Metal tubing fluid lines are sized by which two dimensions, in increments of 1/16 inch?

• A. Outside diameter and length
• B. Outside diameter and wall thickness in 1/8 inch increments
• C. Outside diameter and wall thickness in 1/16 inch increments
• D. Inside diameter and wall thickness in 1/16 inch increments

Answer: (C)

Tubing sizes are defined by outer diameter and wall thickness. The outer diameter determines how the tube will fit with fittings, clamps, and mounting hardware, since those parts grip or align with the outside surface. The wall thickness, together with that outside dimension, sets the inner diameter—what actually carries the fluid—as well as the tube’s strength and pressure rating. Because both dimensions are standardized in 1/16 inch steps, you can reliably select a tube that matches fittings and meets flow and pressure requirements. The inside diameter is derived from OD minus twice the wall thickness, so it isn’t used alone to identify tubing sizes. For example, a tube with an outside diameter of 1/4 inch and a wall thickness of 1/16 inch has an inner diameter of 1/8 inch.