@Hollowpoint- Actually, I too made a mistake. It was getting pretty late last night and I must've been sleepy. I've since corrected to CGA 677, as you say. I wanted to correct that right away before anyone bought the wrong regulator for these high pressures. It is CGA 677 as Hollowpoint says! Definitely *not* the standard SCBA fittings (which are 347). Apologies for the confusion. @Rey - I live in Massachusetts, about 25 miles north of Boston. It is fairly easy to get industrial and scientific things in this area since there is a big city close by. @Brian10956 - I've been filling my 6.8L tank to 4500 psi. Once the big nitrogen cylinder drops under that, I'll just use it to fill to whatever pressure it can and top off the rest of the way with my YH. Good question as to what to do once it drops below about 2800 psi. One thing I'll probably do at that point is run an experiment I've been wanting to do - going to try boosting the input of the YH to see how much that speeds up filling. Just have to re-thread the air input port to something useful. If that works (which it should), I'll just set the regulator to about 150 psi and feed the YH with it. @Eaglebeak - While I wouldn't recommend it, I suppose technically if your fittings don't leak and if they are designed to handle the repeated stress of cycling up and down between atmosphere and 6000 psi, then it would be functional. But the 6K cylinder tank valve is very heavy duty (pic attached) and not conducive to tiny manual adjustments. It's really an open/closed valve. I've never had much success at cracking open a big gas cylinder valve. The regulator is expensive, but as many people here have stated, these high pressures can be dangerous. I'd consider the regulator expense a sunk cost - buy it once and don't think about it ever again. I have regulators in the lab that are 15 years old and still functioning like new. Here is a pic of the N2 tank (red) setup attached to a 45-min bottle via microbore hose. The regulator on the grey UHP Ar tank behind the red nitrogen tank is at least 15 years old and still working perfectly (it's for a 4000 psi tank with 200 psi output).