Scuba Tank Testing and Hydro Tank Service

How do they do Hydrostatic Test my Scuba Tank? 

Your scuba tank is first filled with water from a high pressure water pump and then submerged into a sealed chamber, which is also filled with water. The sealed chamber is connected to a calibrated burette for measuring. Your scuba cylinder is then pressurised to it's test pressure and held there for a maximum of 30 seconds. The water pressure inflates your scuba tank which expands and forces the water out of the sealed chamber into the calibrated burette. After 30 seconds the total amount of water that has spilled into the burette is recorded, this is called the Total Expansion. Now as the pressure in your scuba tank is released and your tank deflates, it shrinks back to almost it's normal or pre-pressure size.  

 The water that was in the burette is allowed to return to the main chamber, but not all of the water will return as your scuba tank has been stretched. the stretching is called Permanent Expansion and the difference between the Total Expansion and the Permanent Expansion is called the Elastic Expansion, lot's of expansion here.