GB/5 pressure variance

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GB/5 pressure variance

Postby Goatherd on Sun Oct 01, 2006 5:06 pm

We just finished installing our new GB/5 and have been using a Scace and portafilter pressure gauge (EPNW) to dial it in. I've never used a pressure gauge before and am wondering if it's normal to have a variance between the on-board pressure gauge and the pressure measured by the portafilter gauge. I'm currently seeing about .8bar difference (portafilter lower) this seems a bit much too me. Is this normal?
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Postby Jim Schulman on Sun Oct 01, 2006 6:26 pm

Commercial pressure guages are rated as +/- 3 pounds plus 2.5% of scale. That tots up to just over 0.5 bar. You may have just gotten unlucky and have two guages near their error limits and erring in opposite directions.
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Postby xristrettox on Sun Oct 01, 2006 7:46 pm

but still .8 is a big difference.

is there any way to check for sure?

is taste the only way to go?
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Postby jepy on Sun Oct 01, 2006 8:29 pm

I got these Lenz brand gauges for some testing, I couldn't take how cheap the gauges I've seen come stock on machines seem to be:
http://www.lenzinc.com/index.cfm?templa ... roduct=161

Mine are the 0-200 psi range, which it says is acurate to within 1%, so we're at .1379 of a bar if my math is correct.
Image
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Postby Jim Schulman on Sun Oct 01, 2006 9:41 pm

xristrettox wrote:but still .8 is a big difference.

is there any way to check for sure?

is taste the only way to go?


The expected error of averaging two poor guages is only 70% of one poor guage.

In terms of operating a machine, a poor guage doesn't make that much difference, since it's much more accurate for adjusting say 1 bar higher (or lower) than whatever it's reading now. It just means one can't easily compare the absolute settings from different machines, unless one measures them all with the same PF guage.
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Postby paul_pratt on Mon Oct 02, 2006 2:07 am

Jim is right, if I pulled 20 gauges (from various brands) off my shelf now they would all have vastly different readings.

Think of it like the phrase used most when Greg`s excellent device first came out - comparing `apples with apples`. Unless we have a standard, one man`s 9bar is not the same as another man`s 9 bar. So like finding a brew temp. use the numbers as a guide and tweak by taste.
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Postby nick on Mon Oct 02, 2006 2:16 am

Sounds like it's time for someone to build Scace Device 2: Electric Boogaloo. 8)

Remember, just like brew temperatures, pressure measurements are going to be only as relevant as how you apply them. Just like brew temperatures, the reading itself (202.2*F, 8.6 bar) is only useful when accompanied by HOW it's measured, WHERE it's measured, and how accurate the measuring protocol.

If and when someone does build a scace-device like standard testing instrument, brew pressures will be more meaningful. However, flow-restrictors (gicleurs) and variances in the plumbing from group-to-group will inevitably lead to a small pressure difference from group-to-group, no matter what the gauge says. Indeed, as unfortunate and cliche as it is, the most significant gauge is gonna be your palate.
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Postby barry on Mon Oct 02, 2006 10:05 pm

nick wrote:Sounds like it's time for someone to build Scace Device 2: Electric Boogaloo. 8)



iirc, it's in the works.
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Re: GB/5 pressure variance

Postby barry on Mon Oct 02, 2006 10:07 pm

Goatherd wrote:I've never used a pressure gauge before and am wondering if it's normal to have a variance between the on-board pressure gauge and the pressure measured by the portafilter gauge. I'm currently seeing about .8bar difference (portafilter lower) this seems a bit much too me. Is this normal?


yes. greg has done a bunch of pressure measurements recently and, iirc, said he found significant difference between the stock gauge reading and the pressure at the portafilter.
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Postby jepy on Tue Oct 03, 2006 12:18 am

I built this temp/pressure device a while back. Mostly just use it for pressure now, since I've installed temp probes inside my groups.
I was suprised to see how the pressures were adjusted at some of the high end cafes around S.F. area, all in the upper 7's

Here's a short clip of how I use it for pressures


http://www.youtube.com/watch?v=KMlCrezRcCg
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Postby Eton on Fri Oct 06, 2006 2:54 pm

jepy,

how are you doin? drop me an email... eton@lacoffee.com

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Postby gscace on Tue Oct 31, 2006 10:08 am

Yeah, well I'm working on it. Actually 1.5% gauges are plenty good enough for the job. That's accuracy of 2 psi, or 0.14 bar. The real problem is twofold. First, gauges being used for this measurement suck for a variety of reasons. Second, the measurement is being made incorrectly. Dealing with the first: Very cheap gauges are being supplied on portafilter pressure gauges that I have seen. I've been investigating incorporating such a device with the thermofilter and the gauges that come on those things have got to go. Additionally, measureing stuff isn't typically taken good care of and calibration of pressure stuff is expensive - so it ain't often done by the typical end user.

With regard to issue number two - the portafilters that measure pressure against a blind adapter or other "no flow" device give you an answer based on NO FLOW. That's the WRONG answer. For example, when you hit the brew switch with no portafilter in the group, the pressure of water coming out of the group is at atmospheric pressure, or 0 bars gauge pressure (the pressure we are talking about here). Lock in the pressure gauge portafilter and you get another answer entirely, because the pump is pushing water against the blind filter. This is called stagnation pressure. The real-life pressure value depends on the water flow rate through the system, and the devices that are restricting flow rate. These devices include tubing between the pump and the group, pressure relief valves, bends in tubing,gicleurs, flowmeters, group solenoids, changes in flow direction within these components, the size of drilled passages inside the group, changes in flow direction within the group, the resistance of the dispersion block and screen, and finally the coffee. And get this - you might have 9 bars of pressure on the group side of the cake, but there's zero pressure on the opposite side of the cake. So the coffee within the cake experiences brewing pressure that is dependent on position within the cake and it will experience pressure of between 0 and 9 bars.

