Author Topic: Extraction bias  (Read 1077 times)

Offline The Big L

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Extraction bias
« on: September 02, 2017, 10:48:53 AM »
Watching the bottom of my naked PF, I find that the extraction tends to start sooner -- and starts blonding sooner -- in the part of the basket toward the PF handle.  In fact it happens so frequently that I suspect there's a consistent bias either in equipment or (more likely) technique.

For equipment biases, I came up with a couple possibilities:

-- Uneven openings in the shower screen due to machining.  Seems unlikely.

-- Uneven openings in the shower screen due to mineral scale and/or coffee oils.  Narrowed openings create high-pressure jets of water that can disrupt the surface of the puck and cause channelling.  Not sure why this would consistently occur on one side of the shower screen though.  And I haven't noticed that it improves with cleaning.

-- Uneven openings in the PF basket (scale, coffee oils).  Larger openings on one side would allow more/faster water to pass through.  Again, I haven't noticed that cleaning improves things.

-- Counter sloping toward PF handle.  Given the pressure in the basket, I really doubt gravity is going to have an effect.

For technique biases:

-- Uneven distribution of coffee in the basket, or uneven tamping pressure.  I've tried rotating the basket 180 deg when filling and/or tamping.  Sometimes this seems to help, but I haven't had consistent success.

I can achieve pretty good results if I make a determined effort to tamp substantially harder on the part of the basket closest to the handle.  Even so, it's relatively rare when I get a very even extraction; I still tend to see the handle-side extracting earlier than the rest.  When this happens I just stop the shot early before the handle-side blonding makes the coffee too weak/bitter.

Any other suggestions on getting an even extraction?  Have you ever noticed/corrected a similar extraction bias in your own shots?

Offline AndyPanda

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Re: Extraction bias
« Reply #1 on: September 02, 2017, 01:48:30 PM »
Do you tamp with the basket in the handle?  You may have a bias in the angle of your tamper.   

Some tamping techniques to try and see if it makes a difference:
Nutation: (wobble the tamper around at an angle rotating around and around gradually decreasing the angle until the handle is vertical)

WDT:  stir the grounds in the basket (before tamping) with a needle poked into the end of a chopstick.  removes any air pockets and breaks up any clumps.

If you tamp with the basket in the handle, try removing the basket and dosing/tamping directly on the basket and then insert it into the handle after tamping.  I do this so I can keep the handle locked in the group and staying hot. (helps if you've relaxed or removed the spring clip that snaps the filter basket in place)

Offline JojoS

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Re: Extraction bias
« Reply #2 on: September 02, 2017, 07:39:53 PM »
It is very possible that it is on the equipment side. The design of the dispersion disc with the boiler water source on the left side through a single hole and distributed to 4 holes plus maybe a bit from the center that are from uneven distance from each other relative to the source can cause the uneven flow. I try to tweak the shower screen on how tightly it is attached to the dispersion disc to get as even a distribution of water flow as I can. Not tight at all works best for me. I suppose some kind of pre-infusion method will also solve the problem. Dispersion disc mod anyone? 

Offline The Big L

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Re: Extraction bias
« Reply #3 on: September 03, 2017, 11:41:39 AM »
Do you tamp with the basket in the handle?  You may have a bias in the angle of your tamper.   

I have noticed that I'm prone to tamping at an angle.  If I'm not careful I can easily end up with the tamped grounds not being level in the basket.  Consequently, I'm very, very careful with this part.  :D

I've been doing the nutating light tamp for a while now.  My rationale is that it helps me set the initial level of the grounds, and then it's easier to see the edge of the tamper relative to the edge of the basket when tamping.  I find this makes it easier for me to get a level tamp.

I dabbled in WDT some time ago, but I wasn't really comfortable with it, and I didn't notice any difference in my shots.  Given that the faster extraction always occurs on the same side, it seems unlikely that any clumps/air pockets would be the cause.  I do vary the orientation of the PF/basket when filling it with grounds.  I figure this would adequately mitigate any basket-filling bias I might have.

I haven't tried filling/tamping the basket outside of the PF.  Might be worth a shot.  Thanks!

