Moisture, when it contaminates hydraulic and lubricating oils, has a degrading effect to both the lubricant and the machine. Some additives adsorb to the water and are removed when the water separates from the oil. Others are destroyed by water induced chemical reactions. Water also promotes oxidation of the oil's base stock. And, water causes rust and corrosion of machine surfaces and reduces critical, load-bearing film strength. In sum, water represents a real risk to your equipment and should be aggressively controlled.
Water coexists with oil in either a dissolved state or a free state. When single water molecules are distributed throughout the oil due to the water's chemical attraction to the fluid, it is in a dissolved state. Numerous factors such as viscosity, base-stock type, base-stock condition, impurities, and additive package determine the volume of water that the oil will dissolve. And, of course, the dissolved volume is a function of the oil's temperature, thus the "humidity" is reported as relative humidity, depending upon the temperature. If the oil has dissolved all the water it can at a given temperature, it is saturated. Dissolved water is very difficult to place under control and does only minimal harm to the machine and oil.
When the oil is saturated and it experiences a temperature decrease, it reaches a temperature below which water will condense into a free form. This is called the "dew point" temperature. Free water is the other state in which water coexists with the oil. Water is in a free state when undissolved globules of water are physically suspended in the oil. Large globules tend to separate to the bottom of the reservoir or sump. However, in mechanical equipment, the shearing forces of gears, pumps, bearings, etc., tend to crush the water into such small globules that a stable emulsion exists. An emulsion is the stable state of physical coexistence of chemically insoluble substances, like oil and water. Additives and impurities that lower the oil's surface tension can serve as agents to strengthen the emulsion. Free and emulsified water pose the greatest risk to the machine and the lubricant and they should be placed under strict control.
There are a number of ways to measure the presence of water in oil, but most of them are complicated, expensive or difficult to use in the field because they employ wet chemistry. An easy way to detect the presence of free and emulsified water, the most dangerous forms of water in oil, is with the hot-plate crackle test. This simple, tried and true method alerts the user to the presence of any free water.
For years, oil analysis laboratories have screened samples with the crackle-test, performing more expensive analysis only when the crackle test is positive.
In this application, the crackle test has been used as a reliable indicator of emulsified water, a go/no-go test. However, with practice and keen eyes and ears, the procedure can be advanced considerably and made more quantitative. Rather than simply listening for the crackle (scintillation), by adding visual observation of vapor bubbles, a rough indication of the amount of moisture present can be obtained.
The revised method is referred to as the "visual crackle". Success in using the procedure depends on practice with varying moisture concentrations in different common fluids. A laboratory syringe and a paint shaker can help create the experimental suspensions. While the visual crackle does not replace the need for other more precise techniques, it does provide vital information when and where you need it. Simple, inexpensive on-site tests such as this can make a real difference in the effectiveness of oil analysis and contamination control.
Fitch, J. C., Oil Analysis for Maintenance Professionals (Coursebook), Noria Corp., 1998.
Komatsu Oil & Wear Analysis (KOWA), 5th Edition, Procedure Manual.