10 bar or 11 bar on the pressure gauge?

[symphonie]

Just when I was planning to reduce the brew pressure (which is on the high side maxed at 12 bar) on my newly acquired coffee machine I stumbled on this video on Youtube.

http://youtu.be/E4y4-E2aL3U

My question is should I reduce the pressure to 10 or 11 bar? which one is more optimal for coffee machine with vibe pumps fitted inside?

[artman]

I dont agree with what they are saying. 9 bar at flow x is the same regardless of how big or small the pump is.

The pressure doesnt influence the extraction as much as other variables, so if you set to 9 or 10 brew pressure you will be fine. if you set to 10 bar (at the group) with no flow it should equate to 9 bar when brewing.

cheers

[Dimal]

If it's a big concern though, you can sign-up for Greg Pullman's Pressure Portafilter, and set it up exactly to what your heart desires...
http://coffeesnobs.com.au/brewing-eq...auge-road.html

Mal.

[MrJack]

I dont agree with what they are saying. 9 bar at flow x is the same regardless of how big or small the pump is.

The pressure doesnt influence the extraction as much as other variables, so if you set to 9 or 10 brew pressure you will be fine. if you set to 10 bar (at the group) with no flow it should equate to 9 bar when brewing.

cheers

This is only true with an OPV - otherwise the pressure v flowrate depends on the pump curve (and power).

There will be pressure drop between the pump and the group, which is dependent on flowrate - so if the machine is set for 10 barg when the line is dead headed (i.e. zero flow) you will only see 9 barg at the group at particular flowrates (i.e. the flowrate at which the pressure drop is 1 bar for your machine).

Thus, if set for a nominal 9 barg (at the group) lower flowrates will result in a pressure slightly greater than 9 barg (but not more than 10 barg), and visa versa.

Personally, I believe that too much emphasis is placed on the importance of extraction pressure and not enough consideration is given to flowrate. Even the Illy "scientific" literature seems to ignore the fact that if you vary the pressure you are usually also varying flowrate (which is a much more significant variable when it comes to diffusion). I've been trying to write a blog post about this for a few months now - I think I'm onto my fourth or fifth rewrite

[artman]

I agree MrJack that flow and pressure need to be considered together, but if the pressure is set to 9bar at the group at a flow of say 30ml/30sec, then it is irrelevant if the pump is a large rotary or small vibe?

cheers

[MrJack]

I agree MrJack that flow and pressure need to be considered together, but if the pressure is set to 9bar at the group at a flow of say 30ml/30sec, then it is irrelevant if the pump is a large rotary or small vibe?

cheers

Not at all. The small vibe pump will be much louder.

[Vinitasse]

I agree MrJack that flow and pressure need to be considered together, but if the pressure is set to 9bar at the group at a flow of say 30ml/30sec, then it is irrelevant if the pump is a large rotary or small vibe?

cheers

I've always pictured a rotary pump as your standard rotary drill... spinning away consistently and smoothly while a vibe pump is more like a small impact driver and if that analogy holds any water at all (pun fully intended) the impact driver effect of a vibe pump might actually be of some benefit when it comes to puck penetration, saturation and extraction

Just throwing a somewhat random thought out there

[symphonie]

I dont agree with what they are saying. 9 bar at flow x is the same regardless of how big or small the pump is.

The pressure doesnt influence the extraction as much as other variables, so if you set to 9 or 10 brew pressure you will be fine. if you set to 10 bar (at the group) with no flow it should equate to 9 bar when brewing.

cheers

I don't understand if the pressure doesnt influence the extraction as much as other variables, why some of the high end machines employs pressure profiling to influence the extraction using variable pressures. I don't know if they make any difference at all in the cup without having used one.

[MrJack]

I don't understand if the pressure doesnt influence the extraction as much as other variables, why some of the high end machines employs pressure profiling to influence the extraction using variable pressures. I don't know if they make any difference at all in the cup without having used one.

Because "variable pressure" is a bit of a misrepresentation. Pressure might be the measured variable, but generally what you are really manipulating is pump speed (and thus flowrate, which subsequently impacts the pressure).

That's my point really - unless you isolate one variable, how can you assess which is the more important one?

[simonko]

I don't understand if the pressure doesnt influence the extraction as much as other variables, why some of the high end machines employs pressure profiling to influence the extraction using variable pressures. I don't know if they make any difference at all in the cup without having used one.

I suspect Artman was more suggesting that the differences between 9 bar and 10 are minimal. The high end machines kick by starting with a substantially lower pressure to softly wet the puck (eg 3 bar), then ramp or step up to around the 9 bar for extraction, maybe a step down at the the end too.

[MrJack]

I suspect Artman was more suggesting that the differences between 9 bar and 10 are minimal. The high end machines kick by starting with a substantially lower pressure to softly wet the puck (eg 3 bar), then ramp or step up to around the 9 bar for extraction, maybe a step down at the the end too.

Still, all about flow/velocity (and, with respect to the step down at the end, compensating for falling viscosity).