acquavivaespresso wrote:I am not trying or pretending to be the clever one : with a blind filter you know exactly the pressure that you have in the brewing circle, and you have the pressure meter fit at the end of blind filter, of course when you actually check the brewing pressure you have to place the meter somewhere along the brewing circle, ideally at the pump output, onto the brewing head itself or anywhere in between : whatever pressure you set with blind filter shall be maximum pressure that you can get in brewing circle (just because above that the OPV spring will collapse and discharge excessive pressure:the very brewing pressure is determined by grind coarseness and quantity, if it stays below the OPV is not working, if it tends to go higher the OPV shall discharge excessive pressure: don't you look at flow rate to know when your brewing is correct ??
Manufacturers claim 15 or 18 bar pumps and that's misleading since you ONLY need 9-9.5 bar to brew espresso, so all pumps SHOULD feature a bypass (OPV) to prevent excessive pressure and overextraction, but then you have to explain to the end user that he is not doing it right if grinding too fine or overdosing does not give a nice extraction : if pressure goes up at least you get some extraction ......is kind of selling a 200 miles an hour car when you can only drive 55 miles ...
I am not a physics expert but it is only as simple as that
Your understanding of pressure relief valve function is oversimplified. So I'd like to help, and also show you why Marshall and Mr. Warren are correct.
Force exerted by a spring is given by K * X, where K is the spring constant, expressed in force per unit of displaced length, and X is the displacement of the spring from the free length. So as the spring is compressed, the force exerted by the spring increases linearly (for a non-progressively wound spring). So the force exerted by the plunger within the pressure relief valve is equal to the spring constant times the difference between the free length of the spring, and the length of the spring when the plunger is resting on its seat. Pressure is force divided by area. The cracking pressure of the pressure relief valve equals the force exerted by the plunger on the seat divided by the surface area of the plunger.
Now lets examine why there is a difference in pressure between pumping against a blind filter and when brewing coffee. When water delivered by a pump is forced through the pressure relief valve, the plunger within the valve is forced away from the plunger seat, as needed to accommodate the volumetric flow rate through the valve. When this happens, the new equilibrium pressure between the water and the pressure relief valve plunger is higher than the cracking pressure because the piston has moved away from the seat and the spring is further compressed. The change in pressure is dependent on the amount of liquid flow through the valve, the diameter of the plunger, the geometry of the outlet orifice that is uncovered by the plunger's motion, and the spring constant. The pump in the DC Mini produces excess flow rate at 9 bars, compared to what is needed to brew espresso. This means that there is always excess water flowing through the pressure relief valve to some degree when the machine is brewing coffee. Vibe pump systems using relatively long-traveling pressure relief valves, such as the one being discussed here, produce large pressure differences between actual brewing and pumping against a blind filter. In the case of pumping against the blind filter, all of the pumped water flows through the relief valve. When brewing, a large fraction of the initial water flow is absorbed by the coffee cake at the start of the extraction process. Then a smaller, but still very substantial fraction of the total flow exits the system through the group and into the cup. Thus the equilibrium pressure imparted by the relief valve is less when brewing than when pumping against the blind filter.
The 2-bar pressure difference that Marshall mentioned and that Mr. Warren measured is certainly within reason, based on my experience with the specific components in question, and my measurements of similar systems.
Hope that helps.
On another topic - not everyone here is an expert in all things coffee, but the collective knowledge of this group is pretty high, with many participants bringing their own personal expertise. We have espresso machine engineers participating here, coffee shop owners, baristas (including the 2007 World Barista Champion), professional roasters, experts in machine design of all types, folks whose tasting skills will blow you away, and a high level of passion. So we rely on each other to fill in the gaps and we learn as much as we can from each other. Our discourse is civil, for the most part. Your tone with Marshall seemed rude to me, and I didn't see you bringing much to the party. I think you owe him an apology. And yes, there is plenty to learn from folks here. This is one of the best espresso resources on the net.