18 mm tubing has preinstalled pressure-compensating 1/2 gallon per hour emitters spaced every 12 inches. Pressure compensating hose puts out the same amount of water over a wide range of pressures. Use when there is more than 10 feet in vertical elevation change or long runs. Use 710 or 1/2 inch fittings. Minimum operating pressure is 15 psi. (An additional $8.50 has been added to the price of 500 foot and 1000 foot rolls to cover additional freight charges.)