Duckbill--I'm not sure if your are kidding. Besides I thought it was illegal to transport a tank that was not up to date, not just across state lines. As in: pick it up and carry it to my car would be transporting it. No?
It's your cylinder, your private property. You can do with it whatever you want as long as it doesn't violate anyone else's rights. JMO.
I could go on and on regarding the legality of departments and regulations as opposed to Constitutionally authorised Congressional law, but I'll avoid a political discussion here and say that the original intent of the DOT (ICC of old) was to oversee interstate commerce (buying and selling across state lines). Regulations were not originally supposed to affect you and me, but now that regulations have wrongfully been given the force of law, and the DOT now regulates intrastate commerce as well, it may very well be that the DOT can cite one for simply transporting one's own cylinder in one's privately-owned vehicle within one's own state. If so, it's sad how far we have gotten from what our founding fathers were willing to die for on our behalf. Just a pet peeve of mine.