Circulating lubricating oils and industrial gear oils, with a few exceptions, contain no or only very low levels of calcium. In engine oils and most hydraulic oils containing zinc, however, calcium is present as an additive. It is intended to detach impurities and keep them in suspension. Since lubricant manufacturers do not usually provide information on additive concentration, in case of doubt the "natural" calcium content of a fresh oil can be determined and compared with the values of the used oil. Several causes are possible for a significantly increased or rising calcium content:

• Ingress of calcium-containing dust, such as lime or cement dust
• Residues from mineral-containing or "additivated" water
• Mixing with an oil containing high levels of calcium
• Entry of a lubricating grease or an assembly paste.

The exact cause of the calcium increase in an oil sample can almost always be deduced from the interaction of the over 20 individual values also determined. If an oil sample shows unacceptably high calcium values, the experienced OELCHECK tribologists will of course address this as part of a diagnosis and comment on any unusual trend. Depending on which irregularities are detected in a sample, these indicate certain causes. The typical correlations are shown in the checklist below. However, because of the widely varying starting points, there can be no generally valid warning and limit value for calcium in a used oil sample. Nor can the changes be limited by rigid values or percentage changes, because there are too many different causes in a large number of different plants. Therefore, the experienced OELCHECK tribologists also take into account the oil grade, machine or equipment type, and trend history in their commentary of the laboratory report when there is a large change in calcium content. Depending on the cause or origin of the calcium, they then suggest, for example, better filtration or a complete oil change.

Typical calcium values in mg/kg (ppm) of different oil types