Air is forced into a specified amount of lubricant creating a foam layer. The air is then shut off and foam is allowed to subside. The time it takes for foam to completely dissipate is recorded. A large amount of foam generated indicates a foaming tendency. A foam layer that takes a long time to dissipate indicates a foam stability problem.
A lubricant's ability to resist foaming is crucial. If foaming becomes an issue, it can overflow sumps and also prevent the lubricant from effectively providing protection against metal to metal contact.
Generally used as part of a comprehensive oil analysis package or as an exception test when foaming issues are encountered. The foam test requires a considerable amount of sample, a minimum of 400 ml is required.