Used Oil Analysis
An effective predictive oil analysis program based on condition monitoring through used oil analysis must determine both machine condition and lubricant condition in a timely manner. Lubricating oil may be used as a diagnostic medium which carries wear debris away from the wearing surfaces. Analysis of the wear debris can, therefore, provide important information about the condition of the internal parts of a machine or engine. In addition, the condition of the lubricant itself is important to understand. Does the lubricant meet specification? Is the viscosity correct? Is the oil contaminated with water, particulates or chemical compounds?
In a modern condition monitoring program based on used oil analysis, a sample, or in some cases several oil samples, are taken from a piece of equipment at a predetermined sampling interval and sent to the laboratory for analysis. Based on the analysis, a diagnostic report is made and a recommendation is sent to the personnel responsible for the equipment. The report may show that everything is normal, warn of a possible problem or make a specific maintenance recommendation. The entire process, from sample taking to the diagnostic report, should take as little time as possible in order to be most effective. If samples are sent to an external laboratory and days or weeks pass before the results are received back, this reduces the effectiveness of the maintenance program as equipment might have already failed before the reports come back. In many environments, such as marine or off-shore oil and gas exploration, sending samples to a laboratory is nearly impossible and certainly not practical.
In a modern oil analysis program, the data generated and collected is also used to provide periodic maintenance summaries. These reports can be statistical in nature and provide an insight to management personnel on the effectiveness of the program, efficiency of the maintenance department, repair status of equipment, recurring problems, and even information on the performance of different lubricants. The military must always have their equipment in a state of readiness, so tracking maintenance is key for these organizations. The United States Marine Corp uses portable oil analysis to maintain their fleets of heavy vehicle and has seen substantial savings from avoiding unnecessary oil changes and detecting maintenance issues before they become catastrophic.
We have been offering instruments and even complete turnkey systems for used oil analysis for 25 years. They include all the instruments necessary for the analysis of machine and lubricant condition. Based on these years of experience we have compiled a comprehensive guide to best practices, techniques and case studies. Please download our complimentary Oil Analysis Handbook.
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Oil Testing Techniques
There are several oil parameters that are typically measured either in the laboratory or in the field to determine the health of the oil and the machinery. In this section we will describe each parameter and how it is typically measured.
The most important physical property of lubricating oil is viscosity. Viscosity determines the load carrying ability of the oil as well as how easily it circulates. The correct balance between high viscosity for load carrying and low viscosity for ease of circulation must be considered for any lubricant and its application. Oil provides benefits in addition to lubrication, and it is vital that it be able to flow under all conditions. When in use, contaminants such as water, fuel entering the oil, oxidation, and soot all affect the viscosity. Therefore viscosity measurement is one of the more important tests for oil in a mechanical system.
Gravimetric Capillary – The most widely used technique for measuring kinematic viscosity is the use of a Gravimetric Capillary that is temperature controlled, usually 40 C for single grade oils, and both 40 and 100 C for multigrade oils. Measurements using capillary viscometers are based on the relation between viscosity and time. The more viscous an oil, the longer it will take to flow through a capillary under the influence of gravity alone.There are several standardized capillaries in use today. Most laboratory instruments employ glass capillaries, or ‘tubes.’ A more recent advancement for field measure of kinematic viscosity employs a split aluminum cell capillary.
The instruments are designed to work as either direct-flow or reverse-flow capillaries. In direct-flow capillaries, the sample reservoir is located below the measuring marks. In reverse-flow types the reservoir sits above the marks. Reverse-flow capillaries allow the testing of opaque liquids and some can have a third measuring mark. Having three measuring marks provides two subsequent flow times and improves the measurement repeatability. For more information about viscosity, please click here.
Particle counting is a critical aspect of any machine conditioning program and there are many tools out there available to monitor and track the quantity and severity of the contamination, be it due to external contamination or machine wear. The specific application and type of particles will often govern what is the best particle counting technique for the job at hand. The continuous cleanliness of a hydraulic system, for instance, is very critical and even very low levels of dirt ingress can clog actuators and valves leading to premature failure. On the flip side gear and transmission systems with lots of moving parts coming together will be able to tolerate many more wear particles than a clean hydraulic system.
Direct Imaging Particle Counters
Direct imaging systems incorporate a solid-state laser configured with a CCD array to create a direct imaging particle counter as depicted in the illustration at left. The laser illuminates the sample, and an optical lens magnifies the laser light. A CCD video camera captures the images of the sample and stores them in memory.
