Tubing is used to transfer liquids, air, or solids under pressure or in a vacuum. It is used across many industries, from food manufacturers to laboratories. While frequently confused with pipe, tubing is different. Pipe and tubing have different sizing standards. Tubing is sized with the actual inside and outside diameter of the tube. Pipe has standard nominal sizing, which does not always correspond with the actual diameter measurement of the pipe. Tolerances for pipes are looser than those for tubes, which means that tubing is generally more expensive to produce than piping. When considering the correct tube for any given application, there are several important product specifications to consider. The first of these is tubing type (heat shrink, hypodermic, etc.). For example, most tubing is dimensionally stable, but heat shrink tubing will change shape and size to protect a wire connection after heat is applied.
Choose the material; it will usually match, or is at least compatible with, the material of the fitting, the media flowing through the tubing, and the external environment. Steel tubing would be a poor choice for transferring water, as the steel would rust. However, steel would be a good choice in high heat applications where plastic tubing would melt.
Then consider shape (round, hexagonal, etc.). Unusually shaped tubes are not pressure rated, but this does include some round tubes. Instead of transferring material, these tubes are used for decorative or structural projects. Round, square, and rectangular tubes are often used for framing, as railing, and for ornamental metalwork.
The next considerations have to do with size. Is it pipe or tube? Pipe is measured on a Nominal Pipe Size, which is based on an inner diameter. However, the nominal inner diameter is not the pipe's true inner diameter. Pipes measured this way also call out a Schedule, which determines the wall thickness. NPS tables are often used to clarify the actual measurements of a nominal size pipe. Tubing is measured according to its actual inner or outer diameter and wall thickness, and may be either in metric or inches.