Re: 20# propane tanks - half full or half empty???

[Rush <jdrush@enter.net>]

Chas. Schlismann wrote:
> If you have the $$ and the tanks are
> reasonably close, you can buy more line and a valve that will allow
> you to run off one tank then switch to the other when the first is
> empty.  For even more $$ you can buy a valve that does this
> automatically.

Can someone tell me what these are called? I'm having trouble googling 
these and I need one now!

Jon