Why difference in pressure between blind filter and brewing?

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acquavivaespresso

#1: Post by acquavivaespresso »

Marshall wrote: After testing ..... James Warren determined that his previous measurements with a blind filter overstated the pressure by 2 bar. So any machines set to a blind filter 9 bar actually produce 7 bar with a continuous water flow
with all due respect to your expertise in espresso please do not try and change physics : 9 bar is 9 bar, in the sense that at such pressure the OPV spring will open and release excessive pressure (keeping pressure in the brew circuit at set pressure, 9 in this case), you would ONLY run at 7 bar if you grind coarser or put less coffee in the portafilter, but in such case the OPV is not working and you know by the flow that you are not brewing at correct pressure : do not run into the same mistake that some manufacturers make, and that's setting the OPV at 11.5-12 bar because the OPV spring is actually taking part of the pressure : pressure is an absolute value where 3 and 9 do not make 12 and 9 stays 9 : the OPV must be set at 9-9.5 if that is to be your brewing pressure, or you get over-extraction:
OPV's can be built in many ways but even in the most professional looking ones the actual working part is the spring and spring hardness is only related to the size of field regulation that you want to achieve.
With ref: to a previous topic related to Fluid o Tech Bypass valve (OPV) not being professional enough, it should be noted that for that very bypass is produced by a hot die stamping mould (WHICH ALONE MAKES IT MUCH BETTER QUALITY than the so called professional OPV pointed out as a better quality one)
Yes, I am learning quite a bit from you guys.


...split from More of my clarity quest with Dalla Corte Mini by moderator...

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Marshall

#2: Post by Marshall »

acquavivaespresso wrote:with all due respect to your expertise in espresso please do not try and change physics
With all due respect, you have made two mistakes:

1. You confused the reporter with the source. I was only reporting what Mr. Warren told me.
2. You assumed Mr. Warren was an ignorant tinkerer. Mr. Warren is the technical director of Dalla Corte USA.

As for physics, I will leave the writing on fluid dynamics to others far more knowledgeable than I. Mr. Scace?
Marshall
Los Angeles

acquavivaespresso

#3: Post by acquavivaespresso »

Marshall wrote:you have made two mistakes ....
When I am wrong I do apologize, still Mr. Warren CANNOT state stupid things; being technical director brings also the responsibility to put out correct information. I see from your posts that you are a kind of authority in the sense that you back your posts with personal experience, so you SHOULD NOT report a report that is utterly mistaken because you become part of it and actually seem to share the opinion.
Still you do not say if you agree with my post, I am not a technical director myself but probably Mr Warren has never built an espresso machine, and anyway the point has nothing to do with espresso machine and is just mis-information : think of the poor guy who buys a DC Mini and sets 2 bar higher (with terrible brew results) because he read Marshall reporting that Mr Warren stated .....

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HB
Admin

#4: Post by HB »

acquavivaespresso wrote:with all due respect to your expertise in espresso please do not try and change physics : 9 bar is 9 bar, in the sense that at such pressure the OPV spring will open and release excessive pressure (keeping pressure in the brew circuit at set pressure, 9 in this case), you would ONLY run at 7 bar if you grind coarser or put less coffee in the portafilter...
I regularly see 0.5 bar spread between blind filter and flow measurements; I attributed it to added resistance to flow when all water is being diverted versus a fraction of same. Those with advanced physics degrees are invited to offer an explanation.
acquavivaespresso wrote:When I am wrong I do apologize, still Mr. Warren CANNOT state stupid things...
While Mr. Warren's claimed delta measurements are greater than I would expect, they don't seem beyond reason. For example, teeing the brew pressure gauge before/after the gicleur will show different readings. Typically the gauge is tee'd in prior to the gicleur, so it will read higher than the grouphead reading. Again, I'll defer to those with advanced physics degrees for a technical explanation of why that is.
Dan Kehn

acquavivaespresso

#5: Post by acquavivaespresso »

HB wrote: I regularly see 0.5 bar difference ....
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
Pietro

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cafeIKE

#6: Post by cafeIKE »

acquavivaespresso wrote:with all due respect to your expertise in espresso please do not try and change physics
Dynamic pressure can vary greatly from static. If the OPV is set at static pressure, it could be much higher than the flow on a puck. The pressure would only be equal if there was no flow through the puck. Think of dripping tap. The water main pressure is about 4-5 bar, but the flow pressure is, for practical measurement purposes, zero. Ditto pre and post gicleur pressures in an espresso machine on a puck or blind basket.
HB wrote: I regularly see 0.5 bar spread between blind filter and flow measurements;
+1. See Digital Pressure Adapter

acquavivaespresso

#7: Post by acquavivaespresso »

cafeIKE wrote:dynamic pressure can vary greatly from static (or vice versa)
I still do not understand if you agree with me, since I am saying exactly the same when I say that blind filter setting sets MAXIMUM pressure in brewing circle before OPV spring collapses and discharges excessive pressure: in a dripping tap the main pressure is the maximum OPV pressure, main pressure does not change if your tap drips os is fully open , not exactly with your espresso machine where the flow rate is determined by puck consistency : so if you set pressure say at 9 and are actually brewing at 8 you have to add coffee or grind finer because 9 is your ideal brewing pressure, below you are under-extracting above you would be over-extracting but the OPV does not let you go above.
Yes I read the ref to meter on E61 group where it shows rightly that brewing pressure is not equal to MAXIMUM (possible) OPV pressure, and I assure you that there is no contradiction.
Pietro

gscace

#8: Post by gscace »

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
Pietro
Hi:

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.

-Greg

acquavivaespresso

#9: Post by acquavivaespresso »

cafeIKE wrote:if the OPV is set at static pressure ....
difficult to say, you assume that the blind filter is set at static pressure , but WHY is the OPV discharging all the water in circle (that's the actual pump flow rate at the set pressure): it could not be more dynamic than that.....
Pietro

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JonR10

#10: Post by JonR10 »

acquavivaespresso wrote:...but WHY is the OPV discharging all the water in circle (that's the actual pump flow rate at the set pressure): it could not be more dynamic than that.....
Pietro, if you will read Greg's post above carefully you may learn that your logic is incorrect. It seems to me that even though you claim not to be an expert you post as if you think you are one. The physics of fluid pressure and flow is not so simple as you seem to think, and so your posts clearly show that you do not actually understand the workings of the "simple" valves in question or dynamic vs. static pressure.

Although I have the education and experience to understand Greg's post in academic terms, to me it ultimately boils down to what shows up in the cup. In my experience, it usually helps to lower the OPV setting from the manufacturer's default. Each espresso machine reacts a bit differently so experimentation is helpful in optimizing taste in the cup for one's personal preferences. I have found that it's best to keep the measurement consistent (that is, if you measure against the blind filter then ALWAYS measure against the blind filter).

But this is why I don't miss having a brew pressure gauge on my machine, it's just not necessary. I have a portafilter pressure gauge I can use for making OPV adjustments but it is most useful for determining the actual result of the adjustment (i.e. max pressure was lowered 0.25 bar from previous setting)


I sincerely hope this helps,
Jon
Jon Rosenthal
Houston, Texas