To check Total Base Number in new and used engine oils The TBN Gage determines the basic constituents (TBN Value) in used Engine Oil. The level of basic constituents comes from vital additives in engine oils that protect the engine components from the deposit and wear. The level decreases with use. If TBN falls below the desired value, the oil would cause corrosive wear & promote deposits. It is a go-no-go test to determine whether the used oil has desired Total Base Number or not. Readings are indicated within minutes on site.