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Setting OPV pressure in espresso machines


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Сообщение 12 Oct 2019, 07:34
Ursego Аватара пользователя
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I experimented a little bit with the externally adjusted OPV (over pressure valve) of my new Lelit Victoria, and then Ascaso Steel Uno w/PID (after a few days of playing with both the machines, I returned them to the store and bought Bezzera Duo DE - by reasons, irrelevant to this topic). Following an advice in coffee forums, I decreased the pressure (using the blind basket) from 11 (the factory setting) to 9. People say, it will be more forgiving. Yes, it's true! It was more forgiving if we are talking about consistency in parameters, which can be expressed mathematically - it was easier to achieve the "magic" 9 bar in manometer. But the shots usually were a little bit less tasty (tasted more "like water") compared to shots with OPV, set to 11 (when the taste was more concentrated, with more nuances).

If the OPV is set to 11, then the actual pressure 9 in the manometer is the natural result of the correct grinding (rather than of an artificial, imitative factor - in fact, cheating). So, I restored 11 bar preferring to achieve 9 bar by playing with grind size rather than pretend, that everything is good, by "enjoying" the "unnatural" 9 bar in manometer (when, in fact, the pressure supposed to be different in the given circumstances). More exactly, I am using the extraction time as the evidence of correct/incorrect grind size for the given brew ratio. So, if the required pressure is higher, then 9, I don't limit it (it is required!!!), and usually get a more tasty espresso. If it's 10 or 10.5 - I am happy too. If it's 9, that it's real, natural 9 - not a result of cheating.

I would compare setting OPV to 9 to a pressurized portafilter. OK, not SO extremely, but by the same principle. In pressurized portafilter, the "correct" pressure is built not by the coffee but by a factor, external to it (the basket). When OPV is set to 9, the story is pretty similar (even though not as extreme, as I wrote above): the "correct" pressure is built not only by the coffee but also by a factor, external to it (the pump is simply not able to produce more pressure). When OPV = 11, the actual pressure depends only on the coffee tablet, so 9 bar signal us that the grind size is correct (there are nuances, of course, but that is the idea less or more). When OPV = 9, then 9 bar on the manometer convey us much less information - maybe, this grind size requires more pressure (which cannot be produced physically), and, to achieve the "real" 9 bar, the grind size should be different.

I was not sure if it's a good idea to write that opinion while many people are against it and recommend to decrease OPV from the factory setting. But I found that I am not alone in the Universe. From here:
the OPV allows the barista to be less accurate. assume there is no pressure release valve (opv) at all; in that case, it would be the grind that determines the pressure at which the water flows through the puck. however, if you can't get the grind perfect enough to regulate the flow to 9 bar of pressure, then the OPV sets the max pressure. if the OPV is set to 11bar, then that perfect grind that would have allowed 9bar of flow would still work perfectly (9bar would release the pressure before the 11bar limit was ever reached). however, if your grind is off (too fine), you go to the 11bar max - and it stays there until you get flow through the puck (or you get nothing). similarly, if you reduce the OPV to 9bar - then 9bar of pressure is the max flow pressure - regardless of the grind.

And from here:
My wife's standard is a 17g dose, 30g yield, cut short at ~23-25 sec. It's just what she likes. Anyways, after adjusting the brew pressure down (from 10.2 to 9 bar), I made her beverage. Same dose, same yield, same shot time, same temp profile, same beans. She didn't even drink half her drink and said it was undrinkable.


Please watch this video - an interview with Luca Bezzera himself! The link is to the video starting 0:46 when the English part starts (the beginning is in German).

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