I've had shots choke a bit and at times I've saved them by reducing the pressure all the way back to PI levels before returning to full pressure. My thought has been that unless you pull up on the arms to create a vacuum the puck isn't going to be disturbed. The pressure will still be positive. Even if you let go of the arms it should be okay, at least that's what I imagine.

A transparent PF would be a great way to visualize this.

Jonk wrote:If you want to pre-infuse you need to tighten up the grind to get the same resistance, that's all.

Just getting back to an old thread comment that came back to my memory when messing with preinfusion this morning (no relation to the thread topic name)

Here is the morning question: when using pre-infusion, does the amount of required tightening of the grind correlates to the duration of the pre-infusion phase?

Cheers

Yes, to some extent. A long or somehow complete saturation of the grounds during pre-infusion will help increase flow through the puck, compared to a partial saturation that you might end up with if you only pre-infuse for a few seconds.

so, in summary:

• to keep the same flow using pre-infusion you need to grind finer

• the longer the pre-infusion step, the finer you will need to grind to keep the same flow

• the more pressure, the faster the flow

Basically, the destination is that place where, for a given grind, you start understanding the profile of pre-infusion and pressure needed to get a consistent flow.

Thanks!

In my experience using the Robot helps a great deal because the feedback is good. It's usually possible to notice just by feel if the resistance is high or low, but you can also have a look at the bottom of the basket.

If resistance is low and drops emerge immediately, push hard and hopefully get a decent shot anyway*.
If it feels like a regular shot, go for a moderate amount of pre-infusion (say 5-10s, I usually just wait until I can see beads of espresso form from every single hole).
If you notice heavy resistance or don't see any droplets form in the normal timeframe, ease up a bit on the levers and wait for a good while before carefully starting the shot.

Pressure is not linear with flow rate, it's more like an M-shaped curve in my experience.

*worst case, if the desired shot volume is exceeded before 15s I usually push all of the water through. If it's a fruity coffee it could end up as a nice allongé.

So much great info to unpack there

Jonk wrote: If resistance is low and drops emerge immediately, push hard and hopefully get a decent shot anyway*

My initial thought would be that if I press harder, more pressure will increase the flow so i will get to the desired volume will finish faster? Is that not the case because there is point where more pressure means less flow? Like at the top of the M-shape you were mentioning ?

Jonk wrote: If it feels like a regular shot, go for a moderate amount of pre-infusion (say 5-10s, I usually just wait until I can see beads of espresso form from every single hole

Can you also gauge how "regular" the shot is gonna be by checking if the espresso drops appear from every single hole at the same time? vs an initial donut or irregular dropping from the holes . This might be more related to puck prep, but i wonder if it is a good indicator too

Jonk wrote: If you notice heavy resistance or don't see any droplets form in the normal timeframe, ease up a bit on the levers and wait for a good while before carefully starting the shot.

Makes lots of sense. I will try this

Thanks. Very useful info

The puck will compress under pressure, so yes - a high pressure can slow things down. The exact relationship is complicated but even at medium pressures you'll see an increase in resistance compared to the [usually low pressure] pre-infusion stage.

You can try brewing this style of filter [2.1]: ..using only slightly coarser grinds than for regular espresso. Then brew with the same grind, slam the levers hard and notice how you can reach high pressure that will quickly decline. In this case of extremes, higher pressure probably led to both higher resistance and higher flow.

But say you were using a bit finer grind, I think you'd find that the increased resistance actually led to slower flow in a shot that peaked at 9 bar compared to if you followed the basic instinct to low resistance and only apply a couple of bars. More importantly, by reaching higher pressure it'll taste like proper espresso and not something from a moka pot or aeropress.

Then if you hit roughly 10 bar (which is not really recommended with the Robot and strenous to do) there's a so called secondary compression of the puck that definitely results in lower flow compared to a 8-9 bar shot.

Hope this makes sense.

mixespresso wrote:Can you also gauge how "regular" the shot is gonna be by checking if the espresso drops appear from every single hole at the same time? vs an initial donut or irregular dropping from the holes . This might be more related to puck prep, but i wonder if it is a good indicator too

Because of the basket shape you should expect to see initial flow from the sides. As long as you're using a flat tamper a donut is actually a sign of good puck prep as far as I'm concerned.

Jonk wrote: Because of the basket shape you should expect to see initial flow from the sides. As long as you're using a flat tamper a donut is actually a sign of good puck prep as far as I'm concerned.

Nice to hear! I had seen a video from Decent about puck prep and the guy mentioned how bad "donut extractions are" and I got paranoid!

Jonk wrote:The puck will compress under pressure, so yes - a high pressure can slow things down. The exact relationship is complicated but even at medium pressures you'll see an increase in resistance compared to the [usually low pressure] pre-infusion stage.

You can try brewing this style of filter [2.1]:
video
..using only slightly coarser grinds than for regular espresso. Then brew with the same grind, slam the levers hard and notice how you can reach high pressure that will quickly decline. In this case of extremes, higher pressure probably led to both higher resistance and higher flow.

But say you were using a bit finer grind, I think you'd find that the increased resistance actually led to slower flow in a shot that peaked at 9 bar compared to if you followed the basic instinct to low resistance and only apply a couple of bars. More importantly, by reaching higher pressure it'll taste like proper espresso and not something from a moka pot or aeropress.

Then if you hit roughly 10 bar (which is not really recommended with the Robot and strenous to do) there's a so called secondary compression of the puck that definitely results in lower flow compared to a 8-9 bar shot.

Hope this makes sense.

Thanks for this detailed description. I am slowly and gradually getting to understand these complexities!

I just saw a video which resonated with your explanation
They tested different brewing pressures with different coffee doses, and they saw that as the pressure increases the extraction times increase accordingly
Their theory to explain it was that

"the carbon dioxide gas produced by the fresh coffee was producing more and more resistance to the water as the pressure increased"

To prove their theory they run the same experiment with stale coffee, which did not show this relationship

They also brewed at 12 bars, which quickly saturated the puck and slowed the flow rate. I believe this is what you were alluding to with the second compression

All really interesting!