These images are analyzed for size and shape. An equivalent circular diameter or ECD is calculated for each image and particle count and size distribution is reported along with ISO codes. Along with particle shape morphology, direct imaging systems provide other particle counting output formats but ISO 4406 is the most common.
Laser light blocking particle counters, or optical particle counters (OPC’s) are the traditional instruments used for in-service oil analysis. A light source, typically a laser, passes through a sample. The light is partially blocked by
particles so less light reaches the photodetector array, resulting in a change in voltage proportional to the area of the particles. The photo detector technology is the same principle used in garage door openers.
Pore blockage particle counters are used as on-site particle counters for in-service machinery oils. They employ a fine mesh whereby particulate accumulates on the mesh. These particle counters are based upon either a constant flow or constant pressure design. Constant flow instruments measure the pressure drop across the mesh while holding flow constant. The constant pressure designs measure the change in flow rate while holding the pressure constant.
For more information on particle analysis in lubrication oils, please click here.
Evaluating the wear condition of equipment is a primary requirement of condition monitoring programs. Oil wetted equipment will generate wear particles throughout its lifetime, the nature and rate of the wear varies from initial break in through to end of life seizure. The technique employed to detect wear and its severity is spectroscopy. Spectroscopy is a technique for detecting and quantifying the presence of elements in a material. Spectroscopy utilizes the fact that each element has a unique atomic structure. When subjected to the addition of energy, each element emits light of
specific wavelengths, or colors. Since no two elements have the same pattern of spectral lines, the elements can be differentiated. The intensity of the emitted light is proportional to the quantity of the element present in the sample allowing the concentration of that element to be determined. Typically these techniques get their names from the method used to excite the elements.
Rotating Disk Electrode Optical Emission Microscopy
A typical method used for the excitation source in modern spectrometers is an electric discharge. The source is designed to impart the energy generated in an arc or spark to the sample. For oil analysis spectrometers, a large electric potential is set up between two electrodes. Two types are commonly used: fixed tungsten or silver electrodes; or disk and rod graphite electrodes. Both operate with an oil sample in the gap between them. An
electric charge stored by a capacitor is discharged across this gap creating a high temperature electric arc that vaporizes a portion of the sample forming a plasma. The light given off as a result of this process contains emissions from all the elements present in the sample.
Another approach for elemental analysis uses X-rays to energize the sample. X-ray radiation of a high enough energy will knock electrons out of the inner shells of elements. These vacancies are filled by electrons with a higher energy level. In order to move down to a lower energy level, these electrons lose energy in the form of emitted X-rays. These emitted X-rays have a specific energy typical of the element being analyzed. The intensity of X-rays produced is proportional to the concentration of elements present.
Flame Atomic Absorption
An atomic absorption (AA) spectrometer is a low cost spectrometer with excellent sensitivity, often employed when only a few elements are monitored. This technique relies on atomic absorption- where a unique atom will absorb light at exactly those wavelengths at which they would emit light when they are excited. An oil sample is prepared by dilution with a solvent, or acid digestion, and this sample is atomized by a nebulizer and introduced into an oxygenacetylene and nitrous oxide-acetylene flame. A radiation source, such as a hollow cathode ray tube, provides the light, usually for the element(s) of interest, and the light is directed through the flame to the detector. If none of the element is present in the sample, the amount of light going through the flame and being measured at the detector is maximum. As the concentration of the element of interest increases (from the oil sample) absorption occurs, and the detector signal decreases.
To learn more about elemental analysis of oil, please click here.
Ferrous alloys make up the bulk of most lubricating machine surfaces. The physical strength and wear properties of cast iron and steel alloys make these good choices for a machine wear surface. The hydrodynamically lubricated surfaces are designed to wear and exfoliate wear particles into the lubricant at a slow and modest rate. These fine particles are generated by abrasion between the surfaces and lubricant forming a constant regenerating layer at the wear surface extremity.
These particles are fine ferrous wear particles and they can be used to indicate when the oil is dirty and needs to be changed or when the forces at the wear surface cause breakdown of the normal lubricating layers and larger more severe ferrous wear particles are produced. The latter scenario sees a breakdown in the normal abrasive wear mechanism at the lubricating surface and a switch over to a more severe adhesive abnormal wear mode. Once the wear surface has been compromised and large adhesive forces take over, which remove larger particles, this can quickly lead to catastrophic failure of the machine if not addressed. Various ferrous monitoring techniques are available to the oil analyst in order to make recommendations based on the physical state of the machine components
To learn more about ferrous monitoring please visit this page.
