Crackle Test: Monitoring Water in Used Oil
Water is a major cause of lubricant failure, component failure and poor machine reliability. Like all contaminants, it is important not only to recognize its presence, but also to take steps to control or eliminate the source of water ingression. If possible, water levels in all equipment shall be kept below the saturation limit, with every effort made to keep moisture levels as low as possible. Reducing the level of water in all types of equipment can dramatically extend the life of the lubricant and the machine.
Water contamination in hydraulic and lubricating oils is one of the most damaging and costly issues in fluid systems. It promotes corrosion, depletes additives, and can lead to premature equipment failure. That’s why detecting water early, before it reaches harmful levels, is essential. One of the simplest, fastest methods to detect and roughly quantify the presence of moisture used by oil analysis laboratories is the crackle test.
Though it’s often seen as a field test, when performed under controlled lab conditions, the crackle test becomes a valuable screening tool in any oil analysis program.
What Is the Crackle Test?
The crackle test is a qualitative method used to detect free or emulsified water in used oil samples. It’s based on a straightforward principle: when a drop of oil containing water is heated on a hot surface, the water turns to steam, creating audible crackling or visible bubbling.
In the lab, the test is standardized with temperature control, calibrated equipment, and experienced analysts who can reliably interpret the results. Note that the crackle test is not an official ASTM standard (there’s no specific ASTM test method that covers the exact procedure). The test is a generally accepted test method in the oil analysis industry and all oils can be analyzed via the crackle test.
How it Works
A hotplate is held at 210 +/- 10oC and a drop of oil is placed in the center. Any moisture present in the oil is reflected in the number of bubbles observed as the water vaporizes.
Fluid Life results are reported as N/PR/PU/PS unless samples are engines or compressors or we have been instructed to follow a different reporting criterion by the customer.
- Negative (N) – no bubbling or sound is observed on the hot plate
- Reportable (PR) – minimal bubbling is observed by the technician (small bubbles (0.5mm) produced and disappear quickly)
- Unacceptable (PU) – pronounced bubbles are observed by the technician on the hot plate (bubbles produced are about 2 mm, gather to the center of the oil spot, enlarge to approximately 4 mm and then disappear)
- Severe (PS) – intense bubbles and sometimes spitting and or crackling are observed on the hot plate (bubbles start out at 2 to 3 mm and then grow to about 4 mm with the process repeating up to 2 times. Violent bubbling and audible crackling are also the results of a severe sample and usually indicate the presence of higher moisture levels).
Interferences
Refrigerants and other low boiling-point suspensions can interfere with crackle test results. Different base stocks, viscosities, and additives may also exhibit varying results. Certain synthetics, such as esters, may not produce bubbles while some oils may contain phenols, which can produce a false-positive. False-positive results can also occur due to gas from screw compressors and soot from diesel engines. Note that this method does not measure the presence of chemically dissolved water.
Every sample that undergoes analysis is reviewed and the current result is compared with previous data from that customer as well as the other testing that the sample received. From this, assumptions can be made about the system accordingly.
What the Results Indicate
The actions that should be taken depend on the oil type, the component, the operating environment and the goals of your oil analysis program. When implementing an oil analysis program, be sure to develop standard operating procedures for addressing abnormal crackle test results.
Flagging may be applied based on customer specific requests. For example, a customer may request that “PR” not be reported. Depending on severity of the visual results, a Karl Fischer Water Titration test would be triggered to provide deeper insight into water contamination issue.
Limitations
While useful, the crackle test cannot detect dissolved water, which requires more advanced lab techniques. It’s also not precise as it does not report ppm values. Even with these limitations, consider including the crackle test for:
- High-volume screening: Quickly triage hundreds of routine samples.
- Preliminary testing: Decide if further lab testing (Karl Fischer, FTIR) is needed.
- Actionable insight: Provide actionable insight into possible issues, especially in time-sensitive operations.