The difference between the Total Expansion and the Permanent Expansion is called the Elastic Expansion. When the Percent Permanent Expansion exceeds the predetermined limits for the cylinder being tested, the cylinder must be condemned and removed from service. A high Percent Permanent Expansion value is an indication that the cylinder metal has lost it's ductility, or that there has been excessive thinning of the cylinder wall and that the cylinder is no longer safe for use. All records of visual inspection and Hydrostatic retest must be maintained until either the expiration of the retest period or until the cylinder is again reinspected or retested, whichever occurs first. During the direct expansion test, the cylinder is completely filled with water and the test connection is then screwed into the cylinder neck.
Water is pumped into the cylinder until the desired test pressure is achieved.
The volume of water which must be pumped into the cylinder to reach the test pressure is measured to determine the Total Expansion. The volume of water that is expelled from the cylinder when pressure is released is measured to determine the Permanent Expansion. Because air has a different compressibility factor than water, air trapped inside the cylinder will cause inaccurate test results. So, it is very important that the cylinder is completely filled with water to eliminate trapped pockets of air. For such cylinders, the Proof Pressure method may be used.