VST basket size and grind volume are at odds - Page 6

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ira
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#51: Post by ira »

Ok, gravity has an effect, but it's not relevant for this discussion. The change in gravity over 1/2" in this context has to be considered zero for all practical purposes.

GDM528
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#52: Post by GDM528 »

ira wrote:to essentially zero at the bottom.
To clarify: zero pressure at the bottom just outside the basket? There must be some pressure just inside the bottom of the basket, given how the coffee flows out faster than what just gravity can account for. My 18g VST basket is >80% impermeable, which creates resistance that will in turn cause pressure to build up just inside. Gist is I'm skeptical that the top of a saturated puck is getting hit with a 9bar hammer during the shot - there are other explanations for the observed compression.

jpender
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#53: Post by jpender »

Yes, absolutely, the effect of gravity is negligible in this context, on the order of 0.001 bar.

The pressure difference between the inside of the basket and outside is also pretty small. Try pouring a measured amount of water through your basket and time it. I think you'll find that it flows a lot faster than your espresso typically does, and that's with the relatively tiny pressure due to gravity. By comparison, a puck that is 1.5cm thick will have an average pressure gradient of around 0.6 bars per millimeter.

Saturated or not, the top of the puck will see the extraction pressure. If that's 9 bar then 9 bar it is. Whether that's like "a hammer" depends on how rapidly the pressure is applied.

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Jake_G
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#54: Post by Jake_G »

jpender wrote:Saturated or not, the top of the puck will see the extraction pressure. If that's 9 bar then 9 bar it is. Whether that's like "a hammer" depends on how rapidly the pressure is applied.
Yes, but remember, in your example below, the next "layer" of the puck will see about 8.4 bar (9 -0.6 bar):
jpender wrote:By comparison, a puck that is 1.5cm thick will have an average pressure gradient of around 0.6 bars per millimeter.
So each layer has roughly the same relative liquid pressure gradient acting on is as another layer, but each subsequent layer deeper in the puck has to support all the layers above.

It is the flow through the puck (and subsequent pressure gradient) that leads to puck compression. While the bottom mm of the puck only has a pressure drop of 0.6 bar or less, it has to support the static (or "solids") pressure of all the layers above it. Of course the floor of the basket has to support the entire force of 9 bar on the top surface but experiences virtually zero fluid pressure.

There is a diagram back in the ramble that demonstrates the transition from cumulative hydraulic drag pressure (called "solids pressure" which is zero at the top of the puck and equal to brew pressure times the surface area at the bottom) to the actual liquid pressure of the brew water.

It isn't quite linear, but yes. Basically atmospheric pressure at the bottom of the puck, but 9 bar of solids pressure exists from all the layers above that are getting pressed down by the flow of water and solids through the puck.

Cheers!

- Jake
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jpender
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#55: Post by jpender »

Jake_G wrote:It is the flow through the puck (and subsequent pressure gradient) that leads to puck compression. While the bottom mm of the puck only has a pressure drop of 0.6 bar or less, it has to support the static (or "solids") pressure of all the layers above it. Of course the floor of the basket has to support the entire force of 9 bar on the top surface but experiences virtually zero fluid pressure.
That's a very good point. There is this idea that in a preinfused puck the incompressible water supports the load thereby lessening the compression and leading to higher permeability. But if the solids see the same pressure differential then that picture can't be right. It must be more complicated.

Now I'm going to have to read that long thread from 2018... :-)

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Jake_G
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#56: Post by Jake_G »

jpender wrote:Now I'm going to have to read that long thread from 2018... :-)
You have my condolences :P
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