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I got a moka pot a couple of months ago and have more or less settled on a formula I like, but one thing still really puzzles me. The quantity that ends up in the top chamber has started varying considerably. The pot is supposed to make enough for about two full cups, but sometimes only about half a cup's worth will come out and then it'll make the 'boiling over' sound that usually tells me it's done. If I try to keep it on the heat, it'll just keep furiously boiling and no more coffee will come into the top chamber. When I get a low yield, it'll always be really strong, so I can dilute it afterwards and the result is roughly the same, but it's still annoying.

I'm finding it very hard to pin down the variables that make it do this. Of course I always fill the bottom chamber the same amount - just below the valve. I have been varying the grind but I will get two wildly different results even between two brews using the exact same grind setting. I have now settled on a medium fine grind. I have switched from cold water to 90°C - but again, when I was trying both of those options out I was getting varying yields for each. The only thing that I don't keep all that precise control over is quantity of beans (I roughly fill to a finger over a certain marker in my grinder) but I don't see how that would influence things that much.

I must admit I don't understand very well the whole percolation mechanism (in my naive brain, it shouldn't work at all!) so I find it hard to reason through it. Any tips?

PS. If it makes any difference, the model I'm using is a Stellar SC63 6 cup

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  • Just for clarification: Moka pots are designed for a specific water-to-grounds ratio, determined by the bottom chamber (fill to a certain level, usually just under the valve, which you do) and the coffee chamber (fill completely, but don’t compress). The only variable that remains is just the grind. Coarser than espresso, finer than pour-over. Can you please clarify: do you fill the coffee chamber always in the same way? – Stephie♦ May 10 '19 at 14:48
  • @Stephie I do fill the coffee chamber almost completely. I lightly tamp it so the coffee isn't heaped, but try not to compress it too much. – Igid May 13 '19 at 12:19
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Yield and the quality of the brew depends on how water is pushed through the grounds. Some things to consider:

  • grind size: the finer the grind, the more pressure is needed to push water through. If you need to build more pressure, it may take longer to get to the same yield.

  • grind consistency: this is related to grind size. If you have larger and smaller pieces in your ground coffee, you may get channelling. To get an idea of what happens, the smaller pieces may be lumped together needing relatively much pressure to push water through whereas the water can easily be pushed along the larger pieces. As such, an inconsistent grind can easily lead to an inconsistent brew (including yield).

  • tamp pressure: do you tamp the grind (or do you level it out and push it with your fingers)? The more packed your grounds are, the more pressure is needed to push water through.

  • Heat, how big you set your flame and how far you set your moka pot compared to the flame will influence how pressure builds. A very large flame with the moka pot directly above it will heat quickly and build pressure faster. This might result in a faster brew. You want to watch out though, you might end up heating the upper chamber too much making the first part of your brew burn making the whole thing needlessly bitter.

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