Why difference in pressure between blind filter and brewing? - Page 3

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#21: Post by The_Left_Hand »

I backflush a Synesso Cyncra every day, at work. With a blind the pressure gauge reads at 10.5; however, during a pull it reads at 8.5.

Logically it's a very simple premise: it's the difference between applying pressure to a solid object versus a porous object. The coffee provides a finite resistance. The blind provides an infinite resistance.
—"What's sleep?"


#22: Post by fizguy »

double_pedro wrote:If so, isn't the point of all of this simply that the blind PF pressure measurement establishes is the maxuimum brew pressure but can't say anything further about the actual brew pressure as this depends on the grind, tamp, dose?
I think that was the point the OP was making.


#23: Post by duke-one »

Are there any machines with the OPV adjustment on the control panel? That would enable another level of control over the brew process. While were at it how about a flow meter readout for brew water in something like ml/s. I don't know if the standard flow meter in auto machines can be connected to a readout or produce a reliable signal at so low a flow but I know from instrumentation world that it can be done. I guess the maximum modern machine will have all parameters on a front panel readout and all user adjustable.


#24: Post by Kleefisch »

For what I see, a flow meter readout or (even worse) an OPV adjustment on the control panel would not be helpful in anyway. It would detract you from one simple rule: While producing, the only parameter is the grind.

An auto dosing espresso machine is a huge step to achieve stability in brew strength. Set it and forget it.
The same goes for the OPV. Thanks to the designers that they usually hide it under the hood.


#25: Post by sbrussell »

A very belated addition--but..
Brew pressure is obviously important, and a machine like a Silvia is factory set very high. So how do we adjust it down to 9 bar? If we use a blind PF, this thread indicates it will be lower. But how much lower? According to the posts from 0.25 bar to 2 bar according to the machine.
So why not crack the screw mount for the pressure gauge open until the flow dribbling out more or less approximates the flow of a shot being brewed, say 20-30 grams in 25 seconds. This would then enable a pressure ajustment more accurate than simply using the pressure gauge tightly screwed into the portafilter. (Not an original idea-I read it some time ago in another thread.)
Any comments?

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#26: Post by cafeIKE »

Another solution is to use a T fitting and a valve to adjust flow to desired rate.


#27: Post by sbrussell »

Much more elegant. But what are the threads? I went to a large hardware store and could not find what the threads are on the bottom of the Portafilter or how to convert to SAE threads. In fact, they couldn't determine the type of threads on the gauge side of the small fitting between it and the bottom of the PF.

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#28: Post by erics »

The threads on the bottom of the portafilter are 3/8-19 BSPP (British Standard Parallel Pipe). You can fit a standard US 3/8" NPT tee whose threads are 3/8"-18 NPT with a little bit of teflon tape or your choice of sealing fluid.

To answer your question in your post subject as to "Why difference in pressure between blind filter and brewing", do a site search on author = gscace and subject = OPV and you will come across a rather elegant (and, of course, correct) explanation for this fact.

Eric S.
E-mail: erics at rcn dot com


#29: Post by sbrussell »

I purchased the parts as shown in CafeIKE's photo and constructed something similar for the Silvia. It was a very tight fit due to how far back the brew head is. (I'll send a photo if someone is curious.)
Result: a 1 oz flow in 25 seconds lowered the brew pressure by a little more than 0.5 bar, from just below 10 to 9. That's what I got after reducing the pressure a little. (It was about 9.5 bar.) It seems that the differential between blind pressure and pressure with coffee flowing differs from machine to machine.

I also timed the rise from 0 to 2 bar with the valve cracked up to the above flow rate: just under four seconds, confirming the possibility of a 3 second preinfusion; cut the pump after 3 seconds, let the satured puck sit for 3 seconds to expand, then pull the shot. I pack about 2 mm below the edge of the basket when doing this--for example 19 g in a 20 g basket allows this amount of head space. Ten g in my single LM basket allows even more head space. I haven't tried the technique with the coffee leveled to the top of the basket so I don't know whether the space contributes to the success of this pre-infusion technique or not.