Condition monitoring programs mainly focus on assessing the wear condition of equipment. Wear particles will be generated by oil wetted equipment throughout its service life. The rate and nature of the wear differ from initial break in through to the end of life seizure. Spectroscopy is the technique used for detection of wear and its severity, and can detect and measure the elements present in a material. The technique is based on the fact that every element is characterized by a unique atomic structure.
The addition of energy causes each element to release light of specific colors or wavelengths. The difference between the pattern of spectral lines between two elements helps to distinguish them from each other. The intensity of the light emitted varies in proportion to the amount of element that exists in the sample, enabling the determination of elemental concentration. Generally, these methods get their names from the technique employed for element excitation.
Flame Atomic Absorption Spectrometer (AAS)
Atomic absorption (AA) spectrometers (Figure 1) are not only inexpensive, but also have superior sensitivity and are often used for monitoring samples containing only a few elements. Atomic absorption forms the basis for this technique, where a specific atom will be involved in light absorption exactly at those wavelengths at which light will be emitted by them upon excitation.
The method involves the preparation of an oil sample by acid digestion, or dilution with a solvent, followed by the atomization of the resulting sample by a nebulizer, and the introduction into an oxygen-acetylene and nitrous oxide-acetylene flame. The light is provided by a radiation source like a hollow cathode ray tube for the elements under analysis and is directed to the detector through the flame.
Figure 1. PerkinElmer atomic absorption spectrometer
If the sample does not contain any of the elements, the amount of light directed through the flame and being quantified at the detector is the maximum. The absorption of light takes place with increasing concentration of the element being analyzed and consequently, the detector signal reduces.
If the sample does not contain any of the elements, the absorption of light will not take place and consequently, all of the incoming light is quantified at the detector. AA spectrometers can analyze typical elements of interest in oil samples, but their application in condition monitoring is limited because only one element is measured by them at a time. The lamp being used needs to be changed and the process described above is repeated to measure the next element. Trained lab technicians who are capable of rotating out the reference lamps and preparing the solutions are required to operate AA spectrometers.
Inductively Coupled Plasma Spectroscopy (ICP)
Inductively coupled plasma (ICP) spectrometer (Figure 2) is another kind of spectrometer widely used for analyzing aqueous solutions and oil samples. In this method, a copper coil placed around the quartz tube carries a rapidly alternating electric current to excite the argon gas passing through the quartz tube to a very high temperature. Plasma refers to a hot, highly ionized gas emitting intense light. An electric spark initiates the plasma that partly ionizes some amount of the argon gas.
Figure 2. Spectro Analytical Instruments ICP optical emission spectrometer
The “dilute and shoot” method is the most widely used method by oil labs for regular oil analysis. An aerosol spray is produced by diluting an oil sample with kerosene (generally at a ratio of 9:1 kerosene to oil) and nebulising with a spray chamber. The next step is pumping just 1% of the sample to the torch to be energized in the plasma.
This method is not only accurate, but also has outstanding repeatability (below 3% RSD). Nevertheless, this method is not suitable for studying particles with a size larger than 5 µm, requiring additional sample preparation using microwave acid digestion. Manual loading is not feasible for ICP as it typically takes 2min to process each sample. Hence, autosamplers are generally employed.
ICP is widely used in high volume analytical laboratories where dedicated technicians and clean argon gas are readily available. The cost of argon gas usage is typically more than $1000 per month, and the gas supply is in either bulk storage or large Dewar tanks. The argon is being used for optics while the device is not in use.
ICP spectrometers are regarded as workhorses, but need a high level of maintenance. Sample volumes more than 250 samples/day are required to compensate for the high overhead costs needed to maintain the system to a degree of analytical readiness.
The advantages and disadvantages of the ICP spectrometer are as follows:
- Limited matrix effect
- High throughput with automation
- Flexibility to test other materials
- Lower detection limits
- Good accuracy and precision
- More complex to operate
- Inefficient on particles bigger than 5 µm
- Needs sample preparation for most lubricants
- Needs special gases
- Needs laboratory environment
Arc/Spark/Rotating Disk Electrode Spectroscopy (RDE)
Electric discharge is typically used as an excitation source in modern spectrometers. The source is intended to impart the energy produced in an arc or spark to the sample. Oil analysis spectrometers (Figure 3) involve setting up a large electric potential between two electrodes. Fixed tungsten or silver electrodes and disk and rod graphite electrodes are the two commonly used types, operating with an oil sample in the gap lingering between them.
