The importance of brew pressure, purpose of adjusting OPV?

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TimEggers

#1: Post by TimEggers »

I've been doing a lot of debating on going to a different machine so that I could get an adjustable OPV valve. I don't know what the pressure is on my Gaggia (I should test for it) and it may not even be a problem (I only make doubles so far). I must admit I may just be fond of the idea of having the ability to adjust the OPV when in reality I may not necessarily need too.

What advantages specifically does the ability to adjust the OPV offer?

What are flavor/shot performance signs that a high-pressure problem may exist?

Does anyone know if it's possible to attach and adjustable OPV (like the new Silvia's for example) to a Gaggia Coffee Deluxe?

My double shots taste pretty good, I'm getting a fairly consistent shot down. Now I'm wondering if an adjustable OPV setup would offer noticeable improvements in my espresso. I find myself to be an espresso purist and only drink straight doubles. I'd like to get the best shot I possibly can with either the machine that I currently have, or one in it's class. If money wasn't an option I wouldn't be asking this and buying a GS3... :wink:

Edit: I ran across this from 1st-Line and now I'm more confused than ever! It reads:

NEWEST Feature: The overpressure valve is now adjustable on our latest batch of Silvias. The overpressure valve is normally set to 11 bar. However, if your grind is just right, the grind should only create 8-9 bar of resistance so the overpressure valve will never open.

Ok so if that's the case then why is an adjustable OPV needed or used? There has to be a reason because people are using them and swearing by them. What does it allow the barista to do? What advantage does it lend? In theory can't the barista adjust the grind (or even the tamp) to compensate for a possible pressure problem? Afterall isn't brew pressure regulated by coffee grind and puck density?

Man I'm so confused...help me out here! :shock:
Tim Eggers
http://www.facebook.com/TimEggers
LMWDP #202

Beezer

#2: Post by Beezer »

I'm not really an expert on OPV valves, but I thought one advantage of an adjustable valve is that you can pull ristretto shots without having the pressure go above 8 or 9 bar. If the valve is nonadjustable and set too high from the factory, then having a really tight ristretto would cause the machine to put too much pressure, which could lead to channeling. Or so I would think. Others may have a better explanation, though.

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erics
Supporter ◈

#3: Post by erics »

Hi Tim -

Firstly, I have no specific knowledge about a Gaggia Coffee Deluxe (e.g., does it even have an OPV?) but I do have some knowledge re Silvia. I do not agree with what 1st Line is saying and here's why. To the best of my knowledge, all Silvia's are fitted with the Ulka EP5 pump and the flow characteristics of that pump are as shown below:

Image

I realize that the definition of a double-shot is a debatable (and very rightly so) subject in and by itself but, strictly for discussion purposes, let's assume that a double-shot puts 60 ml of espresso (including crema) in a cup in 25 seconds when the puck is subjected to a pressure of 9.0 bar. If you look at the graph of the Ulka pump performance, you'll see that it is capable of fufilling this requirement and when it does so, there will be flow in the OPV line back to the tank. Naturally there are tolerances associated with pump manufacturing and, after so many years of operation (even in a home environment), some pumps are a little more tired than others. However, the solid line does represent a production average and the dashed lines represent min/max allowable to get the pump out the door.

So, exactly what pump is installed in your Gaggia Coffee Deluxe?

There has been some discussion here on HB about Salvatore espresso machines and a comment(s) about the machine was that it did not have an OPV. This raised my curiosity and I reached the conclusion (unsubstantiated) that he fitted his machines with the Fluid-O-Tech 1106 pump. As you could see by the performance curve of that pump (if I could locate the graph), it does not need an OPV to regulate its flow. At 9.0 bar, it's flow is "just about right". I'll find that graph and edit it into this post.

I think its "fun" to have the ABILITY to adjust various brew parameters on a machine and try to discern their effect on what's in the cup. If most of your good shots (and you appear to have had plenty) are "X" ounces in "Y" seconds, then you may want to find out what pressure it is that is producing these results. A post by Bob Roseman here on HB detailed his construction of a portafilter pressure gage equipped with a needle valve that allowed him to measure the pressure under brew conditions. I would take a chance and try to find it for you but the last time I did that the post monster ate my words - I am still learning.

Now this does not answer all your questions but my point would be to measure what you have before you modify what you have.

edit - here is the f-o-t 1106 graph:

Image
Skål,

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

hgs

#4: Post by hgs »

i'm no expert, but here's my common-sense take ....

the OPV allows the barista to be less accurate. assume there is no pressure release valve (opv) at all; in that case, it would be the grind that determines the pressure at which the water flows through the puck. however, if you can't get the grind perfect enough to regulate the flow to 9 bar of pressure, then the OPV sets the max pressure. if the OPV is set to 11bar, then that perfect grind that would have allowed 9bar of flow would still work perfectly (9bar would release the pressure before the 11bar limit was ever reached). however, if your grind is off (too fine), you go to the 11bar max - and it stays there until you get flow through the puck (or you get nothing). similarly, if you reduce the OPV to 9bar - then 9bar of pressure is the max flow pressure - regardless of the grind. clear as mud?

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HB
Admin

#5: Post by HB »

I agree with what's been said, but keep in mind that over-pressure valves are just a means of better regulating vibratory pumps; it's necessary because they have variable pressure at different flow rates. Rotary pump espresso machines have far greater flow rates than used for espresso and built-in relief valves, so the barista doesn't have to "guess" what the pressure is. When it flows, it's 9 bar.

