It is funny, the scuba industry makes up all kinds of names for many things including pressure cylinders. It is kind of convenient to call different cylinders: low pressure, medium pressure, and high pressure. But technically and legally they are all “high pressure cylinders”. The definition of high pressure cylinder comes from the CFR (Code of Federal Regulations) and CGA (Compress Gas Association). A low pressure cylinder is like a propane tank. A low pressure cylinder is up to 500 psi. Anything above that is a high pressure cylinder.
I do like some of the distinctions we make in the scuba industry, but they are more of a slang than a legal definition and that is why they are not standard and always changing.
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The US Navy used DA Aqua Masters with 3000 psi aluminum cylinders on a regular bases back in the 60’s and 70’s. The aluminum cylinders were specifically made for the Navy under the mil-spec Mil-C-24316. The cylinders were made from 6061 aluminum alloy pipe. This cylinders were designed to be non-magnetic. The primary fabricator was PST. Bryan has a copy of the Mil-spec in the download section.
I don’t believe the Navy regularly overfilled cylinders (in the 60’s) to a pressure higher than allowed by regulations, but I do not have an easy way of confirming that. I may ask around. I do know that in today’s Navy that would not be the practice.
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All the DOT steel cylinders built under the CFR (Code of Federal Regulations) code 3AA come plus stamped (+) from the factory and they can receive the 10% overfill when there is a plus stamp next to the most recent hydro test date. The + stamp can be renewed during any hydro test, even if it was skipped during some of the previous hydro testing.
All of my steel 72 (some dating back to the 60’s) have fresh + stamps next to their most recent hydro test. I had them all (about 18 cylinders) hydro tested in 2015 when my LDS was closing and they all have the +stamp.
A steel 72 actually holds about 71 cuft of air when it is filled to 2475 psi (that is including the 10% overfill). When they are filled to the stamped pressure of 2250 psi, they actually hold about 64 cuft.
AFAIK, manufacturers in the US have always rated their 3AA steel cylinders by giving the estimated air volume with the 10% overfill capacity.
I don’t know of any manufacturer ever offering a 79 cuft cylinder at 2475 psi (stamped 2250) , but I have no way of confirming this. I do know that the most popular cylinder offered by US Divers and others was the well known steel 72, but this actually held about 71 cuft at 2475 psi with the 10% overfill (stamped 2250).
BTW, the mil-spec for the standard steel 72 is Mil-C-24447. There should also be a copy of that spec in download section.
Here are a few paragraphs from Mil-C-24447
3.2 General. The cylinder shall be for use as the compressed air cylinder in demand type, self-contained underwater breathing apparatus. The cylinder shall conform to DOT Specification 3AA as set forth in CFR, Title 49 - Parts 1-199 and as specified herein.
3.3 Physical parameters. The cylinder shall be of open-hearth steel or equivalent, and of uniform quality, in accordance with figure 1, and have an overall length of 25 ± 3/16 inches , The cylinder 'Wall thickness shall be such as to attain the required buoyancy (see 3. 3.3) and shall have a 0.156 inch minimum thickness.
3.3.1 Service pressure and capacity. The cylinder shall be designed for 2250 pounds per square inch gage (psig) service pressure. Each cylinder shall be hydrostatically tested to withstand 3750 psig (see 4. 5.1.1). After final heat treatment, but prior to final cleaning and painting, the cylinder shall be capable of withstanding the internal hydrostatic test specified in 4.5.1.1. Capacity shall be 71 ± 1 cubic feet air supply at 2475 psig. The cylinder shall have an internal volume of 700 ± 30 cubic inches.
3.3.6 Expansion characteristics. When hydrostatically tested to 3750 psig, the cylinder shall exhibit a total volumetric expansion not exceeding 57 cubic centimeters (cc). The permanent expansion (PE) shall not exceed 5 cc, the remainder shall be elastic expansion (EE).