There are a few reasons for inspecting a cylinder with a straight edge. The two main reasons are for identifying a bulged cylinder and a bowed cylinder.
A Bulged Cylinder
A bulged cylinder will exhibit at least some area of its sidewall being convexed, curved outward. It is possible for the entire sidewall of the cylinder to be bulged, curved outward. The bulge is readily identified as a hump in the sidewall of the cylinder when checking with a straight edge. A bulged cylinder indicates that the metal has yielded and is no longer safe for use. A bulged cylinder should be condemned.
It is very rare for a cylinder to be bulged. A cylinder that is bulged has usually been overheated or overfilled.
A Bowed Cylinder
A bowed cylinder will exhibit an area of the sidewall that is convexed, curved outward, and also an area, 180� opposite the convex side, that is concaved, curved inward. A bowed cylinder is sometimes identified as exhibiting a banana affect. A bowed cylinder can be verified by identifying the convexed, humped, side of the cylinder with a straight edge, then rotating the cylinder 180 and verifying this side of the cylinder is concaved, recessed from the straight edge.
A bowed cylinder sometimes occurs due to variations in the manufacturing process.
For a standard 3000 psi 80 cuft scuba cylinder, Catalina Cylinders considers a bow measured on the concave side of the cylinder acceptable if .085″ or less.