Hydrostatic testing is the most common way to check an SCBA cylinder for leaks or flaws.  During a hydrostatic test, the cylinder is examined to ensure it can safely hold  its rated pressure. SCBA cylinder hydrostatic testing is crucial as such containers can explode if they fail when containing compressed gas.

A hydrostatic test consists of filling the cylinder with a nearly incompressible  liquid—usually water—and examining it for leaks or permanent changes in shape.  The test pressure is always considerably more than the operating pressure to give a margin for safety. Typically, 150% of the operating pressure is used.

Water is commonly used as it is almost incompressible and will only expand by a  very small amount. If high-pressure gas was used, the gas could expand up to several hundred times its compressed volume, running the risk of serious injury.

All SCBA cylinders require periodic hydrostatic testing as required by 49 CFR  180.205. The frequency of the maintenance depends upon the cylinder  material.

Cylinders should be tested every five years and have an indefinite service life until they fail a hydro test.

Cylinders (not including hoop-wrapped) should be tested every five years and have an indefinite service life until they fail a hydro test.   

Cylinders should be tested every three years and have a 15-year service life.   

Cylinders should be tested every three years and have a 15-year service life.   

Cylinders should be tested every three years and have a 15-year service life.

Cylinders should be tested every five years and have a 15-year service life.

Cylinders which show evidence of exposure to high heat or flames (paint turned to a brown or black color, decals missing or gauge lens melted) need to be removed from service and re-hydrostatic tested prior to recharging. If there is any doubt about the suitability of the cylinder for recharge, it should be returned to a certified hydrostatic test facility for examination and retesting. 

Any evidence of a crack, defect or damage requires the cylinder to be removed from service.

Should also be performed on a regular basis as recommended by the 
Department of Transportation (DOT). The visual inspection should include, but is 
not limited to, removing the cylinder valve, inserting a high-intensity light 
probe and angled mirror into the cylinder and examining the inner surfaces of 
the cylinder. This inspection is necessary to aide in identifying defects in the 
inner surfaces of the neck and shoulder area of the cylinder. 
The  DOT requires that hydrostatic retesting and re-qualification be conducted by 
registered agents who have been certified by the DOT and who have been issued a 
valid Re-testers Identification Number (RIN) by the DOT Research and Special 
Programs Administration (RSPA). The recommended visual inspections do not have 
to be conducted by a DOT-certified RIN holder. However, the visual inspection 
should be performed by an individual who has the proper training and is 
competent in performing visual inspections.