EDIT: see comment at end regarding a simple means to completely fill the tank. I don't generally criticize other reviewers, but I think some of the reviewers here (one and two stars) have gotten this confused with a different product. This device will not "freeze up" your grill, as it does not attach to your grill; and there is no "shut off" valve necessary, because your 20-lb tank already has a shutoff valve. I suspect those reviews are for an adapter to attach a 1-lb tank to an appliance that normally takes a larger tank (which I also have, looks somewhat similar to this, and costs about the same). A really important safety consideration: tighten the adapter to the big tank with a wrench. When you are removing the 1-lb tank after filling, you want the little tank to detach from the adapter (so the little tank's valve closes) and not come off with the adapter still on it, in which case the propane inside the little tank is going to blast out. Wrench-tighten the adapter to the big tank, hand-tighten the little tank to the adapter. Rather than repeat other reviews of this simple (and quite overpriced for what it is, although it will indeed pay for itself) device, I'll just give a few hints from experience. The sequence of what tank gets connected first is quite important, but not so much for safety as for operation. You will need to put your 1-pounder in the freezer; physics students will note that the 50 or 60 degree F. drop in temperature will create barely enough pressure differential to even start transferring propane from one tank to another, but the secret is that pressurized propane is a liquid so just a little vacuum gets a disproportionately large transfer. But as I discovered...attach the Mr. Heater to the big tank FIRST because if you attach it to the 1-lb bottle after you take it out of the freezer you'll hear a WHOSH! as air rushes in to fill the vacuum, and you're not gonna get any propane transferred after that! (Cure: leave with Mr. Heater now attached on counter for several hours while the air inside warms back up, remove Mr. Heater, back to freezer, avoid stupid mistake next time.) With the outside temperature around 70 F., I can only manage about 1/3 full. Frankly, submerging tanks in hot and freezing salt water is too much effort for me and I suspect for most people too, and I'll bet most people don't have a vacuum line to draw a vacuum first. In principle if you pulled a vacuum on the 1-lb tank you'd wind up filling completely it full and that would be downright dangerous, but as a practical matter the propane flashes off as it fills and eliminates the vacuum before it's completely full - I've seen people warning never to fill a tank 100% of the way full but actually it's impossible to do unless they override the tank's pressure relief valve (bad). But if getting 2/3-3/4 full takes an ice cream salt bath and a double boiler and a vacuum pump while getting 1/3-1/2 full takes a brief pop in the freezer, I'll settle for refilling a little more often. You'll also hear people say it is illegal to refill or transport refilled tanks. That's not true, at least on a federal level, where the restriction is on commercial transport for sale or use in support of commerce (and even then, what's illegal is actually the mis-labeling of the tank, not the refilling - 49 USC 5124 cites 49 USC 5104, which deals with hazardous tank labels). But that doesn't mean your state agrees, so there could be state or municipal constraints on transport. EDIT: I discovered an incredibly easy way to fill the tank, with no salt ice, hot water, or magic. READ THIS COMPLETELY BEFORE YOU TRY. Do the canister-in-the freezer thing and fill it the expected one-third or so up. Don't put the big tank in any hot water; don't put the little tank in ice water or an ice cream bucket; don't stick needles in a Troll doll. Just put the little tank back into the freezer for a few more hours. Now, repeat the filling and you can refill the small tank as full as you want! This is of course that as you fill, the big tank gets colder (and with less and less propane, it will get colder faster) and the little tank gets warmer (Conservation of Energy), so chilling down the ~1/3 pound of propane you previously transferred creates a heat sink and keeps the small tank colder than the big tank. Physics! Which brings me to an important safety disclaimer: while I know from physics it is impossible to truly fill the tank 100.000% because of the evaporated gas inside, by letting the second fill continue until all transfer finished (no more hissing sound), the 1-lb tank was so full that I could not even hear any liquid sloshing inside when shaking the tank. That's probably dangerously full, and I connected it immediately to my BBQ to run some of the propane back out of it. So either do this carefully or don't do it at all, and remove the tank BEFORE the transfer is completed (in my experiment with a sample size of one, I could hear the transfer clearly slowing down before the hissing completely stopped).
