My experience is that it is VERY HARD  to overfill one of the
disposable tanks from a 20# tank.  I also used a scale and did it by
weight at first.  I found that I could get nowhere near the rated
weight of propane in the tank unless I froze the tank first.  Even
then I repeatedly tested weight and got less than rated weight.  I
finally stopped weighing.  NOTE:  my tank refill gadget is home brew
I got years ago "from a guy."  The ones commercially available now
may fill the small tank faster or something so that it gets
fuller.  I am pretty sure it is illegal to transport the refilled
disposable tank.

Overfilled tanks:  I was once at a festival in a public park and
there were food trailers there.  The 100# tank on one of those
vented.  Let me tell you that was scary.  A huge plume of white vapor
shot out about 20' for about 45 seconds.  People were startled but
knowing what was happening, I was pretty terrified and started
getting the heck away.  The smell of propane lingered strong for 10
or 15 minutes.  I found some cops and told them, by that time it had
stopped, they kind of shrugged and said thanks.  Good thing there was
no source of ignition...

Edward

At 12:03 PM 4/29/2012, Tom Hargrave wrote:
>Coutney,
>
>You refill the green 1 lb cylinders? You need to understand how
>DANGEROUS this is. Those cylinders are filled by weight & not volume
>and when filled with liquid they are never 100% full.
>
>You can easily over fill one and a over filled 1 lb cylinder will
>rupture when it gets hot!
>
>Thanks, Tom Hargrave