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Moisture in hydraulic fluids and lubricating oils has a degrading effect on both the lubricant and the machine. While some additives cling to the water and are removed when the water separates from the oil (water washing), others are destroyed by water-induced chemical reactions (oxidation and hydrolysis).
Water also promotes oxidation of the oil’s base stock, increasing the risk of sludge and varnish formation. Water also causes rust and corrosion of machine surfaces and reduces critical, load-bearing film strength. Water represents a real risk to equipment and should be aggressively controlled.
Water coexists with oil in the dissolved, emulsified or a free state. Free and emulsified water pose the greatest risk to the machine and the lubricant, and they should be carefully monitored and controlled.
There are a number of ways to measure the presence of water in oil. However, most of them are complicated, expensive or difficult to use in the field because they employ wet chemistry. One easy way of detecting the presence of free and emulsified 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. Under carefully controlled lab conditions, the crackle test is sensitive to around 500 ppm (0.05 percent) of water-in-oil depending on the type of oil.
In this application, the crackle test has been used as a reliable indicator of free and emulsified water, as 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), adding a visual observation and rating of the number and size of the vapor bubbles produced allows a rough indication of the amount of moisture present to 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, and maintaining a constant hot-plate temperature around 320°F (160°C). A laboratory syringe and a paint shaker can help create a more homogenous suspension, resulting in more consistent results.
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 onsite tests such as this can make a real difference in the effectiveness of oil analysis and contamination control.
The crackle test is a simple test to identify the presence of free and emulsified water suspended in the oil, provided a few simple rules are followed.
No visible or audible change.
Raise the hot plate temperature to 320°F (160°C). Always use the same temperature.
Violently agitate oil sample to achieve homogenous suspension of water in oil.
Using a clean dropper, place a drop of oil on the hot plate.
If no crackling or vapor bubbles are produced after a few seconds, no free or emulsified water is present.
If very small bubbles (0.5 mm) are produced but disappear quickly, approximately 0.05 to 0.10 percent water is present.
If bubbles approximately 2 mm are produced, gather to the center of the oil spot, enlarge to about 4 mm, then disappear, approximately 0.1 to 0.2 percent water is present.
For moisture levels above 0.2 percent, bubbles may start out about 2 to 3 mm then grow to 4 mm, with the process repeating once or twice. For even higher moisture levels, violent bubbling and audible crackling may result.
Be wary of the presence of dissolved gases, fuel, refrigerants and volatile solvents, which can cause false positives.
Although generally applicable, the crackle test does have some limitations:
The method is nonquantitative.
Hot plate temperatures above 320°F (160°C) induce rapid scintillation that may be undetectable.
The method does not measure the presence of chemically dissolved water.
Exercise extreme caution when performing the crackle test on oils that might contain hazardous gases or low boiling point volatiles (such as ammonia compressor oils), which might produce fumes and vapors that present inhalation and/or serious skin or eye injury upon contact. When evaluating these oils, the hot plate should remain under a vent hood that allows the analyst to conduct the test without coming into contact with fumes or vapors.
The crackle test can be performed with a minimum of investment using the following equipment:
1. Fitch, J. (1998). Oil Analysis for Maintenance Professionals. Tulsa OK, Noria Corp.
2. Komatsu Oil & Wear Analysis (KOWA). 5th Edition, Procedure Manual.
(2001, July-August). “Water, the Forgotten Contaminant.” Practicing Oil Analysis magazine.