An oxygen cylinder must withstand a hydrostatic pressure of 3300 psig to be qualified for service?

• A. Atmospheric
• B. Hydrostatic
• C. Hygroscopic
• D. Vapor

Answer: (B)

Hydrostatic testing uses a liquid (water) to apply pressure to a cylinder, checking its walls for strength and leaks. Because water is incompressible, the pressure reflects true stress on the vessel, so any defects or weakness become evident as deformation or leakage under the test load. Oxygen cylinders are required to be hydrostatically tested to a high pressure—3300 psig in this case—to ensure they can safely contain their service pressure during normal use. The other terms don’t describe a strength-testing method: atmospheric relates to ambient air pressure, hygroscopic refers to moisture absorption properties, and vapor pertains to the gas phase, not a pressure-application test.