During a hydro the cylinder is filled with water and then pressurized to a pre-determined test pressure. This pressure is usually 5/3 of the working pressure (an aluminum 80 is pressurized to 5000 psi). The expansion of the cylinder under pressure is measured and then the pressure is released. The cylinder is remeasured to determine how close it returns to its' original size. If the tank expands too much or does not return close enough to the original size the tank fail the test. Tanks that fail must be marked "reject" unde DOT standards. Filling a tank that has failed a hydro or ahs not been properly tested can result in a fine beginning at $250.00 per cylinder PER DAY! Very expensive.
Water is used to test the cylinders for safety as water does not compress and will not cause a tank to explode if it fails the test. This immersion means that cylinders must be visually inspected after testing and that cylinders intended to be used with oxygen must be recleaned. It is interesting to note that hydro test do not detect the sustained load cracking (neck cracks) to which older tanks are subject. Only the Visual Plus type of eddy testing is currently accepted for checking cylinders for neck cracks. For this reason we test all tanks for neck cracks BEFORE being sent to hydro. Of course, we retest them when they return.
One of Diving Enterprises qualified staff members performs a Hydro on a tank.
