"Roger Shoaf" wrote in message
...
I am thinking about making an espresso maker and one of the design
considerations is the ability to tightly control the temperature of
the
water as it is forced through the coffee. It appears that one of
the week
points of the current designs is the lack of thermal stability of
the brew
head.

As water flows through the head, there is a heat loss from the water
to the
brew head so it is hard to maintain a stable water temp.

To address this I want to introduce water hotter than the desired
temperature into a holding chamber attached to the brew head and
allow the
bleed off heat to soak into the brew head while the water is cooling
to the
optimum brew temperature.

To further complicate matters, the coffee extraction process relies
on a
stable pressure as the water is forced through the coffee, so I want
to use
regulated air or gas pressure to be able to give me a easily
adjustable and
steady pressure.

Now to my question. Since the optimal extraction temperature is
just under
the boiling point, I suspect the heat loss will be such that
starting with
boiling water will not be hot enough, so I want water hotter than
boiling to
start with. This will require heating the water under pressure.

What I need to know is the relationship between pressure and
temperature.
Lets say I wanted to get the water to 225F. How much pressure would
there
need to be to raise the boiling point of water to 225F? I figured
one of
the many steam engine buffs here would know.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the
tube, then
they come up with this striped stuff.


From the steam tables, 20 psia is 227.96 degrees

220 deg is 17.186 psia
230 deg is 20.779 psia

Note these are absolute pressures...