Pressure The pressure that forces the water through the coffee powder depends not only on the density and the granularity of the powder, but also the amount of pressure, which the moka pot is undergoing. If the grounded espresso powder is to fine, the pressure in the lower part of the pot (1) will get to high. Therefore, the water will not vaporise through the funnel shaped cylinder (2/3), it will vaporise through the safety ventil (1).                                                                                                                                                                                                                                                                                                                                                                                                                                                           Category Moka Press Wikipedia This shows how important it is to have the perfect "grind" for your espresso maker. The "Magic of Physic" cannot work with a powder that is to fine or to large. Since pressure is necessary to force the steam through the funnel shaped cylinder and the pipe (3) the grind of your espresso is one of the most important aspects for a strong cup of Moka.