# Extraction time/OPV mod



## Rawk (Nov 17, 2014)

Possibly a stupid question coming up, here goes:

So - I haven't done the OPV mod yet, so if my pressure is say 13bar->14bar, does this mean that my extraction time will be shorter anyway?

That's what I'm thinking.... if the pressure is higher than it should be then surely the extraction time will be faster than it should be, all other things remaining equal?

So in this scenario do you aim to extract for less time than the 24ish seconds recommended?


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## NJD1977 (Dec 16, 2014)

No, it would just mean that you should be grinding finer / tamping harder to keep the extraction time to 25-30s. The rate of flow of water through the puck is the main governor of extraction, not the pressure of that water.

I also read somewhere that there is some counter-intuition with higher pressures leading to quicker extractions because the higher pressure forces the fines in the puck tighter into the basket base and can slow down flow not speed it up - whether this is true or not, I'm not certain.


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## roscoffbean (Dec 22, 2014)

I have just made my own meter. One gaggia classic (old) 12 bar - newer (2010) 14 bar!

if i grind any finer - the newer classic chokes. However at the moment - my extractions are too fast. i'm definitely making cafe lungo - it tastes good! plenty of crema - but definitely lungo not espresso!

So answers to this would be much appreciated - i want to be certain that adjusting the opv to 10 bar is the right thing to do!


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## StuartS (Jan 2, 2015)

After OPV mod (reduced from 13.5 to 10 bar) I had to grind finer. I reckon the higher pressure compacts the grinds in the filter and increases the resistance.

Before OPV I struggled to get a 1:2 brew ratio in 27s because the machine was choking. After OPV mod, no problem. The reduced pressure seems to have improved the flow control.


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## roscoffbean (Dec 22, 2014)

Thanks Stuart


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## AndyS (May 12, 2012)

NJD1977 said:


> I also read somewhere that there is some counter-intuition with higher pressures leading to quicker extractions because the higher pressure forces the fines in the puck tighter into the basket base and can slow down flow not speed it up - whether this is true or not, I'm not certain.


See this post.


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## The Systemic Kid (Nov 23, 2012)

Would be interesting to see how varying bar pressure influences extraction yield.


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## jkb89 (Dec 10, 2014)

The Systemic Kid said:


> Would be interesting to see how varying bar pressure influences extraction yield.


There's a PhD in there somewhere ;-)

P.S: Tried to PM you re. tamper but it wont let me send another message for 10 mins?


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## AndyS (May 12, 2012)

The Systemic Kid said:


> Would be interesting to see how varying bar pressure influences extraction yield.


It's been a while since I've looked at that, but IIRC, a decreasing (lever-like) pressure profile produces a small increase in extraction yield, like 0.5%. Barely tasteable, if at all.


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## StuartS (Jan 2, 2015)

Is it reasonable to compare tamp force/pressure with extraction force/pressure?

If so, tamp force of say 35 lbs over an area of 4in2 (58mm filter) gives a pressure of

This may be the reason why tamp force doesn't affect extraction time as much as you would think.

If my sums are wrong, I'll get my coat.


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