Adjusting OPV vs. Maximum Pump Pressure

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#1: Post by boren »

Some machines provide separate adjustments for maximum pump pressure and OPV. I adjusted the pump on my Lelit Bianca V3 down to 9 bar (instead of the default 10 bar) using the external adjustment screw. This however also reduces water debit, which some consider to be a negative since it extends the time it takes to saturate the puck during pre-infusion and can potentially make extraction less even.

Would it be better to keep pump pressure high to get higher debit, and instead adjust the OPV to limit the machine to 9 bar?

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#2: Post by Jeff »

Peak brew pressure on a rotary-pump machine generally should be set with the bypass adjustment on the pump. The OPV is typically set higher to relieve pressure from, for example, thermal expansion.

The flow through a restriction is related to the pressure across it. Lower pressures will result in lower flow, without regard to what is regulating the pressure.

boren (original poster)
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#3: Post by boren (original poster) »

When you turn on the pump and the group-head is filling up maximum pressure isn't yet reached. As far as I know at this stage the stock pump setting of the Bianca allows 6+ ml/s and 10 bar, while reducing it to limit to 9 bar also limits debit to around 5.2 ml/s. This means that even before maximum pressure is reached stock Bianca debit is higher, causing the puck to saturate at a quicker rate.

If this is true, the only downside (from my point of view) is that the shot might reach 10 bar if the coffee is ground fine enough. Reducing the OPV to 9 bar would prevent this issue, and would only be relevant when high pressure is actually reached and the valve opens up. At lower pressure (e.g. demonstrated to extreme without a portafilter attached) debit should still be the maximum that a stock Bianca can reach, no?