110% agree with you. If we define a vacuum as the boiler has a lower pressure than the atmosphere, then there is no way that valve can allow the boiler to vent to atmospheric pressure ( if it is sealing correctly).Paul_Pratt wrote:It's a safety valve.
The reason why they there is no pressure switch has been discussed countless times before. You will note that older machines always had 2 elements, a high power and a low power. Once up to temp turn to low power and it will idle away with an occasional hiss from the safety valve.
The missing plunger is easy to fabricate I do those all the time. I almost always use the wobbler weight with a pressure switch. If you machine the mating surfaces of the parts or add a gasket they are silent.
Whilst they may vent some false pressure on start up (you would still need to purge anyway, it would never bleed it all out) they will never open and allow the boiler to cool down to atmospheric pressure and you would still have a sealed boiler with a vacuum.
It does effectively regulate the temperature with the constantly energised low power element. The boiling point of water increases with pressure (this is how pressure cookers work) This valve will most likely be calibrated to 1.5bar which means the boiling point is 111.4C or 232.4F. Once the water reaches this temperature the boiling water and steam pressure will pop the valve. Some just sit there and wobble keeping a pretty regular pressure and temperature. The design relies on the lever group acting as a heat sink and cooling the brew water to 93C when it hits the puck. Or if you really think about it. To make sure the brew water is at 93C when it gets to the puck the boiler water must have a temperature greater than that achievable with atmospheric pressure.