sample cell within the coil. Individual particles are measured and sized by a small coil, and the total amount of magnetic material is measured by a second larger coil. The following schematic can be referred for more details.

Experiment and Configuration

The performance of a traditional magnetometer and the Q230 Ferrous Monitor was assessed against a gravimetric ferrous standard. LNF-545 - the Spectro Scientific gravimetric ferrous validation standard - was employed in a 50 ppm concentration. This standard is 76.2% ferrous in a 75 cSt matrix oil.

From the Fe validation standard, six samples were drawn and run on both the LNF Q230 and the traditional magnetometer. Following each measurement, the magnetometer was zeroed by determining a non ferrous sample before the next validation sample measurement. Using Electron 22, the LNF was operated normally to flush following each measurement of samples.

Results and Discussion

The following chart shows the measured values for the six (6) samples together with the 50 ppm reference line:

The table below summarizes the measured values, repeatability, and accuracy of both instruments.

  Q230 MIDAS
Sample 1 46.38 34
Sample 2 41.62 38
Sample 3 50.02 40
Sample 4 47.22 34
Sample 5 51.55 39
Sample 6 52.77 30
Std Dev 4.07 3.82
Avg 48.26 35.83
RSD% 8.44 10.65
Accuracy (average) 3% 28%

 

Conclusion

Compared to the conventional magnetometer design, Q230‘s ferrous measurement is more repeatable and accurate and is also suitable for abnormal wear measurement and trending for monitoring machine conditions. Consider the following statistical results:

  • Compared to the ferrous validation standard, the Q230 Ferrous Monitor showed an accuracy of ± 3%, while the traditional magnetometer showed an accuracy of ±28%
  • Compared to the traditional magnetometer design tested, the Q230 repeatability, as given by the Relative Standard Deviation % (RSD), is 21% better (lower)

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.

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