The 26503-L is a pre-swaged ½ in. OD pump tubing assembly used for connecting the EC155 CO2/H2O Closed-Path Gas Analzyer to the pump module with a recommended distance of 50 to 500 ft. The 25539 Raw Plastic Tubing is the same ½ in. tubing as the 26503-L, but the Swagelok fittings are not included. For distances shorter than 50 ft between the EC155 and the pump module, Campbell Scientific recommends using a smaller 3/8 in. tubing assembly (26504-L).