The application of Fourier Transform Infrared (FT-IR) spectroscopy in lubricant condition monitoring has rapidly increased. Initially, this was driven by mobile equipment operators using FT-IR spectroscopy for crankcase lubricants. In that application, FT-IR spectroscopy monitors common contaminants, breakdown products, and additive package components. However, FT-IR analysis can be applied to a much larger variety of mechanical and fluid systems, such as gear boxes, hydraulic systems, turbines, and other systems found in the manufacturing environment.
STRATEGIES FOR MONITORING CONTAMINANTS BY FT-IR
Using the analogy to metal analysis by emission spectroscopy, a wear metal spectrometer may be configured to monitor bearing wear levels from the lead readings. However, if fluid from a machine using silver alloy based components is tested, monitoring the lead emission line will not be useful in indicating bearing wear. Like wear metal analysis, the proper regions of the spectrum must be monitored to detect the fault signatures of interest.
In FT-IR based condition monitoring, both the bulk matrix and the interactions between the matrix and analyte need to be considered. An example of this is shown in Figure 1. Here three different lubricants, a synthetic ester–based lubricant, a high detergent/dispersent crankcase lubricant, and an EP (Extreme Pressure) additive gear lubricant, all spiked with 1000 PPM of water, are presented. The area highlighted in blue indicates the optimal integrated band area for measuring water contamination in each. The different measurement regions are necessary because water reacts differently in the different systems. Different reactions result in the maximal water response appearing in different locations.
An illustration of how applying a methodology developed for one system to a different system creates misleading results is presented in Figure 2. Here, water measurement trends are presented from a gearbox that is lubricated with an oil containing an extreme pressure (EP) additive. The blue line shows the response for the water in EP additive oils, and is contrasted to the red line showing the measurement optimized for water in crankcase oils. As can be seen, the "Water in EP Oil" measurement generates a strong response from a water contamination problem, while the measurement optimized for crankcase oils only shows a slight deviation from the normal trends.
TO CALIBRATE OR NOT TO CALIBRATE?
Most condition monitoring laboratories running metal analysis spectroscopy are familiar with running periodic calibration standards. Running these standards is used to correct day-to-day variances in the emission characteristics of the plasma and other variances of the spectrometer. However, once a consistent and reproducible sampling method has been set up on a FT-IR, such daily calibration standards are unnecessary, as an FT-IR spectrometer is inherently self-correcting through measuring a background spectrum. This is a spectrum collected on the instrument (ideally including the sampling cell) without any sample present. Subsequent samples collected are then ratioed to this background, correcting for any variances in the source intensity or alignment effects.
As running calibration samples to correct day-to-day instrument variances is unnecessary, the original question of correlating the infrared response to a physical concentration remains. While a series of calibration standards can be prepared to generate a concentration reading instead of a simple infrared response, this can involve a significant amount of additional work for the laboratory. This entails not only preparing the calibration standards, but also documenting standard preparation, tracking standards, ensuring consistent standards quality, and so on. In addition, a proper calibration set for systems that are more complex should include not only the component of interest, but also be blended using a large set of used lubricants. This is necessary to take into account all the interactions and reactions with other components and contaminants which may be found in the fluid. Some laboratories have successfully generated such complex calibrations to predict physical properties like TAN, TBN and viscosity. However, this involves running 150 calibration samples or more, and using complex mathematical techniques such as PCR/PLS (Principle Component Regression / Partial Least Squares). Such mathematical techniques are beyond the scope of this article.
Ultimately, many laboratories are choosing to establish a simpler relationship, using a few prepared standards at or near their previously established alarm limits, and setting an alarm level based only on the infrared response. In the example figures above, an infrared reading of 100 for the Water in EP Oil correlates to a prepared water concentration of 1000 ppm. Thus, the alarm level for the “Water in EP Oil” infrared measurement would be triggered when the integrated area measurement reached 100. This example shows that an alarm would not be triggered on the absolute level, but maintenance action was still triggered based on the rate-of-change of this reading. Establishing such a relationship in a simpler manner preserves the reliability of the infrared measurements without adding in additional sources of error and headaches from standard preparation, storage, tracking, and so on.
ESTABLISHING ALARM LIMITS
An example showing the trend for the antiwear integrated band area is presented in Figure 4, in conjunction with the infrared water response. Note the drop in the antiwear reading at the start of this trend. This is not unexpected, as new lubricants, like new or rebuilt machines, will go through an initial "run-in" process (preferred to the phrase "break-in"). Components of new oil will undergo rapid change as they react with contaminants and breakdown products remaining in the system, and other additives adhere to metal surfaces. After this initial drop from 24 to 16 in the antiwear response, the level remained constant in next sample checked in early December. Sometime between December 4th and the next sample checked on December 22nd, an event occurred which caused the antiwear reading to drop to zero.
Examining the water response from this system immediately indicates the cause of this component loss: severe water contamination. Note that in this example, the severity of the water contamination (nearly 1%) not only adversely affects the operation of the machine, but has removed or destroyed the antiwear component. In this case, simply replacing the additive package, or completely changing the oil, would not itself correct the root cause of the loss. All factors monitored should be used in the process of diagnosing and correcting machinery and lubricant problems.
BASIC TRENDING ANALYSIS