Fourier Transform Infrared analysis (FTIR) is a type of infrared analysis that is frequently used in used oil analysis. It can detect diesel fuel, soot, antifreeze, and more specific to fixed equipment, moisture, and oxidation levels. Moisture is usually reported as a percent by weight and assists in controlling water accumulation in components subjected to high ambient humidity or other sources of moisture entry. Oxidation is reported as an absorbance value and is an indicator of excessive operating temperatures or overextended drain intervals. Equipment manufacturers have specific condemning limits for each of these test criteria.
In order to be effective, IR instrumentation must have an unused reference sample of the oil being tested, so it is imperative that each customer provides unused samples of each of the lubricants being used in their equipment. Each instrument develops a library of reference samples, so that the correct reference can be selected for each sample. Analysis results then will consist of a direct comparison of chemical characteristics between the reference sample and the used oil sample.
Frequency of used oil analysis is usually determined by one of three criteria, or a combination of them:
• The equipment manufacturers’ recommended service interval;
• Ambient conditions; and
• Results of previous analysis results.
When changing oil at the OEM’s recommended service interval, it is a good idea to take an oil sample for analysis at that time. The oil change intervals should be as close to the same as possible, so that meaningful historical trending will be developed. Most used oil analysis laboratories provide wear-metal commenting based on trending — when there are enough uncontaminated samples in the database. However, trending is not as effective a tool when drain intervals are not consistent and testing laboratories are only providing a raw number for wear metals, rather than a wear rate.
Wear rates can be easily calculated in those instances (see Figure 3).
Ambient conditions, such as airborne dust, high humidity, or high/low temperatures, may dictate more frequent sampling, or even reduced drain intervals. Mid-drain samples to monitor on-going conditions such as wear and contaminants will contribute to optimum component life. Wear metal levels from mid-drain samples should not be considered when trending, unless the wear rate calculation method is used.
In addition, mid-drain sampling is beneficial after rebuilds, repairs, or after contaminants or high wear have been identified in a previous sample. Consistent monitoring will determine when the component is ready to return to routine operation.
As important as submitting used oil samples to a qualified laboratory is the method and consistency of obtaining the sample. Samples may be obtained by several methods, including the following:
• Through the drain opening at the time of oil change;
• Through the fill plug if not changing the oil, by use of a sampling pump; or
• By use of an inline quick-connect fitting.
With all of these methods, be sure that the area around the sample opening, whether drain, fill, or quick connect, is clean and dry, and that the oil is as close to normal operating temperature as possible. The method used should be the same each time, if possible.
If the sample is taken from the drain plug at an oil change, allow approximately 1/3 of the oil to drain out, and then catch the sample in a clean, dry sample container. If the sample is taken through the fill plug, insert the pickup tube on the sampling pump about half way into the depth of the oil and fill the clean, dry sample container. If using quick-connect fitting, insert the needle into the valve to fill the container.
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