For sake of completeness... rotary pumps sometimes have what appears to be an over-pressure valve, like the one shown near the bottom of this photo:

Image

Although it's sometimes the same part, in this case it's referred to as an expansion valve. To avoid excess pressure of a closed system, it opens to allow water to escape when the machine heats or if the pump is maladjusted. They are typically regulated to ~12 bar.
Dan Kehn

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TimEggers

#6: Post by TimEggers »

My many thanks to all. The Gaggias do have an OPV but it's strictly an emergency valve set-up. Basically it's there to keep the pump from being damaged.

And Eric I agree with you, I'm not going to mod anything until I can measure what I have. My espresso seems good (to me) so this may not be an issue for me. I just wanted to rule out that I'm missing something. Again I should be focusing on my shot I suppose.

I have seen Mr. Roseman's set-up for measuring the pressure during a brewing situation, it's pure genius.

Again thanks to all. I feel I have learned (and that's what this is all about).

For now I'm not going to worry as much about it and focus on the espresso. I'm going to let my puck do the regulating. The Mazzer Super J is more than up to the task!

(Although this may be the perfect excuse to get the lever machine I have always thought would be fun to have) :wink:
Tim Eggers
http://www.facebook.com/TimEggers
LMWDP #202

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another_jim
Team HB

#7: Post by another_jim »

The OPV situation on higher end homers like the Baby or Silvia is changing.

They used to have simple safety valves. However, if they want to get ESE pod certified, they have to be set at 11 bar. This means that newer models of all machines (not just the ESE versions) are getting better grade OPVs adjusted to 11 bar. This is true of the new Silvia's and may be the case for current gaggia production as well.
Jim Schulman

gscace

#8: Post by gscace »

TimEggers wrote:My many thanks to all. The Gaggias do have an OPV but it's strictly an emergency valve set-up. Basically it's there to keep the pump from being damaged.

And Eric I agree with you, I'm not going to mod anything until I can measure what I have. My espresso seems good (to me) so this may not be an issue for me. I just wanted to rule out that I'm missing something. Again I should be focusing on my shot I suppose.

I have seen Mr. Roseman's set-up for measuring the pressure during a brewing situation, it's pure genius.

Again thanks to all. I feel I have learned (and that's what this is all about).

For now I'm not going to worry as much about it and focus on the espresso. I'm going to let my puck do the regulating. The Mazzer Super J is more than up to the task!

(Although this may be the perfect excuse to get the lever machine I have always thought would be fun to have) :wink:
I can perhaps add a bit to the OPV discussion:

The First Line argument is not really correct. The flow rate required to produce 9 bars of pressure from a system using an Ulka pump does not necessarily coincide with the proper flow rate to produce the best tasting shot. Ulka's pump is a convenient fit between a commercial offering and an application (espresso machines) that gets one sort of in the ballpark, and allows manufactures to skate by, ignoring details that we as folks interested in the best possible taste would have them address. Let's consider a pump whose volmetric flow varies with pressure per the graphs that Eric S uploaded. From the graphs you can see that if the pump is working against high pressures, the volmetric flow rate delivered by the pump goes down. At lower pressure the flow rate produced by the pump goes up. In the case where there is no OPV, the water pressure on the coffee cake is directly linked to flow rate. The coffee cake provides the flow resistance that makes the pressure build upstream of the cake. If a shot is produced by flowing 50cc in 25 seconds, the pressure on the cake will be different than that acting on the cake if the flow is 60cc in 25 seconds. if there is no OPV, then you can't decouple the variables of extraction flow rate, and pressure. A coffee that tastes best at 50cc in 25 secs may be compromised by the fact that the brew pressure is through the roof. Installation of the OPV, or modification of OPVs so that they act like brew-pressure controllers decouples the two variables. Also, pump capacity must be over-specified for systems to have any reasonable lifetime. Pumps don't get more efficient when they age. If you spec a pump to provide just the right pressure for a 9 bar, 60cc / 25 sec shot when new, the pump will not produce that pressure or flow in a year, if the machine has been used with any frequency. Machines that do not have adjustable OPV valves are produced that way because 1) They are being produced to compete in a market that is price-driven, 2) They gotta make compromises in order to compete in that market and they think the degradation to taste caused by fixed brew pressure is less than degradation caused by some other compromise, or 3) The market accepts this and doesn't know the difference for the most part.

I've been working on variable brew pressure profiles recently, and I did some experiments over the last week and a half that are quite useful for this discussion. In these experiments I purposely applied a great deal of pressure to the coffee cake early in the extraction (11.5 bar) in an attempt to extract more from the coffee earlier in the extraction than later. The effect of this was to produce bitter crema, compared to profiles in which the pressure was adjusted to lower values. Crema taste, and the taste in the very bottom of the cup (which contains the first drops of the extraction) became less bitter with less of a pressure hump at the beginning. Taste was very pleasurable at 9.5 bars at the pump, which on my machine ends up being about 9 bars at the cake. I can dial in bitter taste or dial it out by varying early extraction pressure. This demonstrated to me that there is actually a practical pressure maximum. Note that the values we're talking about here are well within the range of Ulka pumps.

Machines that don't have a means of controlling brew pressure through a pressure relief valve would benefit from installation of such a valve, or modification of the existing valve that protects the boiler. It's not as big of a difference as getting brewing temperature right, but it's demonstrable and significant enough that you might consider doing the work if you are capable of mucking around with the guts of espresso machines.

-Greg

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TimEggers

#9: Post by TimEggers »

Hello Greg,

Thanks for the great feedback. I can see your points. I believe that I must first come along in my barista skills before I start modifying machines. I do have a few bids out on some levers on ebay (which will put me in a whole different ball game). For now I'm content where I am until my progress begins to be hindered by my machine.

Knowing me that day is still a ways off! :D
Tim Eggers
http://www.facebook.com/TimEggers
LMWDP #202

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cafeIKE

#10: Post by cafeIKE »