Is pod-compatibility the reason why the OPV on many espresso machines is set to 11/12 bar?

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#1: Post by boren »

It's quite common (and a frequent complaint from James Hoffmann) that the default maximum pressure on many machines is set too high, as high as 12 bars. This is much higher than the recommended maximum of 9 bars, let alone the new trend of using pressure as low as 6 bars.

One theory I've seen mentioned is that high pressure is required to make machines compatible with pods. Is this confirmed? Is there another theory?

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#2: Post by another_jim »

There is a pod connection. Illy set a standard for OPVs of 11 bar for vibe pumps and 9 bar for rotary pumps. This standard was taken over by the various standards organizations in Italy. This was a big step forward at the time, since most home machines in the nineties had no OPV at all.

But why 9 for rotaries and 11 for vibes? This is a slightly complicated story. OPVs set the maximum pressure, not the actual pressure, which is determined by the resistance of the puck and the flow rate through it. For standard pour rates, this is roughly 9 bar. It can go much higher when there is no flow, which happens at the start of the shot. Rotary pumps get up to pressure much faster than vibe pumps. So the OPV difference is to get the same dwell time for both pumps (dwell time is the time between starting the pump and seeing the first drops).

Now this is all true when you use a standard Italian espresso blend with a standard basket and dose. But dwell times change dramatically when using lighter roasted SOs, precision baskets, ultra fine grinds, and all the rest of the contemporary styles. For all this new stuff, there is no overall best OPV setting. Instead you need to be able to program the preinfusion coffee by coffee; although 10 to 15 seconds at 2.5 bar will do well for most fine ground, light roasted coffees.
Jim Schulman
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