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by GTP
547 days ago
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Minor nitpick here, but it's a fact I found interesting when I heard about it. The water inside a Moka doesn't boil, you can indeed observe that the water coming out from the top isn't boiling. What happens is that the air that is left inside the bottom chamber expands due to the heat, pushing the water upwards. |
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The custom where I am at is to load the moka pot with a grind and quantity that produces a significant barrier to the flow of water.
The alert that the brew is finished is the sound of the boiling hot water and steam spraying the coffee through the standpipe into the upper chamber, and it is absolutely under steam pressure, I’d say around 5 to 10 psi.
When the liquid water is low enough that it doesn’t get picked up by the lower tube, you get a significant outflow of pure steam hissing through the standpipe nozzle, and then it’s quiet, as the bottom chamber is now completely dry, as are the grounds when you dump them out.
It could be that if you use a coarser grind or less coffee than is customary here, flow restriction does not occur, and the pressure of the heated air and water vapor is enough to push out all of the water through the coffee without reaching 100c (should only take about 1/6 psi for a flow overcoming gravity to that height) but if you used that method here your coffee would fall under heavy criticism.
The violence with which the flow jets into the upper chamber and the volume and aroma of the steam serves somewhat as a social signal as to the “quality” of the coffee, so there is a strong incentive to heavily load the pots here.
Legends of exploding pots are common, as is precautionary disposal of pots whose threads have become excessively worn.
But I still have no first hand knowledge of anyone witnessing an explosion or even an over pressure venting event (there is a small pressure relief valve on the side of the vessel)…. So I suspect that the risk is not that high.