What is the TOST test?

Oil oxidation stability is a crucial consideration when selecting an industrial lubricant as oxidation will lead to an increase in the oil’s viscosity and will result in varnish and sludge deposits.

The oxidation stability of various industrial lubricants in the presence of water is commonly evaluated by the Turbine Oil Oxidation Stability Test (TOST).

TOST was designed to force lubricants to fail by a specific failure mechanism related to the extreme conditions of high temperature, gross water contamination, and substantial air entrainment. In other words, the test was designed to evaluate oils under a very specific failure mode.

During the test, each reactant and catalyst is exaggerated. Three hundred milliliters of oil is mixed with 60 ml of water, which equates to a mixture with 17 percent water, heated to 95°C (203°F), and agitated by an oxygen flow, in the presence of iron and copper catalysts. Under these conditions, the lubricant will oxidize or hydrolyze, increasing its acidity or acid number (AN), which is the metric used to measure oxidation stability.

The life of the oil is then determined by measuring the time to reach an acid number of 2 mgKOH/g.

Although TOST is a valuable tool for indicating oil life for lubricants that are continuously exposed to gross water contamination, high temperatures, and air entrainment in field service, it has its limitations in determining oil life for oils that are susceptible to other failure modes such as hydraulic oils.