It is well-known that infrared is an extremely versatile technology for oil analysis. IR can provide information on a range of oil characteristics, e.g. contamination, breakdown, additive packages, fluid identity, etc. In all of these cases the response of the oil to specific regions in the infrared spectrum is examined and weighted, each being unique to the characteristic being analyzed.
Infrared spectroscopy of lubricants relies on a very simple method. You observe how much Infrared radiation the lubricant absorbs as a function of the frequency of that radiation. This figure shows such spectra for typical lubricants. That is all we need from the infrared spectroscopy itself – we just need to make sure that an accurate infrared spectrum is acquired. As you can see, different lubricant types and in general different lubricants can have very different spectra. It is these differences we use to turn these spectra into usable information.
For more information on IR spectroscopy of used oil please visit this page.
Fuel Dilution by Surface Acoustic Wave
Fuel dilution in oil can cause serious engine damage. High levels of fuel (>2%) in a lubricant can result in decreased viscosity, oil degradation, loss of dispersancy, and loss of oxidation stability. Fuel dilution is one of the most important lubricant failure modes in internal combustion engines. It usually occurs due to improper fuel-to-air ratio. Fuel dilution can also occur due to excessive idling, piston ring wear, or defective injectors and loose connectors.
■ SURFACE ACOUSTIC WAVE SENSING
The Spectro FDM 6000 Fuel Dilution Meter uses a surface acoustic wave sensor (SAW) that reacts specifically to the presence of fuel vapor.1 It works by the principle of Henry’s law. In a closed sample container, the amount of fuel diluted in the oil is directly proportional to the amount of fuel vapor in the headspace of a closed sample vessel at equilibrium.
To learn more about measuring fuel dilution, please visit this page.
Particle counting is a critical aspect of any machine conditioning program.There are many tools available to monitor and track the quantity and severity of the contamination, whether it comes from external contamination or machine wear. The specific application and type of particles will often determine the best particle counting technique to use. For example the continuous cleanliness of a hydraulic system is critical and even very low levels of dirt ingress can clog actuators and valves, leading to premature failure. Conversely, gear and transmission systems with lots of moving parts are able to tolerate many more wear particles than a clean hydraulic system.
ISO cleanliness code represents the cleanliness of the oil. Each ISO code represents a range of particles per ml of fluid. Table 1 shows common ISO codes and their corresponding particle count ranges.
To learn more about particle counting please visit this page.
Total Base Number/Total Acid Number
TOTAL ACID NUMBER
A high concentration of acidic compounds in a lubricant can lead to corrosion of machine parts and clogged oil filters due to the formation of varnish and sludge. When a lubricant breaks down, acidic by-products will be formed from the chemical decomposition of the base stock and additives in the presence of air and heat. Total Acid Number (TAN) is a measure of acid concentration present in a lubricant. The acid concentration of a lubricant depends on the presence of additive package, acidic contamination, and oxidation by-products. Occasionally, the depletion of an additive package may cause an initial decrease in TAN of fresh oil. However, the accumulation of oxidation by-products and acidic contaminants in an oil over time will always lead to an increase in TAN. This test is most meaningful in industrial machinery applications although sometimes it is recommended in engine applications along with Total Base Number (TBN).
TOTAL BASE NUMBER
Total Base Number (TBN) is a measure of alkaline concentration present in a lubricant. Engine oils are formulated with alkaline additives in order to combat the build-up of acids in a lubricant as it breaks down. The TBN level in a lubricant is targeted for its application. Gasoline engine oils are typically formulated with starting TBN around 5-10 mg KOH/g whereas diesel engine oils tend to be higher (15-30 mg KOH/g) due to the more severe operating conditions. Specialized applications, such as marine engines, may require >30 mg KOH/g. As the oil remains in service, this BN additive is depleted. Once the alkaline additives are depleted beyond a certain limit the lubricant no longer performs its function, and the engine is at risk of corrosion, sludge, and varnish. At this point it is necessary to top-off or change the oil.
To learn more about TAN and TBN please click here.
Spectroscopy is a technique for detecting and quantifying the presence of elements in a material. Spectroscopy
utilizes the fact that each element has a unique atomic structure, and when subjected to the addition of energy, each element emits light of specific wavelengths or colors. If this light is dispersed by using a dispersing element, such as a prism, a line spectrum will result. Since no two elements have the same pattern of spectral lines, the collected light can be analyzed and each element contained in the sample identified. Additionally, the intensity of the emitted light is proportional to the quantity of the element present in the sample, allowing the concentration of that element to be determined. These spectral lines are unique to the atomic structure of only one element.
