O i l   A n a l y s i s   Guide

Two of the most common questions asked of analysts are: "What tests should I run?" and "How do I interpret the results?" The first question is easier to answer than the second. This is the first article of a two-part series that presents details on a variety of laboratory tests and the interpretation of the test results. It discusses the four tests that are performed on every sample at Wearcheck Africa, and commonly performed with other routine analysis programs.

Sample Classification

• Engines
• Drivetrains (gear systems such as manual gearboxes, differentials and industrial gearboxes)
• Transmissions (automatic)
• Hydraulics
• Compressors and turbines

There are also other smaller special classes, such as aircraft engines and refrigeration compressors. At Wearcheck, every sample gets four basic tests:

• 1. ICP spectroscopy,
• 2. Particle quantification,
• 3. Viscosity at 40°C and
• 4. Water screening.

ICP Spectroscopy

There are approximately 30 different types of spectroscopy. One type, inductively coupled plasma (ICP) spectroscopy, measures light in the visible and ultraviolet regions of the spectrum. It is an atomic emission (AE) procedure whereby the diluted oil is passed through an argon gas plasma. The plasma is maintained at a temperature of approximately 8,000°C. In the upper region of the plasma, acquired energy is released as a result of the electronic transitions, and characteristic light emissions occur. Different elements produce different frequencies or colors. The intensity of the light emitted is directly proportional to the concentration of the element. ICP spectroscopy is used to measure the concentration of different elements in the oil.

The elements are divided into three broad categories on the reports:

• wear metals, such as iron from gears
• contaminants, such as lithium, which indicate the presence of grease
• oil additives, like phosphorus, which is found in extreme pressure and antiwear additives

Particle Quantification Index (PQ or PQI)

In this test, each sample is passed over a sensor which measures the bulk magnetic content of the oil. Because iron is the major wear element in virtually all components, the PQI is really a measure of how much iron is present (ferrous density) in the sample, the amounts of other magnetic elements being negligible. The PQI does not mention size - the bigger the number, the more iron. What the PQI is communicating could be interpreted as a concept of mass per capacity or, in metric terms, something like grams iron per liter of oil.

The PQI, unlike the ICP, does not have particle-size limitations. As such, it does not indicate the sizes of the particle. Remember the example of a ball bearing in a sample: a solid ball bearing and the same one ground to powder should give the same PQI.

ISO Viscosity Grade Guide

In 1975, the International Standards Organization (ISO), in unison with American Society for Testing and Materials (ASTM), Society for Tribologists and Lubrication Engineers (STLE), British Standards Institute (BSI), and Deutsches Institute for Normung (DIN) settled upon an approach to minimize the confusion. It is known as the International Standards Organization Viscosity Grade, ISO VG for short.

There are two types of viscosity: kinematic and dynamic (or absolute). Oil analysis concerns itself almost exclusively with the former. Kinematic viscosity is measured in centistokes (cSt) and is a measure of a fluid's resistance to flow or, more simply, its thickness. It must always be quoted at a stated temperature because a fluid's viscosity will change with temperature. At 40°C, a 200 cSt oil is thicker than a 100°cSt one.

Wearcheck carries out a viscosity measurement at 40°C on every sample. A viscosity measurement at 100°C can also be carried out on machines which operate at high temperatures, such as engines and some compressors.

Water Screening.

Water is one of the more common contaminants. If it can be introduced via internal coolant leaks, high-pressure hose cleaning procedures or condensation. Water has several negative effects on the performance of oil, including:

• Formation of rust, which in turn contaminates the oil.
• Increased wear rate from decreased lost load-bearing capacity.
• Creation of weak and strong acids from chemical reactions between additives and base oils.
• Biological formation and growth in low-temperature applications.
• Loss of critical additives and additive function.

It is important that water contamination be kept to the absolute minimum. Seals and breathers should be regularly inspected and maintained. Pressurized cooling systems need to be pressure-tested regularly to confirm their integrity.

Engine samples are screened for water using Fourier transform infrared (FTIR) analysis and every other sample is screened for water using the crackle test. This test involves putting a drop of oil onto a steel surface which is maintained between the boiling points of water and oil. If the oil drop contains water it spits and crackles, hence its name. The crackle test can detect water contamination of less than 0.1 percent, or 1,000 ppm. If a sample fails the crackle test, the actual water content is measured. Once again, tentative limits for water contamination are used (Table 6), although these will vary in situations of abnormal or unusual usage.