Contents

Oil Analysis: Common Tests

As mentioned earlier, there are hundreds of different tests that can be used to evaluate lubricants, hydraulic fluids and other oils. Often two or three different tests can be used to measure the same characteristics. These similar tests vary in cost, accuracy and the time required to complete each test.

Most of these tests are used by lubricant formulators and additive suppliers to determine an oil's suitability for a particular application. Over the years, however, nearly two dozen tests have shown particular value in periodic oil analysis due to their relatively low cost, good accuracy and reproducibility. these oil analysis tests can be grouped into three categories: 1) Simple visual and physical testing; 2) Standardized ASTM procedures; and 3) Identification and examination of contamination and wear metal particles.

Simple visual and physical testing

Appearance -- Perhaps the simplest of all tests, the appearance of an oil can provide a number of distinct clues about oil condition and contamination. Many oils are golden colored liquids that are bright and free of suspended solids when new. A hazy or cloudy appearance often indicates water contamination, while a gradual darkening occurs as the oil is oxidized. Particles as small as 40 microns can be detected by the unaided eye, providing an indication of gross particulate contamination.

This technique is limited by the subjective nature of the observer, and it cannot be used for oils that are dark to begin with. It can be used to detect gross contamination, but cannot be used for trace levels of contamination or to identify what the particulate materials are.

Odor -- Most oils have a bland or non-descript odor. They will develop a more pungent or "burned" odor as they oxidize in service. Any unusual odors can indicate contamination such as fuel dilution from gasoline. Usually the stronger the odor is the greater the oxidation or contamination.

This technique is also limited by the subjective nature of the observer. Some people have a more sensitive sense of smell and react differently to strong odors. Also, vapors can collect over long periods of time in closed systems such as reservoirs or storage tanks. The strong odors given off when such a system is first opened may incorrectly signal excessive oxidation or contamination.

Viscosity (falling ball comparator) -- Viscosity is one of the most important measurements in periodic oil analysis and several companies have developed portable test devices that can quickly and accurately measure viscosity on site. The most common technique is the falling ball comparator in which a sample oil is compared to a previously calibrated reference oil. Identical balls areallowed to fall freely through the reference oil and the oil sample. The relative distance each ball falls is used by comparison to determine the sample oil viscosity. One kit provides a direct reading of the sample oil viscosity, while others require some calculations.

This technique is relatively accurate, with one kit maker advising of a +/-5% accuracy in general, and a 1% accuracy with practice. One limitation is that the oils must be translucent enough to see the balls as they fall through the oil. dark oils or oils that are badly oxidized may not be acceptable.

Blotter test -- This test developed primarily to determine the presence of sludge in crankcase oils. One or two drops of the sample oil are placed on a piece of blotter paper supported by a flat surface. The oil drops will spread out on the surface and eventually dry. If there is a sharply defined ring around the oil wetted area then there is sludge present. If no sludge is present, the oil will fade gradually near its edges.

This test provides some information on the presence of sludge in crankcase oils, and can be used to determine when the oil additives are nearing the end of their useful life. One portable test kit manufacturer has prepared a series of blotter reference standards which attempt to quantify the level of sludge by comparing the sample blotter to reference photos. Even with comparative photos, the test is only a rough measure of the presence of sludge and a moderately reliable determination of the level of oil oxidation. This test does not attempt to identify the contents of the sludge.

Water content (crackle test) -- Small quantities of water (over 0.1%) may not show up as a milky haze, but can be detected by placing a few drops of the sample oil on a hot surface (about 250 deg. F). If water is present it will quickly vaporize and make a crackling or popping sound (be careful, it can sometimes splatter).

This test has more commonly been used for applications which inherently have low water contents such as transformer or turbine oils. It is extremely subjective, and does not indicate the actual level of water present.

Particle count (patch test) -- One of the simplest tests to measure particulate contaminations is the patch test. A predetermined amount of oil, usually 100 ml, is passed through a standard sized filter pad. After the pad has been dried, the appearance of the filter is compared to reference photographs of known particle levels. Higher particle levels produce a darker gray or more highly colored spot.

This test provides a good qualitative measure of the level of particulate contamination, though it does not indicate the relative distribution of large or small particles present, and does not attempt to identify the composition or origin of the particles.

