Im no plumber, but Im pretty sure that this is absolutely incorrect. From what Ive seen, most domestic machines, from a $50 Krups to a $2300 Expobar, use a vibratory pump manufactured by ULKA (or a chinese knockoff). These pumps are not adjustible; they simply pump at 19bar or whatever. Pressure regulation is usually achieved through an overflow pressure valve (OPV), which siphons excess pressure off back into the water reservoir. In some machines, the OPV is adjustable. In cheaper machines, it is not - the OPV is really just there as a mechanism to protect the pump. Often, the machines will be sold as having an 18 bar (or whatever) pump because it sounds impressive, but they could deliver anything, from 12 to 15 bar of pressure (numbers are just a guess). Unfortunately, pressure this high is far from a good thing. Im pretty sure that the higher the pressure, the more exact one must be with their grind/dose/tamp regime in order to get a decent shot time. Worse still, at higher pressures, shots seem to go blonde earlier than at lower pressures. On the weekend, we took Nims (rice) expobar to St Ali with the intention of playing around with some temperature control. Unfortunately, we got the pressure measuring portafilter and found that it was running at 13 bar, so we wasted a lot of time dialling it down to 9. It seemed to be a good deal more forgiving after that.