Swings in shot time with no changes in anything, no spurts or channeling.

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Ken5
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#1: Post by Ken5 »

Hi!

I have a robot and a kinu. I find that I can go many shots with the same shot timing results, then I get a huge swing without changing anything. Same bag of beans, no changes in grinder, use scale, WDT, same tamp, same machine pressure, etc. No spurts, visible channeling, etc

Last 4 shots, again using a cafelot robot,18g grinds to 22g espresso:

25 preinfusion - 30 second shot for a total of 55

25 preinfusion - 28 second shot for a total of 53

34 preinfusion - 34 second shot for a total of 66

35 preinfusion - 34 second shot for a total of 69

What determines my preinfusion above is when it starts to drip and the scale shows a few 0.1 grams of output on the scale.

Really careful to keep everything consistent, is it abnormal to have such big swings?

Thanks!

Ken

lessthanjoey
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#2: Post by lessthanjoey »

Not normal. You're grinding very fine for it to take that long to get drips and then take that much longer to get ristretto output. Is preinfusion with a couple of bars applied? Or water just sitting there at atmospheric pressure?

How are you controlling pressure during the extraction phase? That seems like a secondary source of variation, but I'm still a little stuck on how unusual this shot setup is in general though.

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sadbox
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#3: Post by sadbox »

The first thing to remember is that we are all terrible at consistent prep.

Can you describe your WDT method? I am a proponent of the "deep WDT" method, where you mix all the way down to the bottom of the bed. The very recent study by Jonathan Gagne showed that this method showed reduced variation in puck resistance vs. a shallower WDT.

Can you also describe your overall extraction profile? What are you doing during preinfusion? What pressure are you aiming for? Do you taper through the shot?

How are you preheating? The temp of the pull can have an impact on shot times as well. Do your shots pull faster or slow as you pull subsequent shots?

Lastly, have you tried grinding any coarser? It's possible that you are simply grinding too fine for your grinder, and your variation is caused by channels opening and closing inside the puck. Total shot time of >60s isn't strictly bad, but it is at least a little surprising given your dose and yield. Channels won't necessarily be visible with a bottomless portafilter.
LMWDP #674

Ken5 (original poster)
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#4: Post by Ken5 (original poster) »

Thanks Joe and James!!

The four shots I posted above were my first attempts at a smaller ratio. Perhaps I did go a little finer than I should have. I was extremely surprised and happy to get a good tasting shot in 28 seconds on my first attempt! Having swings in times makes it hard to make instant decisions on changing things based on one shot. :( Even before going to a lower ratio I had differences between shots. I do understand that it is hard to get perfect preps each time.

I just grinded coarser and got a preinfusion of 15 and a shot time of 32 for the 22g cup. Was 'one' of my best shots and I was happy with the lower preinfusion time. Shot was so good, and so small :( , that I made another shot and the shot time was the same, but the preinfusion time went up to 25 seconds, quite a bit longer that the 15 seconds I got just prior, but still not as bad as my 3rd and 4th shots. Tasted good, slightly less so than the shot just before. This last shot did have some minor spurts which I would have thought would have decreased times.

Joe, the long preinfusions did not bother me as people that use the robot state that these are common. Preinfusion is at 2 bars pressure. For the shot I go to a little below 9 bars pressure and maintain the same pressure throughout the shot. I am trying to be consistent at this point, plus I don't know enough yet to try different pressures through the shot as some do. Shot times are not what a machine would give, but everyone is saying that levers are more forgiving and one cant go by what a pump machine does. Confused...

James,

I am using levercraft ultra wdt. At the beginning I go to the bottom of the basket with a slight upward motion, then I do a Spirograph motion and move that up to the top half. Once mixed and level I place the tamper level with the basket and try to always press down the same.

I am going to 2 bars for preinfusion and a little under 9 bars for the duration of the shot.

Preheating on my robot is done by using the pressurized basket in my portafilter and pressing the boiled water completely out of the basket, into the cup I use which also heats that up. Pouring the boiled water into the prepped single wall basket is extended past what is needed to fill the basket in order to account for the cool basket. Letting the water overflow and run down the portafilter helps heat that up too, but as Jeff mentioned in another thread... the poor conductivity of the stainless steel and the extremly small contact between the basket and the portafilter via a very narrow lip on the basket there is very low heat loss there anyways. The beans that I am using recommend 203-205 temps.

This morning I did grind a little coarser, great shots! Though there was a big swing in preinfusion time for the second shot.

Ken

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sadbox
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#5: Post by sadbox »

The temps, WDT, etc look good!

One thing you might experiment with is tapering the pressure on your shots. As your coffee is extracting your puck will up providing less resistance, so maintaining that fixed pressure will increase flow through the brew. That will increase the chance of channeling, and couple be some of the variance you're seeing.
LMWDP #674

Ken5 (original poster)
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#6: Post by Ken5 (original poster) »

Thanks again James!

Before trying 1:1.2 there were times where the levers started dropping faster towards the end of the shot. Could definitely tell just from feel that there was channeling going on. I did at those times soften up a bit. With these shots yesterday and today there was no drop in resistance. Actually I was surprised by how instantly the shot stopped when letting up on the levers. Also had to press the excess water out of the basket where as when the grind was coarser with the 1:2 ratio it would drip out by the weight of the levers in a few minutes.

Just made a third shot with the same settings as the first shot this morning, which was one of my best shots. The time of the shot and preinfusion was similar to that great shot and was really excited for it, but this time the shot was not good at all. The fact that the shots were exactly the same in every way, but didn't taste the same, the fact that no changes in settings, prep, and beans the timings can change, then change back, but taste is different with the similar shots make it really hard to figure this whole art out.

Oh... with the prep of the basket I forgot to mention that after wdt'ing I tap the basket down a few times.

Ken