For the hydrogen atom, with an atomic number of 1, the spectrum is fairly simple. On the other hand, the spectrum of iron with an atomic number of 26 is much more complicated with many emission lines in the visible spectrum corresponding to the many possible electronic transitions that may occur. If more than one element is present in the sample, spectral lines of distinctively different wavelengths will appear for each element. These lines must be separated in order to identify and quantify the elements present in the sample. Usually only one spectral line among many is chosen to determine the concentration of a certain element. This line is chosen for its intensity and freedom from spectral line interference of other elements. To accomplish this, an optical system is required.
To learn more about elemental spectroscopy, please visit this page.
Coolant in Oil Test
Glycol is used to cool engines and other components in vehicles. Glycol can get into engine oil through faulty seals and contaminate engine oil and transmission fluid (if an intercooler is used). Gycol is a particularly nasty oil contaminant and can be challenging for a laboratory to detect. Depending on the oil temperature, the glycol coolant may break down rapidly or over time. This instability is a major challenge for determining the true glycol content
in the oil at a given time, and is the major reason why field and lab tests often do not agree with each other.
Using the FluidScan immediately after sampling the oil often gives you the best opportunity to detect glycol contamination in oil. Another test used to find telltale signs of coolant contamination is elemental spectroscopy. Elemental testing can find the metallo-organic corrosion inhibitors that are present in high concentrations in the glycol coolant, but not native to the oil formulation. Sodium, boron, potassium and silicon are commonly added to coolant for corrosion inhibition.
For more information about glycol contamination, please visit this page.
The most important physical property of a lubricating oil is viscosity. Viscosity determines the load carrying ability of the oil as well as how easily it circulates. The correct balance between high viscosity for load carrying and low viscosity for ease of circulation must be considered for any lubricant and its application. Oil provides benefits in addition to lubrication, and it is vital that it be able to flow under all conditions. When in use, contaminants such as water, fuel entering the oil, oxidation, and soot all affect the viscosity. Therefore viscosity measurement is one of the more important tests for oil in a mechanical system. For machine condition monitoring, kinematic viscosity, defined as the resistance to flow under gravity, is the established method.
To learn more about viscosity and how it's measured, please visit this page.
There are many ways to measure particles in oil including laser direct imaging, light blocking and pore blockage. Spectro Scientific products actually use all three of these techniques.
Laser Direct Imaging - The LaserNet 200 Series uses laser light direct imaging to count and classify images by size and type. It is the most versatile of the particle counting methods and offers the following advantages:
- No coincidence effects – below 5 million p/ml
- Accurate to 1um resolution
- No calibration required (intrinsically correct)
- Additional shape classification
Light Blockage - The MicroLab Series incorporates a light blockage particle counter as one of the test it runs on engine, transmission and hydraulic oils. The advantages of light blockage particle counters include:
- Accurate for contamination control
- Easy to automate
Pore Blockage - These employ a fine mesh whereby particulate accumulates on the mesh. These particle counters are based upon either a constant flow or constant pressure design. Constant flow instruments measure the pressure drop across the mesh while holding flow constant. The constant pressure designs measure the change in flow rate while holding the pressure constant. The FieldLab 58 incorporates a pore blockage particle counter as one of its tests. The advantages of pore blockage are:
- No interference from additives or water
- No interference from soot
- No degassing to remove air bubbles
- The accumulated particles can then be analyzed by some other means, such as XRF
To learn more about particle measurement in oil please visit this page.
Ferrous devices can be broadly broken down into total ferrous monitors and ferrous particle monitors. The total ferrous monitors will tell the analyst the total ferrous content in the oil and also give an idea of any transitions into a more severe wear regime. Spotting the transition from normal wear to severe or abnormal wear depends on the accuracy of the device. Periodic sampling of a closed loop lubricating system will always see a steady increase in fine total ferrous material until the oil is changed. These devices act as good screening tools for additional testing in both labs and end user environments because the measurement is quick and easy to perform.
The ferrous particle monitors are particularly useful in identifying critical wear transition points and break down of film thicknesses. These are the most important devices in identifying large particles and stopping any further damage to the machine and wear surfaces.
To learn about measuring ferrous wear, please visit this page.
