REFRIGERATION IS CALLED AND MEASURED BY THE OD.
Refrigeration Copper tube is used in air conditioning and refrigeration systems due to its high thermal conductivity. Copper is used because it's light, strong, corrosion-resistance tubing. Coppers long lasting and maintenance free characteristics make copper the leading choice for plumbing, heating, cooling and other mechanical systems. 3/8" OD Refrigeration tubing has a .032 wall thickness. Brass fittings are typically used with Refrigeration Copper tubing.
ASTM Specifications Refrigeration copper tubing are B280