Modern Analysis Techniques for Lubricating Oil Condition Analysis
Elemental analysis is used to determine the concentrations of 15-25 different elements, ranging from wear metals and contamination to oil additives. This technique operates on the principle of Atomic Emission Spectroscopy (AES).
In AES, individual atoms within a sample are excited using a high-energy source. These atoms absorb energy and transition to a higher electronic state. Due to quantum physics, excited atoms rapidly release this gained energy, primarily by emitting light. The frequency (and thus wavelength) of the emitted light is characteristic of the atom's electronic structure. By measuring the amount of light emitted at specific wavelengths for elements like Iron, Copper, Zinc, and Sodium, their concentrations can be determined. The unit of measurement is parts per million (PPM).
Limitations:
AES requires the excitation of individual atoms, meaning samples must be fully vaporized for all atoms to be measured. The probability of a particle being vaporized and analyzed using AES drops rapidly for particles above 5 microns, and an AES spectrometer is almost blind to particles exceeding 10 microns. When analyzing elemental analysis data, it's crucial to observe the trend line (the change in elemental concentrations over consecutive samples) rather than just the absolute values.
There are two main types of AES instruments commonly used in oil analysis laboratories:
- Inductively Coupled Plasma (ICP) Instrument: In this instrument, oil is injected into a high-temperature argon plasma, where atoms are vaporized, excited, and subsequently emit light. Only particles smaller than approximately 3 microns can be measured.
- Rotating Disc Electrode (RDE) Instrument: Here, oil is vaporized and excited using a high-voltage discharge between an electrode and a rotating carbon disc. The detection limit is slightly higher at 8-10 microns.
FTIR spectroscopy is a versatile tool used to detect common contaminants, lubricant degradation by-products, and additives.
An infrared spectrometer works by passing an infrared beam through a fixed thickness of oil, typically 100 micrometers (0.1 mm). First, a new oil sample is tested to establish a baseline reading. Then, a used oil sample is tested. Oil contaminants and additive molecules absorb some of the infrared radiation at specific frequencies, while soot and other particles absorb radiation across all frequencies. After testing, the frequency spectrum of the used oil is compared to that of the new "reference" oil. This comparison reveals changes in the oil's condition from its virgin state, allowing for recommendations.
Working Principle:
One infrared beam goes to a stationary mirror and then back to a beam splitter. Another beam goes to a moving mirror. The motion of the moving mirror creates a variable total path length compared to the stationary mirror's beam. When both beams recombine at the beam splitter, the difference in path lengths creates constructive and destructive interference, forming an interferogram. This recombined beam then passes through the sample, which absorbs different wavelengths, subtracting specific wavelengths from the interferogram. The detector reports variations in energy over time for all wavelengths.
Particle count is a critical aspect of oil analysis, with the most common unit for reporting fluid cleanliness being the ISO Code system (4406:99). This system determines the number of particles in 1 ml of sample across three size categories: less than 4 microns, 6 microns, and 14 microns.
There are three basic methods for determining the absolute number of particles in a given sample:
- Optical Microscopy (ISO 4407): This is the original method for determining fluid cleanliness levels, where particles are manually counted to assess the cleanliness of the bulk sample.
- Automatic Optical Particle Counting (ISO 11500): This is the most widely deployed method for determining fluid cleanliness. All instruments, whether handheld units or full lab instruments, use either a white light source or a laser for detection.
- Pore Blockage Particle Counting (BS 3406): Two types of instruments use this method:
- One instrument measures the flow decay across a membrane as it becomes plugged while pressure is held constant.
- The second measures the rise in differential pressure across a screen while the flow rate is held constant as it becomes plugged with particles.
Acid Number (AN) and Base Number (BN) are key indicators of oil quality, used to monitor the accumulation of acids and the depletion of the base additive package in lubricating oil. A significant rise in acid number or a decrease in base number may indicate a deterioration in oil quality due to chemical reactions, oxidation, incorrect oils, or additive depletion.
Potentiometric Titration is the most widely accepted technique for measuring both Total Acid Number (TAN) and Total Base Number (TBN). This method is highly accurate and can measure a variety of sample types regardless of color or contamination. However, it involves the use of solvents and requires careful technique.