135 gallons is a lot of weight already on the inside wall, pushing outwards.
We're naturally assuming that the wall is thick enough to withstand the
pressure of that weight from within. Therefore, we're also assuming the
amount of low pressure on the upper air surface (created by the gentle pump)
is minimal (just enough to draw trapped gasses to the surface and suck them
out: it only takes a -1 lb/sq in. less than ambient over a period of several
hours to do the trick), while the actual weight of 135 gallons of fluid
keeps the outward pressure on the wall to prevent collapse (implosion).

Even so, Lum's posting below probably has a point if you're already using a
transfer pump. If you're racking or transferring with gravity (as some
wineries do), then a pump may still be an alternative option, in the way Lum
mentioned.

Whatever pump you choose, it will be a good idea to research the physics of
it all (and maybe experiment with a tank of water first).

Jeff




"MikeMTM" > wrote in message
s.com...
> Jeff Chorniak wrote:
> > Try a vacuum pump.
>
>
> Careful! On a thinwall tank you may well collapse the sidewall. They're
> designed to resist outward pressure, not inward. Think of the strength
> of a 2 liter soda bottle against bursting vs. how easy it is to deform
> it with your fingers. No experience to go by, just some basic engineering.
>
> --
>
>
> Mike MTM, Cokesbury, New Jersey, USA
>
>
>