On Apr 5, 10:41 pm, wrote:
I have a 500 gal. propane tank in my yard for heating. I am changing
suppliers so the old company is going to come and get their rental
tank. The company I have been doing business with are real a$$holes,
so I want to make sure there is no gas left in the tank. The tank
contains about 5% and the heating season is almost ended. If there is
any gas left, I'd like to fill all of my 20lb cylinders and if needed,
I can borrow several 100lb ones from a neighbor. I can always use
these, and I want to make sure the bulk tank is empty, but dont want
to just waste it. I have the pipe adaptors for both tanks and a
certified for propane hose. Do I just connect from tank to tank, or
should I do it after the bulk tank regulator?

Yes, I know that the 20 Lb cylinders should not weight more than 20
lbs more than the empty tank, and I have something to get a rough
weight measurement on them. I'll underfill them just to be safe, and
they will remain outdoors after I do it. Of course all this will be
done outdoors too.

Has anyone ever done this? I've filled the refillable torch bottles
from 20lb cylinders in the past, so I dont see this all that much
different other than the amounts.


OK it's been a while since I took physics, or chemistry for that
matter, but let's see. In the tank there is propane, some of it
gaseous, some liquid. The pressure is high enough to liquefy it at
ambient temperature. As you draw off some gas (for your furnace, or
whatever) the pressure goes down a little, and some of the liquid
'boils' into gas. Until the liquid is all gone, then when you take
out more gas the pressure just goes down.

So if you take gas off the top to fill up a smaller tank, it will fill
the smaller tank until the pressure in it is the same as the pressure
in the larger tank. I think if the two tanks are the same
temperature, it will stop with only gas in the smaller tank. But if
the smaller tank is colder than the bigger tank, gas will condense
into liquid form in there, gradually filling it up. (As it condenses,
more gas will migrate over from the bigger tank.) All speculation
here.

Another possibility might be if you could somehow stick your filler
tube down to the bottom of the large tank, so that the pressure would
force liquid out instead of gas. Like a seltzer bottle. -- H