Exchange of money really isn't the key. You got to remember, the DOT's madate is fairly narrow, and the regs regarding tanks are not intended to promote the safety of all cylinders in all uses, but to regulate their safety when used as packaging for the transport of products in commerce. So buying the tank for your own use doesn't enter it into commerce, nor does filling it, even if the tank is subsequently transported, since the tank and the contents are not being transported in commerce at the time it is sold, filled or serviced.
There's a fascinating DOT letter somewhere concerning the use of DOT specification high pressure cylinders for fuel tanks in alternate fuel vehicles, which says clearly that these cylinders do NOT fall under DOT regs, and do not have to be routinely hydrotested, because they aren't being used in commerce. This is despite the fact that they are routinely filled commercially. If the owner was to SELL any of the contents, the letter says, then the cylinders would fall under DOT regs and require hydrotesting.
It is often said here, as you do, that a owner can fill a privately owned tank regardless of its hydro or inspection status since it is not being used in commerce. This is true, but there's also a DOT letter that clearly states that it is even legal for a commercial entity like a diveshop to do the same, for the same reason - the diveshop is not transporting the tank in commerce, but just filling it. The whole transaction takes place in the diveshop. If the tank owner was then to take the tank somewhere and sell the contents, by, say, renting the tank, then it would have been transported in commerce - but it would be the tank owner, not the diveshop that filled it, who was doing so, and hence the tank owner's responsibility to see that the DOT regs were being met, not the diveshop's.
There are a lot of gray areas, but since there are not DOT tank police waiting at each street corner and dive spot, they are probably not worth worrying about.
captain:
True, if the filling, transporting or any other use results in the exchange of money than they are considered being in commerce and fall under DOT regulation. A private indivudal who owns, fill and tranports his own personal tanks is not bound by any regulation or law to follow DOT regulations which include not being required to have them hydroed or visually inspected..
