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.