Brew pressure and its effects on espresso - Page 2

Want to talk espresso but not sure which forum? If so, this is the right one.
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malachi

Postby malachi » Aug 20, 2008, 11:53 pm

Anthony wrote:I am familiar with Hairbender (originally from Portland), and when I was in Portland last month on my migratory trip to the Division St. Stumptown, I had a couple ristrettos. I sensed (in my newbie way) what was probably more on the higher brew pressure side (though I realize there are a lot of variables)—actually it seemed a litter nuttier (almost almond-like).


Division Stumptown machine (used to be) run at 8.75BAR
"Taste is the only morality." -- John Ruskin

Anthony

Postby Anthony » Aug 22, 2008, 11:04 am

Oh, I see. So, to understand better what you were writing earlier, would you say this is something like a middle-high pressure? In any case, I assume this setting on that machine works well for Hairbender, or what the Baristas there want to bring out in Hairbender.

Thanks for the references you posted, Jim. More edifying nighttime/weekend reading!

To start off my little quest, I have just lowered my brew pressure to a static 9.5 bar—beginning there. I am overdosing and pulling normals and ristrettos (Dancing Goats about 5-6 days from roast date, at about 200 F--so pulling the shot with the digital read at the adapter site at about 203-204 F). For the sake of consistency and to see what would happen, I pulled a couple shots with the old brew pressure with these beans (at 10.5 bar); I did not adjust the grind at first with the new brew pressure, but it feels like I want to tighten it a bit. (This seems counter intuitive, but there it is.) I am not having the "stalactite" problem so far at all, and I do not see any pinholes like I was seeing before [I should say, it was not a problem with these beans even at the higher bar this time, but was a problem a few days ago.].

I just don't have the refined taste buds and vocabulary to express the experiences, but something changed. For example, the texture was a little denser and had a softer finish, not elongated, and was more "forward" on my palate than before (even a little "fruiter"). There was no astringent after taste like I was getting sometimes previously. And at this brew pressure, the tiger striping even seems to be a little deeper and I am not getting what seemed to me before to be a pre-mature blonding with a normal shot.

Let me ask a question that seems so evident as to be naive, but it is a matter of confirmation. When I am pulling a ristretto, my brew pressure is right around the static setting (like I would have with a blind PF), and when I am pulling a normal, it is around 9.0 bar, so 0.5 bar less. Does that sound right?

Not trying to set my taste-buds according to brew pressure, but eventually my brew pressure according to taste, for now, nevertheless, I am wondering "what happens when ...".

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HB
Admin

Postby HB » Aug 22, 2008, 11:17 am

Anthony wrote:I just don't have the refined taste buds and vocabulary to express the experiences, but something changed. For example, the texture was a little denser and had a softer finish, not elongated, and was more "forward" on my palate than before (even a little "fruiter"). There was no astringent after taste like I was getting sometimes previously. And at this brew pressure, the tiger striping even seems to be a little deeper and I am not getting what seemed to me before to be a pre-mature blonding with a normal shot.

Nicely explained, I could not have described the effects of a brew pressure reduction better.

Anthony wrote: Let me ask a question that seems so evident as to be naive, but it is a matter of confirmation. When I am pulling a ristretto, my brew pressure is right around the static setting (like I would have with a blind PF), and when I am pulling a normal, it is around 9.0 bar, so 0.5 bar less. Does that sound right?

Yes, the effective brew pressure is determined by the lower of the over-pressure valve setting and the pump pressure at a given flow rate. From the FAQs and Favorites Digest, see Vibe pump's OPV (over-pressure valves) explained and I still don't get it: Why adjust the OPV? for a detailed explanation.
Dan Kehn

gscace

Postby gscace » Aug 22, 2008, 1:30 pm

Anthony wrote: Let me ask a question that seems so evident as to be naive, but it is a matter of confirmation. When I am pulling a ristretto, my brew pressure is right around the static setting (like I would have with a blind PF), and when I am pulling a normal, it is around 9.0 bar, so 0.5 bar less. Does that sound right?.



