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question about pressure
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dspear99ca
Senior Member


Joined: 17 Jan 2013
Posts: 93
Location: BC, Canada
Expertise: I love coffee

Espresso: Gaggia Coffee
Grinder: Mazzer Mini
Posted Mon Jan 28, 2013, 1:49pm
Subject: question about pressure
 

Many posts advise one to set brew pressure to 9.5 Bar with a blind portafilter, some calling this "static pressure", and implying if not stating outright that the pressure when brewing would somehow be different, i.e. that it will drop to ~9 Bar.  The way I see it, the OPV is going to regulate pressure in the boiler at 9.5 bar if that's what it's set to.  Putting grounds in a portafilter will require more inlet pressure to maintain the same boiler pressure, but the OPV will compensate by increasing the incoming water pressure to maintain 9.5 Bar, no?  How can it be otherwise?  I am waiting for an adjustable OPV for my machine so I want to know where I should set it.  I am leaning towards 9 Bar on my guage.
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dyqik
Senior Member


Joined: 7 Oct 2011
Posts: 383
Location: Cambridge, MA
Expertise: I love coffee

Espresso: Bezzera BZ07 PM
Grinder: Baratza Virtuoso Preciso...
Vac Pot: Cona D
Drip: Bona-Vita, CCD, Aeropress.
Roaster: Gene Cafe, Modded Poppers
Posted Mon Jan 28, 2013, 2:20pm
Subject: Re: question about pressure
 

The OPV is generally before the boiler/HX, group valve and dispersion screen - there is a loss of pressure between the OPV and the group when there is liquid flowing, roughly proportional to the flow rate through the boiler and group.  If the group is sealed off, then the flow rate is zero, and the pressure at the OPV is the same as at the group.  With a small flow through the group (through coffee), the pressure at the OPV is higher than at the group by some amount.

Since the pressure isn't that critical, subtracting 0.5-1 bar is accurate to compensate for this for most purposes.  If you want to be super precise, use a pressure gauge at the group with a needle valve that be adjusted to provide the correct flow rate through the group in parallel to the gauge you are measuring the pressure with.
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germantownrob
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germantownrob
Joined: 2 Dec 2007
Posts: 2,137
Location: Philadelphia
Expertise: I love coffee

Espresso: Duetto 3, A Dead Oscar
Grinder: Vario-W, Preciso w/Esatto,...
Drip: Brazen
Roaster: Diedrich IR-1, HT B
Posted Mon Jan 28, 2013, 2:27pm
Subject: Re: question about pressure
 

This was posted today for me about rotary settings but it works for you as we'll. Click Here (www.home-barista.com)
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MJJ
Senior Member


Joined: 2 Jan 2013
Posts: 37
Location: Montreal
Expertise: I love coffee

Espresso: Rancilio Silvia PID
Grinder: Baratza Vario
Drip: Hario V60
Posted Mon Jan 28, 2013, 8:31pm
Subject: Re: question about pressure
 

Has anybody tried the pressure gauge from Espresso Parts?

http://www.espressoparts.com/V_512

I'm wondering if it would be any better than a homemade one. It also doesn't have a needle valve.
  I'd like to lower the pressure on my Silvia V3 and want to see where the starting point is. At the moment I can't get the portafilter spout off, even after clamping it in a vice.
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