Top off
Top off turns up in 2 fields: tribology, HVAC. Here is what it means to the people in each one.
Top off in tribology
To add oil to bring a gearbox, sump or reservoir back up to its correct level.
To add make-up oil replacing what has leaked, misted away or been sampled out. Top-offs are where most cross-contamination happens: the wrong oil grabbed from a shelf, a dirty funnel, an open bucket. Good practice uses sealed, labeled top-up containers and filtered transfer, and logs every top-off, because frequent topping on one machine is a leak signal in itself.
"Who's been topping off the press, the reservoir took ten gallons this week and nobody wrote it down."
Top off in HVAC
To add refrigerant to a system that is low on charge without finding and fixing the leak.
Topping off gets the customer cooling and guarantees another call later, because refrigerant does not get used up: if it is low, it leaked. Good techs leak search before adding refrigerant, and rules for larger commercial systems require leak rates to be tracked and repaired. The phrase carries a mild sneer when one tech describes another's work.
"Last company just topped it off every June for four years, there's a leak in the evaporator and nobody looked."