This morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator/Moka Pot on a gas top.

Diagram of the Italian Percolator Picture of the Italian Percolator

Chamber A is filled with cold water, and B with ground coffee. As you heat the water, steam builds up, increasing the pressure in A. This then forces water up through B, where it is soaked up by the coffee. Once the coffee has expanded, it provides more resistance, and as such the pressure in A increases further. At this point, the water is then forced through B, and up the pipe into C where it then bubbles out to fill the chamber. This makes total sense to me.

However, a problem occurs when you fill A above the safety valve shown in the second picture. The water in chamber A boils much earlier, and is then forced out through the valve, and also from B where the pot screws together. Very little water actually flows up into chamber C.

In my understanding, if you have two chambers connected by a pipe and you increase the pressure of the lower one, then surely water should flow into the upper chamber at a faster rate. It seems like this effect has an optimum pressure, above which an increase in pressure is actually detrimental to the effect. Why should this be the case, and is there a name or term for this process?


1 Answer 1


There are two things going on with your valve. Of course it is a safety device - if anything goes wrong (blocked) you want to prevent steam pressure building up so much that it literally explodes.

But I believe the height of the valve also plays a role. The formation of steam vapor bubble is the coffee is not a continuous process - the water locally superheated, then a (largish) bubble forms at a nucleation point. This means there is a sudden violent expansion of the water (the saturated vapor pressure in the superheated liquid is well above an atmosphere). Normally the space above the liquid absorbs this (the bubble expands and its pressure drops while the pressure on the volume above increases slightly). But when there is not enough volume (filled above the valve) the pressure will rise above the design pressure: so water will leak out of the valve and potentially other seals (which are not designed to withstand high pressure either). Of course the same pressure will drive the water very rapidly through the coffee above. This may be what makes the coffee taste bad.

  • $\begingroup$ Why would filling water above the valve mean that chamber A gets to a higher pressure? I don't follow that unless for some reason the valve doesn't open to water as easily as it does to steam. I suppose that's possible. But even if that's the case, how does this explain chamber C filling less than when chamber A is filled below the valve? $\endgroup$
    – DanielSank
    Aug 30, 2016 at 8:42
  • $\begingroup$ @DanielSank there is less ballistic volume to absorb the pressure spike after a bubble nucleates, which raises the pressure (for a given $\Delta n$, if $PV=nRT$ then $\Delta P=\frac{\Delta n ~RT}{V}$ - smaller $V$, larger $\Delta P$); further, the (volumetric) rate at which liquid can escape the valve will be much lower (viscosity) than for steam; then at the higher pressure water leaks out of the "seal" between A and C and that is where a lot of water is lost- at least that's my observation, and that of OP. $\endgroup$
    – Floris
    Aug 30, 2016 at 8:47

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