Hello, I have been reading a lot of old threads about pressure and how important is not to exceed too much the 9 bar mark. Well, what I would like to know is the following:

Parting from the following premises:
- The puck flow resistance increases through the brew
- If the pressure in the water circuit increases, so does the brew flow
- The 9 bars that push against the puck are the natural generated pressure in a water circuit which's only exit is the puck and which has to exit at +o- 2ml/sec
- During the brew, as the puck flow resistance increases, so must the water circuit pressure in order to maintain the brew flow-rate of aprox. 2ml/s

What I don't understand is:
How is it that adjusting the OPV could in any-way improve the shots quality without affecting the amout of coffee produced??

theCoffer wrote:Hello, I have been reading a lot of old threads about pressure and how important is not to exceed too much the 9 bar mark. Well, what I would like to know is the following:

Parting from the following premises:
- The puck flow resistance increases through the brew
- If the pressure in the water circuit increases, so does the brew flow
- The 9 bars that push against the puck are the natural generated pressure in a water circuit which's only exit is the puck and which has to exit at +o- 2ml/sec
- During the brew, as the puck flow resistance increases, so must the water circuit pressure in order to maintain the brew flow-rate of aprox. 2ml/s

Hi Martin. Interesting post, but all these points are debatable:
- As extraction proceeds, solids are removed from the puck. Resistance to flow tends to decrease rather than increase towards the end of the pour.
- Somewhat surprisingly, experiments by folks like Andy Schechter indicate that flow rates are maximal around 9 bar, and decrease with both lower and higher pressure.
- The "natural" pressure of 9 bars may be correct for a normale double flowing at 2ml/sec, but does not hold true for all pours (e.g., ristrettos).
- Again, flow resistance decreases as solids are extracted from the puck. There is no need to increase pressure to maintain flow; quite the contrary. Lever fans glory in the decreasing pressure profile of their machines.

theCoffer wrote:What I don't understand is:
How is it that adjusting the OPV could in any-way improve the shots quality without affecting the amout of coffee produced??

The OPV sets a limit on the maximum pressure, generally agreed to be optimal at around 9 bar. For normale doubles, this is not much of an issue. But ristretto pours can result in higher pressures if not limited by an OPV.

This particular lever fan glories in the declining pressure profile of the spring lever, and has often mimicked this profile on the manual lever. As John remarked, with the removal of the good stuff from the coffee bed, the bed presents less resistance, and so less pressure is called for as the puck becomes depleted.

Maintaining the highest pressure for an arbitrary length of time, i.e. without examining the ongoing quality of the extraction, can result in over-extraction, as the extraction continues even after the puck has been depleted of the good stuff.

The home-barista does well to ignore every parameter whose purpose is the production of requisite beverage volume; volume has nothing whatsoever to do with the excellence of the espresso.

OK, that would leave my doubts more than clear, thanx John and thanx 888

timo888 wrote:The home-barista does well to ignore every parameter whose purpose is the production of requisite beverage volume; volume has nothing whatsoever to do with the excellence of the espresso.

I thought a lot about this when I spent a week or so experimenting with singles last month. I decided that the experience of espresso enjoyment for me required having enough volume that I could sit, sip and savor the coffee thoroughly. Standing by my machine and throwing back a single like the Italians (or worse, a teeny-tiny ristretto single) was not nearly as enjoyable as a sit-n-savor double of a reasonable volume. So, no, for me a tiny puddle of coffee extract is not an excellent espresso. I spend some effort on grind and mass to produce 1-1/2 to 2 oz of coffee.