Constant level oiler

A constant-level oiler is used to check the lubricant quantity (e.g. in rolling element bearings). As long as the reservoir contains oil, the oil level in the bearing housing is sufficient to ensure reliable lubrication.

The reservoir should be kept at least one third full in order to ensure that the oil level in the bearing is guaranteed for a prolonged period of time. 

See Fig.1 Constant-level oiler