Espresso brew pressure - conflicting answers

#1: Post by **BaristaMcBob** » March 3rd, 2021, 5:33 pm

Luca Bezzera says that the 9 bar range is for big rotary machines. Machines with ULKA vibe pumps should run at 11-12 bar. I tried for months to get my BZ10 to produce a decent shot at 9-10 bar. I even purchased a new grinder and a new VST 20g basket. The extraction was always way too fast and no grinder settings will remedy that, because as soon as I make it finer, the pressure increases. At 12 bar, it's a perfect 30-second shot.

Any thoughts?

#2: Post by **lessthanjoey** » March 3rd, 2021, 5:39 pm

That's just because your machine isn't using the opv to regulate pressure. I think espresso is significantly better at 9 bar or lower and this is an unfortunate choice on some machines that's worth rectifying

#3: Post by HB » March 3rd, 2021, 5:43 pm

BaristaMcBob wrote:Any thoughts?

I'm not following everything in your post, but I still don't get it: Why adjust the OPV? from the FAQ may help clarify, specifically this:

HB wrote:The OPV serves the same purpose as the bypass valve of a rotary pump: To set the maximum brew pressure. Rotary pumps are capable of moving significantly more volume at espresso brewing pressure than a vibe pump. In the case of a rotary pump, the bypass valve allows water to flow from the outlet side to the inlet side; in the case of a vibe pump, the over-pressure valve allows water to escape to the reservoir, effectively reducing the maximum pressure because of the flow rate / pressure relationship:

From Flow rate of a rotary pump espresso machine

If you do some calculations to determine the flow rate / pressure intersection for the vibe pump (left curves), you'll find that it conveniently works out to around 9 bar -- if you're pulling doubles. As Jim notes, an OPV acts as a "safe release" for ristrettos where the pressure would otherwise rise to 10+ bar given the slower flow.

For even more details, see purpose of adjusting OPV?

For vibratory pumps, the brew pressure reading that matters is at flow rate speeds. If it's against a blind basket, you're measuring the OPV open set point, not the actual brew pressure. If the onboard pressure gauge is upstream of the gicleur/orifice in the grouphead, you're not measuring the actual brew pressure either (it's typically 0.5 to 2 bar lower than the onboard gauge, depending on gicleur size, specific pump model, and OPV setting).

March 4th, 2021, 12:01 am

Thank you for that very helpful explanation.
Just to clarify something in my original statement - what I meant was "why adjust the OPV down from the factory setting of 11 bar down to 9 bar - as if 9 bar was the golden number?" I see people doing that in youtube. After all, one could easily reduce the pressure by easing up the grind. If the grind is too fine, then all that's happening is the machine is choking at 9 bar as some of the water is diverted back into the tank.

Your explanation was very helpful because I have not considered the affect of the flow rate.

#5: Post by **Jeff** » March 4th, 2021, 2:21 am

I also find that I prefer the flavor profile of a shot somewhere below 9 bar. I had my E61 set to a blind pressure of around 8 bar for most of its years of enjoyment, mainly on medium-dark roasts. I seldom go above 8 bar in my Robot. The light-roast coffee I'm pulling this week in my current machine tastes "flat" at 8 bar in the basket (probably around 9 bar blind) and is much more enjoyable to my taste at around a 6 bar peak.

Every coffee, every machine is a bit different, as is everyone's preferences. Lighter roasts behave very differently than darker ones. I'd try it and see with the kind of coffee you enjoy. Dial in at the stock setting, take notes on the flavor and the fraction of great/good/fair shots, for several days. Then try it with, say, an 8 bar setting, dialing in again and then taking notes the same way with the same coffee.