Most oxygen storage cylinders are painted green, but yellow  and other colors may be used as well. They are certified to  Department of Transportation (DOT) specifications. To  ensure serviceability, cylinders must be hydrostatically  tested periodically. In general, a hydrostatic test consists  of filling the container with water and pressurizing it to  5 ⁄ 3  of its certified rating. It should not leak, rupture, or deform  beyond an established limit.  Figure 16-9  shows a hydrostatic  cylinder testing apparatus.  Most cylinders also have a limited service life after which  they can no longer be used. After a specified number of  filling cycles or calendar age, the cylinders must be removed  from service. The most common high-pressure steel oxygen  cylinders used in aviation are the 3AA and the 3HT. They come  in various sizes but are certified to the same specifications.  Cylinders certified under DOT-E-8162 are also popular for  their extremely light weight. These cylinders typically have  an aluminum core around which Kevlar$^{®}$ is wrapped. The  DOT-E 8162 approved cylinders are now approved under  DOT-SP-8162 specifications. The SP certification has  extended the required time between hydrostatic testing to 5  years (previously 3 years). [ Figure 16-10] The manufactured date and certification number is stamped  on each cylinder near the neck. Subsequent hydrostatic test  dates are also stamped there as well. Composite cylinders use  placards rather than stamping. The placard must be covered  with a coat of clear epoxy when additional information is  added, such as a new hydrostatic test date.  Oxygen cylinders are considered empty when the pressure  inside drops below 50 psi. This ensures that air containing water  vapor has not entered the cylinder. Water vapor could cause  corrosion inside the tank, as well as presenting the possibility of  ice forming and clogging a narrow passageway in the cylinder  valve or oxygen system. Any installed tank allowed to fall  below this pressure should be removed from service. The design of the various oxygen systems used in aircraft  depends largely on the type of aircraft, its operational  requirements, and whether the aircraft has a pressurization  system. Systems are often characterized by the type of regulator  used to dispense the oxygen: continuous-flow and demand  flow. In some aircraft, a continuous-flow oxygen system is  installed for both passengers and crew. The pressure demand  system is widely used as a crew system, especially on the  16-7 Figure 16-10.  Common cylinders used in aviation with some certification and testing specifications. Figure 16-9.  This test stand is used for hydrostatic testing of oxygen  cylinders. The water-filled cylinder is lowered into the barrel on the  left where it is pressurized to the proper level as monitored via gauges  mounted on the control panel. A displacement container on the top  left of the control board collects water from the barrel to measure the  expansion of the cylinder when pressurized to ensure it is within limits. Oxygen Systems and Regulators DOT 3AA DOT 3HT DOT-E-8162 DOT-SP-8162 DOT 3AL Steel Steel Composite Composite Aluminum Material 1,800 1,850 1,850 1,850 2,216 Rated pressure (psi) 5 3 3 5 5 Required hydrostatic test Unlimited 24 15 15 Unlimited Service life (years) N/A 4,380 N/A N/A N/A Service life (fillings) Certification Type