Standardized ASTM Procedures

Over the years the American Society for Testing and Materials (ASTM) has standardized a large number of important oil analysis tests. Standardized tests provide two important benefits: improved accuracy per test and greater reliability between different labs. By improving the accuracy, the test data has become much more meaningful in determining oil and equipment condition, and in identifying those cases where corrective action should be taken. The greater reliability of these tests has enabled different labs to plants to compare data from the same types of equipment, and develop overall performance trends.

ASTM has prepared a formal outline of each test including the equipment required, test procedures and the expected levels of accuracy and reliability. The tests should be set up and run in the controlled atmosphere of a laboratory to achieve the best results, a requirement that is sometimes difficult to meet in the plant. To fulfill this need, dozens of independent labs, throughout the Untied States can run these tests, and will analyze the oil sample for a moderate cost.

The following ASTM tests are often used in periodic oil analysis. The levels of accuracy are approximate and individual results can vary between labs, but they are good indication of what to expect.

Viscosity (ASTM D445) -- This procedure measures the time for a given volume of oil to pass through a specific size orifice at a given temperature (often 100 deg F) It is accurate to within 0.5%.

Total acid number (ASTM D974) -- This procedure determines the level of acidity by mixing in an indicator solution and then adding potassium hydroxide (KOH) until the solution changes color. The acidity is expressed in the milligrams of KOH required to neutralize a gram of oil (mgKOH/g). It is accurate to within 15%.

Total acid number (ASTM D664) -- Some oil samples may be too dark to use a color indicator, and for those cases this procedure measures the change in electrical conductivity as the KOH is added. The acidity is also expressed as mgKOH/g. This procedure is accurate to within 4% (an improvement over D974).

Total base number (ASTM D2896) -- This procedure determines the level of alkalinity in an oil sample, which indicates the ability of the oil to continue to neutralize corrosive acids. The test measures the change in electrical conductivity, and the results are expressed as mgKOH/g. Accuracy is to within 15%.

Total base number (ASTM D664) -- As in ASTM Test D-2896, this procedure (see above) is used to determine the level of alkalinity in an oil sample. This method is considered by most to be superior to ASTM D-2896 in its ability to more accurately indicate the oil's serviceable condition. Accuracy is to within 4%.

Water content (ASTM D1744) -- This procedure determines water content by reacting the oil sample with Karl Fischer reagent. It is particularly accurate for small quantities of water (in the parts-per-million level). It may not be effective for samples with over 1000 ppm water present. Results are expressed in ppm and accuracy is to within 10%.

Identification and examination of contamination and wear metal particles

In addition to the standardized ASTM tests, there are several other oil analysis tests that have found widespread application in the last ten years. They have been oriented toward counting identifying the thousands of small particles of dirt and wear metals that are found in nearly all oil systems. By determining particle levels and their relative sizes it is possible to detect equipment problems before there is extensive damage. An analysis of the types of material present can indicate the source of the contamination or wear metals, and allow early corrective action. These are five tests that have been used most frequently.

Wear metals analysis (atomic absorption) -- In this procedure the oil sample is burned in a high temperature flame, and this equipment detects how much energy was absorbed by a particular chemical element such as iron or tin. The equipment is specifically calibrated at different levels for different elements, and as such provides a high degree of accuracy for each element examined. An analysis for ten wear metals would therefore require ten passes through the equipment.

This procedure provides the greatest level of accuracy per metal analyzed, but it is time consuming. Some time can be saved by running several dozen oil samples for a specific element and then changing the equipment to the next element. Some labs run so many samples that they can provide same day service on a routine basis.

Wear metals analysis (emission spectroscopy) -- In this procedure a small oil sample is also burned but the detection device measures the different levels of light emitted. The equipment is calibrated to simultaneously measure the emitted light from as many as 18 different wear metals and contaminants. In little more than a minute a complete metals analysis can be completed, with accuracy to within several parts-per-million.

This procedure provides somewhat less accuracy than atomic absorption, but it can be completed much more quickly. It can be particularly effective in monitoring trends in wear metal levels for gasoline engines and gas turbines.

<<Prior (Page 2)

>>Next (Page 4)