Hello Betty, Thank you for your inquiry! The H2O Redundant Scuba Air System 4 Aluminum cylinder has a working/fill pressure of 3000psi. The Alternate Air Source system is available with a 13 or a 19 cubic foot tank. You can determine the size of the cylinder by a marking of “S13 or S19”, which is usually stamped after the hydro date on the tank.
http://www.scuba.com/scuba-gear-33/043189/H2O-Redundant-Air-System-4.html If you have other questions, need help or would like to place an order, please contact SCUBA.com by calling 800-247-2822 <> 949-221-9300 or e-mail us at: info@scuba.com