Lubricant Condition Monitoring Using FluidScan IR Spectroscopy - Determining Lubricant Degradation by Total Acid Number (TAN) or Oxidation

In order to demonstrate the effectiveness of the FluidScan handheld condition based maintenance system for measuring lubricant physical properties, several initiatives were undertaken to compare results obtained from the FluidScan to those obtained by independent laboratory techniques. This application note is one in a series that demonstrate FluidScan's ability to analyze in-service lubricant physical properties as required by an effective predictive maintenance program.

FluidScan's ability to determine lubricant degradation as compared to wet chemical titration and also to laboratory FT-IR spectroscopy is the subject of this application note. The test results show that the FluidScan has the capability of providing high quality analytical results for in-service lubricant degradation.

FluidScan - Handheld Maintenance System

The FluidScan is a handheld condition based maintenance system that protects machinery by determining when a lubricant needs to be changed due to excessive contamination or degradation. Its detection capabilities can determine lubrication contamination, degradation and cross contamination at the point of use by measuring key oil condition parameters in both synthetic and petroleum-based lubricants and fluids. The FluidScan uses a new innovative and patented flip-top sampling cell.

The FluidScan analyzes lubricants and fluids using infrared spectroscopy, a technique that has found wide acceptance as a primary test for contamination and degradation. It performs the analyses with similar accuracy to laboratory instruments, but does so on-site as a handheld device.

Why Total Acid Number or Oxidation?

Total Acid Number and lubricant oxidation are analytical tests used by oil analysis laboratories to determine the deterioration of in-service lubricants. The more acidic a lubricant is, the further its degradation has proceeded. Oxidation is a form of lubricant degradation that occurs as lubricant molecules are exposed to oxygen over long time periods and is accelerated by high operating temperatures. As oils or hydraulic fluids break down, they generally form acidic by-products that can be corrosive to metal components, accelerate wear, form deposits and increase viscosity.

Thus as fluids degrade, the levels of corrosive acids increase along with the danger of component failure. Some fluids are already acidic in their formulation. Therefore, effective monitoring requires comparison to new oil or to previous samples. Wet chemical titration techniques can be used to accurately determine lubricant degradation. However, most modern laboratories apply simpler, and less expensive tests such as FT-IR or infrared spectroscopy to measure oxidation and acid number because these techniques eliminate the need for a complex chemical analysis.

Total Acid Number (TAN) and Oxidation by FluidScan

In order to demonstrate FluidScan's ability to determine lubricant degradation, synthetic and petroleum base in-service oils were collected from various Navy platforms. The samples came from two families of lubricants, MILPRF- 23699 a synthetic polyol ester oil and MIL-PRF-2104 a petroleum base oil. The oil samples were then analyzed by the FluidScan and the results were compared to acid number and oxidation measurements for the same samples as supplied by Navy Oil Analysis Program (NOAP) laboratories.

IR Spectrometry in Detecting Lubricant Degradation

Lubricant degradation of synthetic lubricants is measured by IR spectrometers in the range of 3180cm-1 to 3750 cm-1. Figure 1 shows the IR spectra response of FluidScan on lubricants with varying degrees of lubricants ranging from 0 to 20 acid numbers. As can be seen from the figure, FluidScan can easily differentiate among the degrees of degradation.

Scans on In-Service Oil Samples to Demonstrate FluidScan
Figure 1. Scans on In-Service Oil Samples to Demonstrate FluidScan's Sensitivity to Varying Levels of Lubricant Degradation

Scans as shown in Figure 1 are of interest to experiments and scientists, but not necessarily to the maintenance professional. For that reason, the FluidScan is calibrated to provide readings in units that are standard in the industry and easy to store and use as the basis of trends in predictive maintenance programs. To accomplish this, chemometric calibrations are stored in FluidScan to provide excellent correlation to acid number and oxidation. For an explanation of chemometric calibration, see side bar on next page.

For this comparison, the acid number analyses on the MIL-PRF-23699 in-service oil samples were performed by a Navy Oil Analysis Program laboratory using wet-chemistry titrations. The results in mgKOH/g were then compared to FluidScan analyses as shown in Figure 2..

NOAP and FluidScan readings of Total Acid Number (mgKOH/g) in used MIL-PRF-23699 samples.
Figure 2. NOAP and FluidScan readings of Total Acid Number (mgKOH/g) in used MIL-PRF-23699 samples.

The correlation between the tests performed on the FluidScan and by the NOAP laboratory for acid number were excellent with a correlation factor over 99%. The MIL-PRF-2104 samples were analyzed to ASTM Standard Practice E-2412 by a Navy Oil Analysis Program laboratory using a bench-top Fourier Transform Infrared spectrometer (FT-IR). The same samples were analyzed with FluidScan and the results were compared as shown in Figure 3.

NOAP and FluidScan readings of Oxidation (abs/mm-2) in used MIL-PRF-2104 samples
Figure 3. NOAP and FluidScan readings of Oxidation (abs/mm-2) in used MIL-PRF-2104 samples

The correlation between the lubricant degradation data based on oxidation analysis with the FT-IR spectrometer and FluidScan was excellent with a correlation factor over 96%.

Chemometrics Applied to the FluidScan Chemometrics

Chemometrics can be defined as the application of mathematical, statistical, graphical or symbolic methods to maximize the chemical information that can be extracted from data. Chemometrics uses mathematical and statistical methods to improve understanding of chemical information thus providing spectroscopists with efficient ways to solve the calibration problem for analysis of spectral data.

Chemometric Calibrations of FluidScan

Chemometrics calibrations are used in the FluidScan, because as documented in ASTM E2412, infrared signatures relating to TAN in gas turbine oils have interferences from water contamination. Thus, in order to obtain quantitative readings with the FluidScan, chemometrics calibration is used to automatically subtract the effects of the presence in water from the TAN area of the spectrum. To accomplish this, a database of used MIL-PRF-23699 with a wide range of TAN and water values were gathered and principal components regression was applied for this analysis.

Linear Regression Analysis on Total Acid Number (TAN)

Linear regression analysis is performed on the absorbance surrounding the region of TAN activity, ie. 3180 to 3750 cm-1. This process in general involves the following steps:

  • Gather FluidScan spectra of a wide range of used samples.
  • Gather corresponding laboratory TAN and Water readings.
  • Choose the spectral region of interest where the property has the highest correlation (eg. 3180 to 3750 cm-1 for TAN).
  • Perform principal components regression on the spectral region with the corresponding laboratory readings.

This process yields a set of numbers which, when multiplied by an unknown spectra of turbine oil, yields the property of interest (eg. TAN) in the units of interest (mgKOH/g).

Summary

The tests and data as described in this application note demonstrate that the FluidScan can provide high quality analytical results for lubricant degradation based on acid number or oxidation at levels comparable to those provided by laboratory instruments.

The FluidScan provides beneficial information on fluid degradation that can be used in maintenance recommendations based on oil analysis. By providing such results at the site of the equipment, corrective action can be taken in a more timely manner without having to wait for analysis results from a central laboratory.

Spectro Scientific

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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