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On Jan 11, 10:10 am, Steve wrote:

It's not steam pressure that forces the water through the grounds but
air pressure. Air will expand at temps far lower than 212 degrees.

Now it's getting fun ;-) Let's say initial air temp is 20C, heated to
90C. That's 70C delta. Expansion should be proportional to temp,
relative to abs. zero. So each added degree would add 1/(273+20)
fraction of volume. My calculator says 70/293 is about .24, or 1/4. So
it would take four cups of air to push one cup water out by the time
90C is reached. Do those contraptions have that much air space?

Yeah, that sounded a little fishy to me, too.

As
Wolfgang noted, this expansion doesn't wait until the water is 90C, it
happens all the way up the temp scale, so I'd think it'd be "better"
to wait and let the steam pressure push the water.

Agreed, bearing in mind that Wolfgang also noted that one doesn't need to
bring the full volume (or anything near that much) to boiling temp, and that
pressure is relieved by the water moving through and out of the system,
thereby mooting any supposed notable increase in the boiling temperature.

Wolfgang