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.