ASTM D6595 Method for Wear Metals and Contaminants in Used Lubricating Oils
ASTM D6595 Standard Test Method for Determination of Wear Metals and Contaminants in Used Lubricating Oils or Used Hydraulic Fluids by Rotating Disc Electrode Atomic Emission Spectrometry
4. Summary of Test Method
4.1 Wear metals and contaminants in a used oil test specimen are evaporated and excited by a controlled arc discharge using the rotating disk technique. The radiant energies of selected analytical lines and one or more references are collected and stored by way of photomultiplier tubes, charge coupled devices or other suitable detectors. A comparison is made of the emitted intensities of the elements in the used oil test specimen against those measured with calibration standards. The concentrations of the elements present in the oil test specimen are calculated and displayed. They may also be entered into a data base for processing.
5. Significance and Use
5.1 Used Lubricating Oil - The determination of debris in used oil is a key diagnostic method practiced in machine condition monitoring programs. The presence or increase in concentration of specific wear metals can be indicative of the early stages of wear if there are baseline concentration data for comparison. A marked increase in contaminant elements can be indicative of foreign materials in the lubricants, such as antifreeze or sand, which may lead to wear or lubricant degradation. The test method identifies the metals and their concentration so that trends relative to time or distance can be established and corrective action can be taken prior to more serious or catastrophic failure.
6. Interferences
6.1 Spectral - Most spectral interferences can be avoided by judicious choice of spectral lines. High concentrations of additive elements can have an interfering influence on the spectral lines used for determining wear metals. Instrument manufacturers usually compensate for spectral interferences during factory calibration. A background correction system, which subtracts unwanted intensities on either side of the spectral line, shall also be used for this purpose. When spectral interferences cannot be avoided with spectral line selection and background correction, the necessary corrections shall be made using the computer software supplied by the instrument manufacturer.
6.2 Viscosity Effects - Differences in viscosity of used oil samples will cause differences in uptake rates. Internal references of the instrument will compensate for a portion of the differences. In used oil applications, the hydrogen 486.10 nm spectral line has become the industry standard for use as an internal reference. Without a reference, trended data on used oil will be adversely affected if the sample base stock has a different viscosity from the base line samples.
6.3 Particulate - When large particles over 10 µm in size are detected, the analytical results will be lower than the actual concentration they represent. Large particles may not be effectively transported by the rotating disk electrode sample introduction system into the arc, nor will they be fully vaporized by the spark.