Offline The Big L

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Re: Extraction bias
« Reply #4 on: September 03, 2017, 11:48:44 AM »
It is very possible that it is on the equipment side. The design of the dispersion disc with the boiler water source on the left side through a single hole and distributed to 4 holes plus maybe a bit from the center that are from uneven distance from each other relative to the source can cause the uneven flow. I try to tweak the shower screen on how tightly it is attached to the dispersion disc to get as even a distribution of water flow as I can. Not tight at all works best for me. I suppose some kind of pre-infusion method will also solve the problem. Dispersion disc mod anyone?

Interesting!  I have noticed that the water coming from the group head doesn't always come out evenly.  But I assumed the flow is fast enough, and the pressure is high enough, that the surface of the puck experiences +/- evenly distributed force from the water.  Maybe that's a bad assumption.  I wonder if drilling more holes in the shower plate would help?

Loosening the dispersion disk is easy enough -- I'll try that tomorrow and see if it makes any noticeable difference.

Offline JojoS

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Re: Extraction bias
« Reply #5 on: September 03, 2017, 07:34:27 PM »
It is the screw that attaches the shower screen to the dispersion disc that I tweak. The 2 bolts that attaches the dispersion disc to the boiler base is tightened normally. FWIW, I have observed that the flow of water without the shower screen is seldom even with the holes farther from the source flowing faster. Not sure if adding more holes to the dispersion disc will improve things. 

Offline The Big L

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Re: Extraction bias
« Reply #6 on: September 04, 2017, 07:03:52 PM »
I loosened the shower screen screw a little, but I found that it became very loose after pulling the shot and cleaning up afterward.   I think my post-shot routine of backflushing water with a blind basket might encourage it to loosen pretty quickly.

Afterward I looked at the flow of water out of the shower screen with it tightened vs a little loose.  It changed somewhat when loosened, but in both cases it does appear that there is more flow toward the front of the machine than the back.

I think I'm going to play around with my distribution technique a bit.  For quite a while I've eschewed shaking the portafilter horizontally, or tapping the side to try to settle the grounds into the sides of the basket.  I may give that another go.

Offline cmawp

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Re: Extraction bias
« Reply #7 on: September 05, 2017, 03:35:40 AM »
My preference is not to tap once I've distributed. The danger of tapping the side is you may introduce a fault in the puck, particularly between the side of the puck and the pf, which may go some way to explaining the uneven extraction. This can still happen, even when the tamp compresses any potential puck faults.

Perhaps have a go at distributing and engineering out the tapping?

The challenge will be to ensure you still have the required dose in your pf after distributing.

Offline The Big L

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Re: Extraction bias
« Reply #8 on: September 05, 2017, 08:39:55 AM »
My preference is not to tap once I've distributed. The danger of tapping the side is you may introduce a fault in the puck, particularly between the side of the puck and the pf, which may go some way to explaining the uneven extraction. This can still happen, even when the tamp compresses any potential puck faults.

I've been seeing the extraction bias without tapping the side of the PF/basket, and I was suggesting that tapping could alleviate it.  It sounds like you're saying the reverse.  Maybe I'll experiment with some very gentle tapping and see what happens.

Offline The Big L

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Re: Extraction bias
« Reply #9 on: September 05, 2017, 08:42:17 AM »
After thinking carefully about my distribution technique, I think (my implementation of) the stockfleths move to level out a mound of ground beans is inadequate, and results in lower density of ground beans around the edge of the basket.  Lower density near the edge + uneven water flow from group + biased tamping could explain the extraction pattern I'm seeing.

This morning I tried something different.  When I emptied the ground beans from a measuring up into the basket, I distributed around the edges first, then the center.  There was no mound of ground beans rising above the top of the basket, so I couldn't do the stockfleths move.  I then did a nutating tamp followed by a firm tamp as usual.

Extraction pattern was still biased toward the PF handle, but only slightly, and definitely less than I'm used to seeing.  Notably there was a more uniform emergence of beading between the edge and the center.  Hopefully I can build on this and improve things further!

Offline The Big L

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Re: Extraction bias
« Reply #10 on: September 13, 2017, 07:11:03 PM »
For the last week or so I've been gently tapping the sides of the portafilter near the basket after filling with ground beans and leveling with the edge of my finger.  On average this is resulting in a much more even extraction pattern than I was seeing before, so I'm going to stick with it for now.

It's much easier to pull a good shot when one half of the basket isn't in the first 1/2 of its extraction while the rest is already starting to blonde!

 

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