the visual crackle does not replace the need for other more precise techniques, it does provide vital information when and where it is needed. Simple, inexpensive on site tests such as this can make a real difference in the effectiveness of oil analysis and contamination control.


The hot plate 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.


1. Raise the hot plate temperature to 320°F (160°C). Always use the same temperature.


2. Violently agitate the oil sample to achieve an homogenous suspension of water in the oil.


3. Using a clean dropper, place a drop of agitated oil on the hot plate.


The diagnostic or maintenance engineer should look for the following:


1. If no crackling or vapour bubbles are produced after a few seconds, no free or emulsified water is present.


2. If very small bubbles (0.5 mm diameter) are produced but disappear quickly, approximately 0.05 to 0.10 percent water is present.


3. If bubbles of about approximately 2 mm diameter are produced, gather to the centre of the oil spot, enlarge to about 4 mm, then disappear, approximately 0.1 to 0.2 percent water is present.


4. 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.


He should 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.


1. The method is non- quantitative.


2. Hot plate temperatures above 320°F (160°C) induce rapid scintillation that may be undetectable.


3. The method does not measure the presence of chemically dissolved water.


The crackle test can be performed with a minimum of investment using the following equipment: -


1. A hot plate capable of achieving and maintaining 320°F (160°C) surface temperature.


2. A paint shaker (or equivalent) for oil agitation.


3. An oil dropper tube or laboratory syringe.


WARNINGS


The diagnostic or maintenance engineer should know that all oil has to be considered as hazardous and he should avoid oil contact with his skin, eyes and mouth and a sample should never be taken from an engine that is running.


He must exercise extreme caution when performing the crackle test on oils that might contain hazardous gases or low boiling point volatiles which might produce fumes and vapours 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 vapours.


The analyst must wear protective eye goggles, gloves and a long sleeved overall. The test must be performed in a well ventilated area.


This article first appeared in the Institution of Diagnostic Engineers magazine late last year and is published here with their kind permission.


The Report • March 2020 • Issue 91 | 81


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