Pass
Pass turns up in 2 fields: thermal spray, boilers. Here is what it means to the people in each one.
Pass in thermal spray
One sweep of the gun across the area being coated; the coating is built up pass by pass.
Single traverse of the gun over the surface, which lays down a thin layer, often 10 to 25 micrometers. Total thickness is controlled by counting passes, and the step between adjacent strokes, called the index, sets the overlap. Thickness per pass, overlap and time between passes all affect heat input, residual stress and banding.
"We're getting a thou per pass, so it's twelve passes to hit 0.012, then measure."
Pass in boilers
One run of combustion gases from end to end through a boiler before they turn or leave.
In a firetube boiler, the furnace is the first pass and each bank of tubes the gases travel through afterwards is another pass, so boilers are sold as two-, three- or four-pass. More passes lengthen the gas path and keep velocities up for better heat transfer, at the cost of more draft resistance. Baffles and turbulators inside tubes are another way to wring heat out of each pass.
"Third pass tubes are clean but the second pass is packed with soot, that's where the heat's going up the stack."