Machinery Lubrication magazine published by Noria Corporation
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www.machinerylubrication.com | May- June 2015 | 27 FTIR Calibration Pros and Cons Based on the recent two-year assessment, the mixed-mode calibra- tion can deliver statistically ASTM-identical data at rates roughly equivalent to operating nine to 10 dedicated titrators for each analysis type. Tables 4 and 5 summarize and compare the key variables of the two analytical approaches (ASTM and FTIR). With one analyst capable of analyzing nearly 500 samples by FTIR per 8-hour shift, the advantage is clearly in favor of the FTIR system. Although the mixed-mode calibration was developed to produce results statistically identical to ASTM D664 and D4739, accuracy is intrinsically limited by the uncertainty of these reference methods. The uncertainty and bias in the FTIR methods can be minimized by using a large set of in-service oil samples (more than 200). The FTIR methods also benefi t from excellent precision in contrast to the more compli- cated indirect titration-based methods. While the stoichiometric approach was found to be superior in accuracy relative to the neat-oil PLS-only approach, this does not mean the PLS-only method should be completely discounted as a potentially useful procedure. It is possible that this method may serve to provide adequate tracking estimates of ASTM parameters in more limited situations. Unfortunately, there is no published information or performance data available for the chemometric PLS-only approach. The advantage of this method is that it can make use of FTIR instru- mentation that many laboratories already have in place. Neither the FTIR PLS-only nor the stoichiometric acid number/base number approaches are sanctioned by any governing bodies, ASTM or otherwise. Therefore, any laboratory using either of these methods can only present the results to their clients as an accurate, precise and cost-effective means of obtaining acid and base number data. ML Table 4. Comparison of Key Performance Characteristics Between the ASTM and FTIR Methods Table 5. Reproducibility Comparison of Acid and Base Number Results ASTM D664 FTIR ACID NUMBER Sample preparation time 120 seconds per sample 60 seconds per sample Samples per hour 4-6 60 Daily startup and preventative maintenance time Variable (up to 1 hour) < 15 minutes Waste disposal volume ~130 ml ~25 ml ASTM D4739 FTIR BASE NUMBER Sample preparation time 120 seconds per sample 60 seconds per sample Samples per hour 4-6 60 Daily startup and preventative maintenance time Variable (up to 1 hour) < 15 minutes Waste disposal volume ~90 ml ~25 ml BASE NUMBER MIXED-MODE BASE NUMBER FTIR REPRODUCIBILITY ASTM D4739 REPRODUCIBILITY 6 1.3 3.55 10 1.3 4.52 15 1.3 5.46 ACID NUMBER MIXED-MODE ACID NUMBER FTIR REPRODUCIBILITY ASTM D664 REPRODUCIBILITY 1 0.20 0.44 2 0.20 0.88 3 0.20 1.32