Yep. That sounds right. There are two things at play here. First, when no water is flowing through a plumbing system, the pressure is the same throughout. In order to get water to flow through a pipe you have to have a pressure differential across the water. Imagine that you are holding your finger over a garden hose so that no water squirts out. The force of your finger pressed on the end of the hose exactly opposes the water mains pressure and the net force on the water is zero, meaning that no water moves down the hose. Once you release your finger, the pressure at the open end of the hose is at atmospheric pressure, while the pressure at the water mains connection is the water mains pressure. Now there's s big force imbalance and water flies out of the pipe. In the case of your espresso machine, the flow when pouring a ristretto is pretty small, so the measured pressure is gonna be similar to the static case (when pumping against a blind filter). As soon as substantial water flow occurs, such as when brewing normal espresso, the pressure drop along the flow path increases, and the pressure at the group is reduced. The magnitude of the pressure drop that is reported by the gauge in your machine depends on where in the flow path the gauge is tied into the system. If it's close to the OPV you may not see much difference (but read the next section). If it's near to the group you'll for sure see a substantial difference.

Second, we have to consider the behaviour of pressure relief valves that rely on spring pressure and a plunger to regulate the pressure of a system. Spring force on the plunger is given as k*X, where k is the spring constant, given in pounds per inch of displacement (or Newtons per meter in SI units), and X is the amount that the spring is compressed from its free, uncompressed length. The important concept is that the more you compress the spring, the more force the spring exerts. When you are pumping on a blind filter, or pulling a ristretto, the spring is compressed more than when you are pulling a normal shot, because more water is being forced through the OPV. As a result the spring puts more pressure on the plunger and the OPV regulates at a higher pressure than when less water is flowing through the OPV, the case when relatively more water flows through the group. Depending on the OPV design,and the capacity of the pump, the difference in regulated pressure can be pretty large. I once measured a system of vibe pump and OPV that produce 4 bars difference in regulated pressure between pumping on a blind filter and actually brewing coffee.

For espresso machines, the flow-dependent pressure drop through the plumbing system and the difference in regulated pressure due to relative changes in spring force imparted to the OPV plunger work in the same direction, unfortunately toward more pressure variability. As relatively more of the total water flow goes to the group, the regulated pressure (from the OPV) goes down, and the pressure is further reduced as the water flows down the flow path to the coffee. The effect is greater with vibe pump machines than with rotary pump machines. In rotary pump machines, the internal pressure relief valve in the pump has a very large bypass port and the pump itself has a very high flow capacity compared to a vibe pump. The difference in flow between a ristretto and a normal espresso does not change the position of the plunger much, resulting in less difference in regulated pressure compared to vibe pump / OPV systems.

Constant pressure regulation requires alternative regulation techniques, such as a system of variable speed pump, electronic pressure transducer, and process controller. One of the advantages of the system that Eric S referred to in his light reading suggestions is that the regulated pressure at the pump is highly reproducible, resulting in greater ease in maintaining consistency.

Getting to the real nut of the matter - The idiosyncrasies of espresso machines are interesting if you are so inclined, or if you are a machine designer or engineer. It's often useful to know why things work the way that they do and to be aware of how and why variability is introduced into the brewing process. I don't know what you're capable of mechanically or electronically, or if you're a super-taster or what, but at some point you'll prolly wanna decide for yourself what you can and can't taste, how much of an effect pressure variability is on your particular setup, and how much money and effort you wanna spend on seeing if you can do better. Generally pressure variability is not the tallest nail sticking out of the board. Whack on the tall ones first, taste carefully, draw your own conclusions. You seem pretty smart.

-Greg

Anthony

Postby Anthony » Aug 25, 2008, 10:08 am

Greg:

Thanks for taking the time to respond and to explain these points on brew pressure, really, with such clarity. Would constant pressure regulation eventually have to take into account the variable resistance of the puck after it is infused, or rather, during the process of infusion?

