Moisture in hydraulic fluid and lubricating oils has a degrading effect on both the lubricant and machines. While some additives cling to the water and are removed when the water separates form the oil (water washing) others are destroyed by water-induced chemical reaction (oxidation and hydrolysis). Water also promotes oxidation of the oils base stock, increasing the risk of sludge and vanish formation. Water also causes rust and corrosion of machine surfaces and reduces critical, load-bearing film strength. Water represents real risk to equipment and should be aggressively controlled.
Water co-exists 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 are employ wet chemistry. One easy way to 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 users to the presence of any free water.
For years, oil analysis laboratories have screened samples with 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 5000ppm (0.05 present) of water-in-oil depending on the type of oil.
In this application the crackle tests as been used as a reliable indicator of free and emulsified water, as a “go/on-go” test. However, with practice and keen eyes and ears, the procedure can be advanced considerably and made more quantities. Rather than simple listening for the crackle (scintillation), adding a visual observation and rating of the number and size of the vapour bubbles produced allows a rough indication of the amount of moisture present to be obtained.
The reversed 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 degrees farenhite (160 degree Celsius). 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 can be performed with a minimum of investment using the following equipment
- Hot plates capable of achieving and maintaining 160 degree Celsius surface temperature.
- Paint shaker (or equivalent) for oil agitation.
- Oil dropper tube and lab syringe.
The crackle test is a simple test to identify the presence of free and suspended in the oil, provided a few simple rules are followed.
No visible or audible change.
Very small Bubbles (0.5 mm) produced and quickly disappear.
Bubbles approximately 2mm are produced, gather to center, enlarge to 4mm and disappear quickly.
Bubbles 2-3mm are produced growing to 4mm process repeats, possible violent bubbling and audible cracking.
-) Raise the hot plate temperature to 160 degree Celsius. 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.
- No cracking or vapour bubbles are produced after a few seconds, no free or emulsified water is presents.
- Very small bubbles (0.5mm) are produced but disappear quickly, approximately 0.05 to 0.1 percent water is present
- Bubbles approximately 2mm are produced gather to the center to the oil spot enlarge to about 4mm, then disappear, approximately 0.1 to 0.2 percent water is present.
- For moisture levels above 0.2 present bubbles may start out about 2 to 3 mm then grow to 4mm, with the process repeating once or twice. For even higher moisture levels violent bubbling and audible cracking may result.
- Be wary of the presence of dissolved gase, 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 temperature above 160 degree Celsius induces 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 presents inhalation and/or serious skin or eye injury upon contact. When evaluation these hot plate should remain under a vent hood that allows the analyst to conduct the test without coming into contact with fumes or vapors.
- Wear protective eye wear and long sleeves
- Perform test in a well-ventilated area