Figure 3. Spectro Scientific MicroLab Series
A high-temperature electric arc is produced following the discharge of an electric charge stored in a capacitor, forming a plasma by vaporizing a fraction of the sample. The light emitted because of this process consists of emissions from all the elements that compose the sample. It is possible to separate these emissions into individual wavelengths and measure them using a properly designed optical system.
The graphite electrode (Rotating disk RDE) method is commonly used for oil analysis across the world, and is regarded as the standard for the United States Department of Defense Joint Oil Analysis Program (JOAP). Various organizations have adopted JOAP to perform multi-elemental analysis easily and robustly with lab quality results.
RDE Spectrometers (Figure 4) can concurrently analyze up to 31 elements within a minute without using gases or solvents. This user-friendliness, as well as repeatability of 3 to 6% RSD, makes them an instrument of choice for condition monitoring teams. Systems are generally installed in less than ideal lab environments or field workshops, where they deliver reliable operation for years.
Figure 4. The Spectro Scientific SpectrOil spectrometer
Fixed tungsten or silver electrodes are used in another variant of this technique, which is often utilized for flow-through systems for sequentially performing various oil analysis tests on an oil sample. The purpose of these systems is to integrate various oil analysis tests with the objective of obtaining a clear picture of the oil condition, including contamination, viscosity, chemistry, and wear particles. Typically, these systems will compromise some accuracy and repeatability to facilitate a complete set of oil tests to be run.
The advantages and disadvantages of the RDE Spectrometers are as follows:
- Easy to operate
- Readily available consumables
- Robust; runs in non-laboratory environment
- High sensitivity of wear metals up to 10 µm
- No sample preparation
- Matrix effect on certain lubricants
- Detection limit good for fuels and oils, not as low as ICP or AAS
- Repeatability poorer than ICP
- Needs separate calibration curve for blending quality control
- Inefficient on particles larger than 10 µm
X Ray Florescence Spectroscopy (XRF)
X-rays are used in some techniques to energize samples for elemental analysis. Electrons will be knocked out by a high-energy X-ray radiation out of the element’s inner shells. Electrons with a higher energy level fill these vacancies. These electrons come down to a lower energy level by losing energy in the form of emitted X-rays. The energy of these emitted X-rays varies specifically with respect to the element of interest. The intensity of the X-rays emitted varies in proportion to the elemental concentration.
Energy dispersive XRF instruments (Figure 5) are employed for routine elemental detection in oil analysis. Monitoring the concentration of the additives is a typical application of XRF for ensuring quality control of new oils. The preparation of a liquid sample involves making a sample accessory first utilizing a mylar film that is stretched across the two ends of a polyethylene cylinder.
The oil sample is carefully placed in the cylinder before clamping on the top film cover. The next step is placing and irradiating the sample chamber. The time varies based on achieving the desired level of settings and accuracy. However, it normally takes 3-5 minutes. XRF is comparable to atomic spectroscopy for high-concentration additives, but spectroscopy is preferred for trace wear metals that remain suspended in the oil.
Figure 5. Oxford Instrument XRF spectrometer
The advantages and disadvantages of the XRF technique are listed below:
- Minimum sample preparation
- Analysis of large wear particles
- Wide dynamic range for high concentration elements such as additives
- Several minutes test time
- Poor detection limits for certain key elements such as boron and silicon
- Penetration depth must be considered
- Licensing requirements and additional safety training
- Constant correction is required for matrix effects from oil and local environment
Wear metals in oil can be measured by a number of methods, and each technique has its own benefits and drawbacks. The selection of a method for analysis should be based on the range of interest, the volume of resources and samples available, and the wear metals of interest. Spectrometric oil analysis can be applied to all closed loop lubricating systems, including those found in hydraulic systems, compressors, gear boxes, transmissions, gas turbines, gasoline and diesel engines.
Collecting periodic oil samples from the equipment to be monitored is a good practice. The selected spectrometer should have the ability to perform sample analysis for trace levels of metals worn from moving components, as well as for extraneous additive element and contamination levels. The ensuing data can be used as a measure of identifying whether the wear is normal or a potentially severe issue in its early stages. The following data needs to be considered while selecting a spectrometer for wear metal measurements:
- The operating costs and reliability over time
- The ranges and performance of elements of interest
- The number of samples that need to be tested on a specific day, month, or year
- Availability of human resources to maintain and run the instrument
- The different elements, additives, and contaminants collected from various equipment and oils of interest
This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
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