Comments on: Talking About Pressure Profiling

By: Richardcranium247

Richardcranium247 — Fri, 04 Mar 2011 17:16:00 +0000

*Comment removed due to violation of comments policy*

May I remind people – no personal attacks that contribute nothing but animosity.

-JH

By: Tom Jagiello

Tom Jagiello — Sun, 21 Feb 2010 13:51:52 +0000

Less pressure indeed means lower flow rate, but from what I’ve experienced on my rig it also means steadier more even extraction from the whole puck ie. late blonding. Perhaps at that stage of the extraction the coffee gave away most of it’s ‘value’ and increasing the contact time is needed to pull something more out of it. Whether the ‘more’ is good or not is the real question (something that probably depends on the type of coffee used).

Regards,
Tom

By: Graeme

Graeme — Sun, 21 Feb 2010 13:29:13 +0000

RE: Footnote #1 The period of declining pressure in the final third meant that we were extracting the nearly-spent coffee less aggressively, yet still adding volume to the shot – aiding balance, and reducing negative flavours. Of course you could easily tear this theory to pieces – I just couldn’t find a better way to explain it.

Why do you think declining pressure equals less aggressive extraction? I think it’s just the flow rate that is “less aggressive”? This would imply the contact time is greater in that final third. Did you compare this with a shot with no declining pressure in the final third, implying “normal flow rate”, and therefore shorter time to yield the same volume?
I had thought the brew pressure only overcomes the hydraulic resistance produced by the puck and it was flow rate and hence contact time which affects the extraction. I think that when brewing at a constant pressure there is no significant pressure gradient across a coffee particle so there is no extraction of stuff out of a coffee particle due to pressure. I do think pressure does other things (eg emulsification) to the coffee extract but essentially espresso is produced by washing the surface of specifically sized coffee particles with a specific volume of water in a specific time.
However, maybe it’s possible that changing the brew pressure during brewing does produce a pressure gradient across a coffee particle. This would result from the pressure “front” moving across the particle. Maybe then there will be extraction of stuff from inside, or at least deeper inside, the coffee particle? This effect may produce the difference observed in flavour.
What do you reckon?

By: Define: Pressure Profiling « La Marzocco USA

Define: Pressure Profiling « La Marzocco USA — Fri, 19 Feb 2010 21:51:14 +0000

[...] summary, this post is only meant to put some thoughts on pressure profiling, given some of the questions out there. If anything, this hopes to serve to get more discussion (and experimentation) going to [...]

By: SB

SB — Mon, 01 Feb 2010 06:03:07 +0000

James — Interesting that your default pressure profile is fairly similar to what an E61 group does automatically.

By: Bluecold

Bluecold — Wed, 27 Jan 2010 21:24:43 +0000

That residual pressure would be gone after a few drips because water isn’t compressible. Maybe some more because the puck expands as it isn’t compressed anymore, but nothing serious. The flow stops very soon after the lever bottoms out on my La Peppina. The drips after a lever bottoms out are not something you want in your cup anyways.
Maybe the VA groups don’t have pretensioning. Is the first few cm of pulling down the lever very easy?

By: James Hoffmann

James Hoffmann — Wed, 27 Jan 2010 20:25:32 +0000

But as far as the coffee is concerned – it would experience a linear drop to zero.

Its resistance determines the rate of decline in pressure, and even though the spring may no longer be expanding it has created pressure that will continue in a linear fashion to drop to zero as the pressure is released through the coffee itself.

I figure this is the only way to get a linear decline that I see on my Scace 2 – as if the spring completely extended then the last part of the pressure profile would likely be a slower drop in pressure.

By: Bluecold

Bluecold — Wed, 27 Jan 2010 14:07:30 +0000

They do return to zero, but not linearly. For example, a lever could concievably start at 12 bar, drop linearly to 6 and then hit the block. The spring never is in relaxed state in most lever machines. Check the graphs in the Slayer link. The LaMarz starts at 11, drops linearly to a bit less than 5 and then hits the block.

By: James Hoffmann

James Hoffmann — Wed, 27 Jan 2010 11:24:28 +0000

Surely every lever machine has to return to 0? Otherwise, as they have no three way valve, there would be quite horrific portafilter sneeze when you take the handle out.

By: Bluecold

Bluecold — Wed, 27 Jan 2010 00:15:09 +0000

Not every lever has the same pressure profile. It depends on the spring and on the pretension applied. If the pressure drops linearly to zero, you’ve got zero pretension. La Marzocco’s lever started at 14 bar and dropped to 7 according to the Slayer guys. So the La Marzocco lever has quite a bit of pretension.
http://www.slayerespresso.com/2007/12/10/lever-espresso-extraction/
And the HX levers have a preinfusion pressure which is the same as line pressure, while the dipper groups have boiler pressure preinfusion. Then you’ve got the Fellini preinfusion which is also something else.
So ‘your’ profile is probably quite similar to another lever machine out there. And wildly different from a lot of others.
Also, the cimbali guy probably didn’t answer because it would be embarrassing to admit that the machines they made 50 years ago did basically the same thing, but automatically >:)