Pressure reading when brewing versus blind filter

#1: Post by **Socalsteve** » March 23rd, 2013, 1:08 pm

Hi all,

I searched the forum, but couldn't find the answer, so...

On my Alex Duetto I adjusted the pressure to 9 bars while using a blind portafilter basket. When I go to pull a shot, it only goes to about 7 to 8 bars. Is this normal or do I need to adjust the pressure up a bit more?

As always, thank you all in advance!

Steve

#2: Post by **Carneiro** » March 23rd, 2013, 1:29 pm

If this is a vibratory pump this is normal, you should adjust the OPV with a blind filter to a little bit more, let's say 10 bar.

Márcio.

#3: Post by **genovese** » March 23rd, 2013, 2:29 pm

Socalsteve wrote:On my Alex Duetto I adjusted the pressure to 9 bars while using a blind portafilter basket. When I go to pull a shot, it only goes to about 7 to 8 bars. Is this normal or do I need to adjust the pressure up a bit more?

You're certainly in the ballpark. I don't know about normal, other than what tastes normal to you. I can tell you my numbers (Duetto II bought 03/2011):
9.5 blind
9.0 shot
8.3 open group
If you run tests bracketing your current setting and find a taste improvement, change it. If not, forget about it.

#4: Post by **Beenbag** » March 25th, 2013, 4:48 pm

9.5 blind
9.0 shot
8.3 open group

Err, since 8.3 bar is effectively ZERO pressure at the puck surface,.. doesnt that suggest to you that the gauge isn't telling you much about the REAL pressure on the puck when brewing ??
Have you ever checked with a PF pressure gauge at the normal flow rate ?

http://users.rcn.com/erics/ · #5: Post by **erics** » March 25th, 2013, 6:09 pm

Err, since 8.3 bar is effectively ZERO pressure at the puck surface . . .

Richard - You getting things a little reversed

1.0 bar = 14.50377 psi or 14.5 psi amoungst friends. That bar reading is a gage reading and so would be correctly stated as 14.5 psig (pounds per square inch gage). At sea level and when the barometer is reading reasonably close to 29.92"Hg (14.696 psi), that 1.0 bar gage reading is equivalent to a pressure of about 29.2 psia (pounds per square inch absolute).

Only very rarely would espresso discussions get into "pounds per square inch absolute". What to remember is that (gage reading) + (atmospheric pressure) = (absolute pressure)

#6: Post by **genovese** » March 25th, 2013, 6:26 pm

Good point, which can also be inferred from the convention of basing the scale of the gauge (or other device) at 0 (=rest).

#7: Post by **genovese** » March 25th, 2013, 7:13 pm

Beenbag wrote:Err, since 8.3 bar is effectively ZERO pressure at the puck surface,.. doesnt that suggest to you that the gauge isn't telling you much about the REAL pressure on the puck when brewing ??
Have you ever checked with a PF pressure gauge at the normal flow rate ?

Executive summary: no; yes.

Not the most flattering lighting for a spattered machine, I admit. I'm impressed that the gauges agree so well, even given static (flow = 0) conditions like this. In any other case (flow > 0), a gauge reading is dependent on where in the system the gauge is attached. Since there's no strict standard on plumbing (or quality of) the OEM gauge, different machine models can show different pressures on their OEM gauges. It would be nice if they all read pressure on good gauges right above the coffee, but in this case the OP and I happen to have the same model, and I had to infer that he was reading the same OEM gauge, since he didn't refer to any other. To paraphrase Johnny Cochran, "if the words don't refer, you must infer." Since the Duetto's OEM gauge attaches at the pump output, there's a lot of possibly restrictive hardware between there and the group outlet. Eric would know better, but my expectation is that the bottleneck (with group open, meaning PF removed and pump running) is the gicleur in the upper valve of the E61, and that is what is reflected in the reading of 8.3 on the OEM gauge.

#8: Post by **Beenbag** » March 27th, 2013, 12:09 am

erics wrote:..
..
. What to remember is that (gage reading) + (atmospheric pressure) = (absolute pressure)

erics,
I will rephrase..
If you had a loose/untamped/channeled puck in that machine the gauge would read 8.3 bar !
What would you expect a gauge to read if it were reading from the front port on the top of the group ?...
Blind ..?
Shot...?
Open group..?

genovese wrote:... my expectation is that the bottleneck (with group open, meaning PF removed and pump running) is the gicleur in the upper valve of the E61, and that is what is reflected in the reading of 8.3 on the OEM gauge.

Yes, i agree, but that is not what you need to know. You have to assume that those restriction(s) will influence the gauge reading at any significant flow rate.

#9: Post by **genovese** » March 28th, 2013, 9:02 pm

Of course, and I said as much. I realize the "8.3 bar" value is meaningless to you or anyone with a different machine, but I directed it to the OP, whose hardware is identical to mine. With PF removed, the influences of bean, grind, dist, tamp and preinfusion are removed, and the gauge reading is an objective measure of pressure at the pump outlet, again, not to belabor it, but for identical hardware.

I don't know why the OP reports a bigger blind vs. coffee pressure differential; my OEM gauge reading never drops below 8, not even with open group, and my mushroom/gicleur apertures are clean. Not to say that Izzo can't change these at any time as they see fit. My numbers are a bit closer together: