Counterintuitively, adding more water to the lower chamber will not allow more coffee to be extracted at the same temperature; in fact, adding water reduces the volume of air whose expansion forces the water of the boiler up in the funnel, so that in typical operating conditions the volume of coffee is proportional to the volume of air in the lower chamber. On the other hand, the volume of coffee obviously cannot be much greater than the initial volume of water. The recommended "just under the safety valve" fill level thus produces near-maximum yield for any given final temperature.