Closed constant-level oilers use a pressure-balancing line between the oiler and the lubricant housing to exchange air without venting to the outside atmosphere. These oilers are useful in applications where a closed system is needed to elevate protection against the ingress of dirt and moisture. They are primarily used in dirty industrial environments such as paper mills and concrete plants where oil reservoirs, machinery housings, and gearboxes must remain closed to prevent contaminants from entering the lubricants and damaging moving parts.