samuellaw178 wrote:The maximum temperature you get in the coffee cake seems to be at 124 Celsius for Brikka and 118 Celsius for Moka(preheated). Water doesn't exceed 100C unless the pressure is higher than 1atm**. Which means, indirectly, your experiment indicates that the pressure in Brikka goes up to 2.3 bar and 1.9 bar for the Moka... very enlightening...
It doesn't work quite that way because there is also air inside the moka pot. The pressure is the sum of the water vapor and air partial pressures. In the espresso world this goes by the name "false pressure". During the brew cycle the water vapor pressure keeps rising and the air pressure drops, but never to zero. This is the reason H. Barista said in his video that initial water volume has an impact on water temperature.
The other thing is that the steam tables assume liquid-vapor equilibrium. In the moka pot the temperature in the head space is significantly lower than in the water. I measured differences of 10-15°C between water and vapor; Navarini shows similar values. It turns out that the vapor temperature is a better indicator of the pressure when using the steam tables. This by itself would lower the pressure estimate by at least 0.6 bar. But taking into account the air pushes it back up part of the way. Navarini measured about 1.5 bar gauge pressure in his experiments.
It's a very interesting system from a thermodynamic point of view.
When the pot sputters (Navarini called this the strombolian phase
) the water, vapor and grounds temperatures converge. So at the end the water temperature actually is a good indicator of vapor pressure. Here is Navarini's plot of measured pressure with calculated air and vapor pressures superimposed. The green line is liquid flowed.