Leak Testing Notice from the FDA:

According to FDA guidance and industry standards, soap and water solutions used to leak test are not recommended and may even prove dangerous. Soap contains animal fats ( lard ) which burns easily and explosive-like in oxygen. Synthetic soap such as dish washing soap (Dawn, Joy, etc.) contain petroleum ( oil ). Some soaps contain both. These solutions leave residues on the valve that, when they come in contact with high pressure oxygen, can ignite and cause a fire.

The FDA requires 2 leak tests during the fill process, one while the cylinder is filling and another on the cylinder after they have been filled. This test must be done on every cylinder, to ensure that the contents of the cylinder is not leaking out, and that the patient will receive the full contents of the cylinder.

Applied recommends using a Certified Medical Grade Leak Detecting Solution.