Again, for uniformity there must be agreement on where the measurement should be taken. The obvious answer is for all of us to talk about pressure right above the cake, and recognize that this value sets up the pressure distribution within the cake. Next, we have to do the measurement under conditions that mimic actual brewing, because stagnation pressure is different from pressure experienced during brewing.

About a year ago I did a machine review for a company that was using a fluid-o-tec vibe pump coupled with a pressure relief valve that set brew pressure. Stagnation pressure was set to 9 bars by the manufacturer, yet the actual pressure under flow conditions set by the same parameters used to develop the flow values for WBC temperature testing produced brew pressures that were half that - 4.5 bars. This was measured using a modified thermofilter with a pressure port and a rather expensive needle valve for adjusting water flow. Using this device I am able to map out pressure / flow curves for pretty much any espresso machine. Since there was a disparity in my Linea between what I was measuring at the brew boiler pressure gauge and the value at the group under brewing, I performed a flow analysis that predicted the pressure drop across various components on the way to the group, just to see what fluid mechanics theory predicted in terms of pressure drop. The values were very interesting. A typical group solenoid of the type that connects via the flat flange has two 90 degree bends in it, and relatively small passageways. A simple analysis predicts about 0.3 bars of pressure drop across the solenoid at flow rates consistent with the WBC-desired espresso production rates. Coupled with pressure drops across flowmeters, changes in direction within groups, etc, it's pretty easy to see that the stagnation pressure value is pretty worthless, particularly when comparing machines. Two machines of different manufacture could have wildly different brew pressures above the cake, while producing identical stagnation pressure values.

I was given a paper recently that was published in the Journal of Agricultural and Food Chemistry entitles "Influence of Water Pressure on the Final Quality of Arabica Espresso Coffee, Applicat6ion of Multivariate Analysis". Authors are Andueza, Maeztu, DePena, Bello and Cid. It was published by the American Chemical Society in 2002. As I recall the gist was that brew pressure was a pretty important parameter whose effect on taste was less significant as pressure increased. In otherwords, low pressure produced relatively poorer espresso than over-pressure. I don't know what the sensitivity threshold is for brew pressure - in other words, how much difference is allowable before one can taste a change. But I bet that differences on the order of 1-2 bars can be tasted, and that's smaller difference than existing measurement schemes produce, comparing stagnation pressure to real-world operating conditions.

-Greg (There - I've spilled my guts on ths now) Scace




nick wrote:Sounds like it's time for someone to build Scace Device 2: Electric Boogaloo. 8)

Remember, just like brew temperatures, pressure measurements are going to be only as relevant as how you apply them. Just like brew temperatures, the reading itself (202.2*F, 8.6 bar) is only useful when accompanied by HOW it's measured, WHERE it's measured, and how accurate the measuring protocol.

If and when someone does build a scace-device like standard testing instrument, brew pressures will be more meaningful. However, flow-restrictors (gicleurs) and variances in the plumbing from group-to-group will inevitably lead to a small pressure difference from group-to-group, no matter what the gauge says. Indeed, as unfortunate and cliche as it is, the most significant gauge is gonna be your palate.
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Postby paul_pratt on Tue Oct 31, 2006 9:41 pm

Greg that's pretty vague, can you be a bit more specific? :lol:
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Postby Michael_Teahan on Wed Nov 01, 2006 6:50 pm

The 9 bar pressure rating/setting on commercial machines is based upon the ability of the pump and motor to maintain that pressure against the restriction provided by the gicleur. Increase the flow by energizing more groups and the pressure will generally begin to drop off. Increasing the flow rate of the pump motor usually requires more horsepower and tends to be problematic for pumps at lower flow rates.

Big pump and low flow is bad. Small pump and high flow is bad. Gotta find the right balance. A 3/4 bar fluctuation on the pump side of the gicleur (where the gauge likes to be, before the HX) is not likely to affect the brew pressure as the actual pressure inside the group is necessarily lower than that.

The pressure in the chamber is a function of the restriction provided by the density and distribution of the coffee and has nothing to do with the gicleur, which is simply a metering device of sorts.

The difference you see in the gauge on the filter holder and that on the machine can also be attributed to the expansion of water in the heat exchanger which is generally the same as that of the boiler. Heat water under pressure of 9 bar to a temperature of 253 degrees and expansion is going to tak place. You should be able to see the pressure change very slightly in the PF as they elements kick in.

This is why the expansion valves are adjusted under pressure with a blind filter approaching 10 bar where the element just kicks off to allow a few drops of water to pass through the expansion valve. This is the highest pressure the machine's heat exchangers will experience under normal operation and is about the functional limit of the solenoid valve.

I do not know the exact figure that the heat adds to the pressure on the gauge, only that it is well known and common on commercial machines from a testing and service standpoint. Take the pump pressure on the machine and add the steam pressure, knocking off a little for heat loss and it should be pretty close.

I don't have a better explanation for the phenomenon, but it isn't a bad gauge. Unless they are all bad. Every one of them. Ever.

That's my story and I'm sticking to it.

Anyone want to volunteer an E61 machine for a real time extraction temperature and pressure readout from inside the brew chamber while extracting actual coffee?

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Oops

Postby Michael_Teahan on Wed Nov 01, 2006 6:52 pm

GB5's are probably a little different, with the expansion a litle more subtle as the temps are a bit lower.

They still run expansion valve to the drain from the main water circuit, though. Gotta be something to it then.

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