The 26504-L is a pre-swaged 3/8 in. OD pump tubing assembly used for connecting the EC155 CO2/H2O Closed-Path Gas Analzyer to the pump module with a recommended maximum distance of 50 ft. The 26506 Black Raw Plastic Tubing is the same 3/8 in. tubing as the 26504-L, but the Swagelok fittings are not included. For distances longer than 50 ft between the EC155 and the pump module (up to 500 ft), Campbell Scientific recommends using a larger ½ in. OD tubing assembly (26503-L).