Since no engine is 100% efficient, products other than carbon dioxide and water will be formed during combustion. One such product produced from incomplete combustion is soot. Soot is a mass of mainly carbon particles that are typically spherical in shape. As soot levels rise, the soot particles begin to clump together and become more dangerous. The soot levels will continue to increase and the particles clump together until it reaches a level great enough to precipitate out of the oil. This precipitation will both increase the viscosity of the oil and attach itself to the engine surfaces which will significantly increase wear on the engine. This precipitation can also lead to filter plugging. Performing regular soot checks can realize cost savings by extending drain periods, reducing used oil disposal, and extending the life of diesel engines.
Thermal Gravimetric Analysis (TGA) is the method used in ASTM D5967 to measure soot content in oil. TGA is a fairly time-consuming lab method that requires pure gases and ovens and does not lend itself to testing in the field. IR spectroscopy is a simpler method and has a comparable ASTM method D7889 for measuring soot content through the use of grating IR spectroscopy. This is the method used by the FluidScan handheld analyzer.
To learn more measuring soot in oil, please visit this page.
Oxidation, Nitration and Sulfation
Oxidation, nitration and sulfation can only be measured using spectroscopy. Other methods, such as viscosity changes or impedance can be used to infer that changes have occurred due to oxidation, nitration or sulfation, but to truly understand each parameter, spectroscopy must be used to analyze the oil.
Infrared spectroscopy uses a radiative source, a detector, and a computer to study the interaction of matter and light. Oxidation and nitration products appear as peaks in the IR spectrum between 1600 and 1800 cm-1. Sulfation products appear as peaks in the IR spectrum around 1120-1180 cm-1. Because there are no absolute reference standards for oxidation, nitration, and sulfation, the results are always compared to those of new oil. For example, if the nitration peak around 1650 cm-1 becomes significantly more intense as engine oil is sampled over a specified time, then nitration has occurred, possibly due to improper air/fuel ratio.
There are test methods for laboratory grade FTIR measurement as well as for portable field testing. ASTM E2412 describes the standard practice for FTIR measurement of these properties. In addition, specific test methods have been defined for oxidation (D7414), nitration (D7624), and sulfation (D7415). For monitoring oil chemistry in the field, ASTM D7889 uses a grating infrared spectrometer like the FluidScan® which is easy to operate and does not require an experienced technician.
To learn more about measuring oxidation, nitration and sulfation, please visit this page.
Fuel dilution is a critical lubricant contamination issue that can result in expensive engine damage. There are several methods available to measure fuel dilution. Viscosity is a great screening method that is traditionally performed as part of the testing suite for used lubricants. Direct methods include GC, flash point testing, and SAW sensing. The best method to use depends on the application need.
Viscosity is an indirect method that only suggests there might be fuel dilution occurring. It can't directly measure fuel dilution and the viscosity could be changing for other reasons.
Flash point testing is simple and inexpensive, but it can be dangerous and an experienced operator is needed to interpret the results.
Gas chromatography is a very accurate method and has several ASTM standards associated with it, but it is a laboratory method and difficult to implement in the field.
Surface Acoustic Wave (SAW) sensing is quick, easy, safe and accurate. It requires no solvents and provides direct readings of the percentage of fuel dilution in oil.
To learn more about fuel dilution, please visit this page.
We have all heard the saying, “Oil and water don’t mix.” Unfortunately that doesn’t necessarily apply to lubrication oils. Water can exist in several states in lubrication oils and can do quite a bit of damage to valuable assets if left unchecked. In this guide we explore the challenges posed by water in lubrication oils and discuss the methods available to reliability professionals for measuring water in oil.
Water contamination in industrial oils can cause severe issues with machinery components. The presence of water can alter the viscosity of a lubricant as well as cause chemical changes resulting in additive depletion and the formation of acids, sludge, and varnish. Water testing is always a part of any lubricant condition monitoring program. Water contamination in industrial oils with strong water separation properties has been historically difficult to measure with any technique.
To learn more about measuring water in lubrication oils, please visit this page.
New Oil Analysis
New lubricating oil analysis is primarily a quality control process. It is important for lubricant blenders to verify the levels of additives and contaminants during the production phase, and equally as important for lubricant users to confirm specifications prior to use. Less than 40% of lubrication professionals test incoming oil before use. There are many good reasons for testing oil as it is delivered and portable, on-site tools make this job easy. For more information, click here.
For more information, click here.