To your final comments: Of course, eventually I would like to produce (and reproduce) great tasting coffee, but getting there is also pretty fun, too! I have already found brew pressure significant enough to alter the taste at least within one bar, and this is something I never really knew before, though your reminder that brew pressure is not the most significant variable is helpful. What I find intriguing about brew pressure is that it relates to the environment in which espresso is produced. As such, it appears to be one of the few things—after one figures out what it is doing, perhaps through some experimentation—that can remain more or less part of a stable environment when dialing in other things.

Aside from this, the nuances of espresso are fascinating, and I admire the ability of those who can find their way around the subtleties of coffee either intuitively/experientially and/or systematically. Finally, to invoke Socrates (though outside of Hemlock, I am not sure what he drank!), the unreflective life is not worth living; in that spirit, pulling a good shot entails for me in part also knowing, as you put it, "why things work the way that they do and to be aware of how and why variability is introduced into the brewing process." (Though sometimes, heck, I'll just settle for tasting a good shot.)

Anthony

Anthony

Postby Anthony » Sep 08, 2008, 4:47 pm

After some experimentation, I have followed the suggestion of several here (again, thanks for the advice) and lowered my brew pressure again, now to a static 9.0 bar (according to the on board gauge). I have been noticing a consistently more viscous ristretto (sometimes within a more nutty/carmelly range) across several different blends with which I am familiar. I have also noticed far fewer problems with channeling compared to when it was set at 10.5 bar.

This brings me to my simple question. Why do the distributors of such espresso machines pre-set the brew pressure for something like 10.5 bar and not e.g., 9.0 or 9.5 bar? They cannot be assuming that we are all making lovely triple basket ristrettos like Jon Rosenthal!

Anthony

User avatar
cafeIKE

Postby cafeIKE » Sep 08, 2008, 5:39 pm

The pressure is set Hi to avoid "My machine doesn't work" tech calls.
At 11 bar, with commercial coffee or sub any grinder we'd be caught dead with, you'll always get something in the cup.

It's kind of like buying a new target pistol. Those that know what they're doing will sight it in.
Those that don't can't hit the broad side of a barn unless they're standing on the latch.

Anthony

Postby Anthony » Sep 08, 2008, 6:43 pm

:lol:

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another_jim
Team HB

Postby another_jim » Sep 08, 2008, 7:21 pm

cafeIKE wrote:The pressure is set Hi to avoid "My machine doesn't work" tech calls.
At 11 bar, with commercial coffee or sub any grinder we'd be caught dead with, you'll always get something in the cup.


And you would know this ... how?

Here's the real story, once again.
  • The desirable pump pressure for espresso is roughly 8 to 9 bar. Below that, one loses crema, above that one gets extra bitterness.
  • Espresso machines are routinely adjusted against a blind filter. This leads to the two problems Greg mentions: first the pump puts out less pressure at espresso flow than at zero flow, and second, there is flow resistance between the pump and the puck, which can drop the pressure before the puck is reached.
    Vibration pumps have a steeply declining pump curve that puts them roughly in the right range for doubles, but not for singles. Rotary pumps have very flat pump curves
  • Because of its flat curve, a rotary pump is adjusted to around 9 bar against a blind filter. Flow resistance will drop this pressure, but not by much, and it will remain between 8 and 9 bar.
  • Vibe pumps are adjusted to 11 bar, since the pump curve will drop them by 2 bar in any case.
  • OPVs, the valve which allows the pressure to be adjusted, are variable in quality, especially the ones for vibe pumps, as Greg found out. Adjusting the pressure at the high end compensates for possible defects.

The service and adjustment standards for espresso machines, as done in Italy, may not be perfect, but they are based on long experience. Only a fool would assume they are systematically wrong. Sometimes, Italian manufacturers make odd assumptions about the US market, and deliver machines not tuned as well as they would be in Europe. This is not the case for the 9 bar and 11 bar settings for rotary and vibe pumps, which is the standard in Europe as well.
Jim Schulman

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cafeIKE

Postby cafeIKE » Sep 08, 2008, 7:23 pm

another_jim wrote:And you would know this ... how?

Horse's mouth. From more than one manufacturer.