dspear99ca Senior Member Joined: 17 Jan 2013 Posts: 88 Location: BC, Canada Expertise: I love coffee
Espresso: Gaggia Coffee Grinder: Mazzer Mini
Posted Mon Jan 28, 2013, 12:49pm 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.
Posted Mon Jan 28, 2013, 1:20pm 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.
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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