The title line says it all. I know um.. empirically that it's ok, but I'd like to hear a confirmation of safety and/or a more extensive empirical experiment.
The tank in question is a BBQ tank measuring about 1 ft in diameter and about 1 1/2 ft in height.
Edit
Let me just add car gasoline tanks as a reference for comparison. The lore from folks who own a car they do not drive in winter is to fill the tank in Fall, then to park the car in the garage with the tank full. (The typical garage is not heated, but it absorbs heat from the adjacent structure, and so as an extreme the temperature can get close to freezing—32F/0C.) If the tank is left half-full, water condenses (and so Harper's description makes sense), but it condenses inside the tank. Needless to say, come Spring the engine is not too happy to be fed gasoline contaminated with water.
Will water condense in a similar way inside a propane tank left outside in freezing temperatures (of as low as 15F/-10C, say)?
Best Answer
I'm going to be a little bit contrary and say, that depends. If the temperature where you live gets below -44°F, then, yeah, you will need to keep it a few degrees above that. According to articles here and here the boiling point of propane is -44°F (-42.4°C), so in order for your propane to exist as a vapor, the temperature of the propane need to be kept above -44°F. And I'm guessing that to generate at least some usable pressure, it would need to be 10 - 20 or so degrees above that, and that doesn't take into consideration that fact that the temperature of the propane will drop as it turns from liquid to vapor (as Harper points out). Simply moving the tank indoors temporarily away from any ignition sources would probably suffice.