Offset between the surfaces of two members at a butt joint, from out-of-round pipe, mismatched wall thickness or poor fit-up. Excess hi-lo makes it hard to fuse the low side of the root and causes incomplete penetration, so codes cap it, typically at 1/16 to 1/8 inch. Fitters measure it with a hi-lo gauge and fix it with clamps, rotation or counterboring.
Hi-lo is an eighth at the bottom, rotate the pipe a few degrees and see if it comes in.
Also heard as
- high-low
- misalignment
- offset
Related terms
Fit-up
Positioning and holding parts for welding with the right gaps, alignment and clean bevels.
Incomplete penetration
A root left unfused because the weld never reached all the way through a joint meant to be fully penetrated.
Dog
A small steel lug tacked to a part so a wedge or bolt can pull pieces into alignment during fit-up.
Root pass
The first weld pass in a joint, laid at the root, which sets the quality of everything above it.