The pressure in the tank is very high, um, 200+ psi comes to mind.  The
appliances need 11" water column to operate.  That's about 1psi.

A pressure gauge on the tank will not show how much propane is left because
the pressure doesn't drop until all the liquid has turned into gas.  Then
it's too late...

Edward

At 08:23 PM 2/17/2006, you wrote:
>Why would it have to go before the regulator?
>
>I was imagining a shutoff valve at the disposable bottle's connection, and
>if the main tank it empty, just close that valve and open the valve for
>the little propane botttle.  You don't need a regulator for the little
>bottle right?