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Inspecting and Testing for Your SCUBA Tanks: Hydrostatic Tank Test

Inspecting and Testing for Your SCUBA Tanks: Hydrostatic Tank Test

Written by Leisure Pro Staff
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Published on August 3, 2011

This is a continuation of yesterday’s article on air cylinder maintenance. 

So, now your tank has been inspected for damage and corrosion and been deemed A-OK by the technician. If it’s been five years since your last thorough inspection, you are now due to have testing performed on your air tank that is required by federal law. In the US, it is regulated by the Department of Transportation. Each country has its own rules in regard to the maintenance, use, and transportation of pressurized vessels. There are risks associated with pressurized air cylinders, so the regulations will be similar no matter where you are. To address these risks, a hydrostatic test is performed on your cylinder.

The purpose of a hydrostatic test is to locate any flaws that would put the diver in danger, much like the visual inspection. Here the technician is looking for leaks in the tank. The hydrostatic test takes it a step further from the visual inspection and physically tests the limits of the cylinder by use of water and pressurization. The tank is filled with water, and submerged into a sealed chamber. A burette, which is a type of measuring cylinder, is connected to the sealed chamber to record the water displaced during pressurization.

The tank is then pressurized to 5/3 of its normal pressure, which is found stamped on the cylinder. For 30 seconds the tank is pressurized, causing it to inflate and displace water from the chamber into the burette. The amount of water spilled is recorded as the total expansion. The water is returned into the chamber from the burette, but not the entire amount, because even though the pressure has been released and the tank begins to deflate, it still has been stretched a bit. The amount the tank has stretched to after deflation is called permanent expansion. To determine whether the tank is a pass or a fail, the permanent expansion is subtracted from the total expansion, which provides the technician with the elastic expansion. If the value of the elastic expansion exceeds the limits set for your cylinder, it will be condemned, and marked as such or with Xs over the DOT specifications. If the tank passes the hydrostatic testing, it will be stamped with the date and ID number for the shop that carried out the testing. See you in another 5 years!

Hydrostatic testing is mandated and regulated on such a high level for public safety. While SCUBA air tanks are made of durable metals, there are inherent dangers in dealing with highly pressurized gases within a metal missile. Compromised tanks can not only cause trouble underwater, they can cause damage and injury if one is dropped or happens to tear, causing the gas to propel the tank haphazardly around a room, with not much that can stand in its way. These are tests that not only keep people safe, they keep people alive!

Leisure Pro Staff

Leisure Pro Staff

Marketing Director for LeisurePro