Good Description of a hydro-test: American Paintball hydrostatic
"The second burette from the right now reads 65ml. The water has been pushed up the burette by the tank expansion due to the pressure on it of 5000 PSI. This must be a stable reading. In other words, the water level in the burette cannot change for at least 30 seconds. This could be due to a continual expansion of the metal of the tank which would be reasonable cause to fail the cylinder. |
After the burette level is recorded. The pressure is released from the cylinder. The cylinder will not return to the size it was before the pressure was applied because it was permanently stretched. The burette to the left reads 2.4 ml. That 2.4 ml represents the amount of permanent stretch. If the tank stretched and did not return close to its original size, it would mean the metal of the tank was not resilient enough to be safely used to contain high pressure. This SCUBA tank must return to at least 10% of the upper reading. SCBA bottles must return to 5% usually."