Light off
Light off turns up in 2 fields: thermal spray, boilers. Here is what it means to the people in each one.
Light off in thermal spray
To ignite the spray gun, whether striking a plasma arc or lighting an HVOF or flame torch.
Booth phrase for starting the gun: a plasma gun lights off on a high-frequency or high-voltage start that establishes the arc, while HVOF and flame guns ignite with a spark or pilot after a purge. A clean light off settles to stable voltage or chamber pressure within seconds. Repeated failed light offs point to worn hardware, wet gas or a bad ignitor.
"Light off and let it settle a minute, I want the voltage flat before the first pass."
Light-off in boilers
The moment a burner ignites, and also the verb for lighting it.
A good light-off is quiet and immediate: spark, pilot, main flame, no puff. A hard light-off comes with a thump, a puff of flue gas out of the inspection ports, or a shudder through the breeching, and means fuel accumulated before it lit. Causes include delayed spark, low pilot, wrong low-fire position or too much fuel at ignition, and repeated hard light-offs crack refractory and loosen tubes.
"Every light-off on number three comes with a bang you can feel in your chest, nobody leaves until we find out why."