Tubing shall be clearly marked, "FIREFIGHTERS AIR SYSTEM" and "HIGH-PRESSURE BREATHING AIR" using signs or self-adhesive labels. Signs shall be 1 inch (25.4 mm) high and shall be secured to the tubing. Signs shall be made of copper alloy, stainless steel, or plastic and engraved with 3/8 of an inch (9.5 mm) letters with a 1/16 0f an inch (1.6 mm) stroke lettering. Signs or labels shall be placed at not less than 20 foot (6096 mm) intervals and at each fitting, whether the tubing is concealed or in plain view. Tubing shall have a sign or label at an accessible point.