Why 9?


Lower pump pressure means 3 interesting things. More gentle water flow - less chance for channelling. More even saturation of the grounds. And less fines migration.

Considering the arguments above, how on earth everybody got stuck on 9 bars for so long?

History
The first coffee machines made "espresso" more like filter coffee. Successively the pressure was higher and higher. Which allowed for about 30s extraction of concentrated beverage known as espresso.

It's clear, that taste wasn't the critical factor. So, let's say the goal was to extract more. Understandable, but extraction percentage in relation to pump pressure is Gauss's graph. Therefore, as you increase pressure, you get higher extraction, but only to some point. After that, increasing pressure lowers extraction.

The reason for that is uneven extraction. The peak point of the graph depends on coffee used, grinder, baskets, distribution, tamping and hundreds other factors. The thing is, usually the peak point is around 8 bar with perfect technique (the one you do competing).

Café environment
Pulling 100 shots per hour, for 10 h on shift is a different case. You have to be consistent. And consistency derives from minimising unpredictability. Pump pressure is one of them.

Let's assume you have a decent tamp - firm, level and consistent. But everyone makes little mistakes from time to time. The tamp no. 793 of the day might be of lower quality. Lighter and not perfectly level. 9 bars would cruelly channel it into 22s shot (everyone is weighing shots, right?). Not acceptable. But 6 bars would gently go through the puck extracting servable coffee. The shot would not be perfect, but you wouldn't tell the difference by watching the shot.

The example above shows how lowering the pump pressure minimises an obstacle in achieving even extraction. Mitigating negative factors is a key to consistency. It also allows you to focus on different factors. Such as temperature or distribution.

Extraction
The ultimate factor is taste. Always. Taste is correlated more to evenness of extraction that extraction. The question is, can you taste uneven extraction?
Tasting this is like a reawakening. Once you get used to 6-7 bar pressure, pull a shot on 9-10 bar. Even if the shot doesn't seem to be channelled it's gonna be hell in your mouth.

Some critics may say, that getting used to 6 bars worsens your technique. Just like getting used to gravimetrics or volumetrics deteriorate to stop your shots manually. There's a hint of truth in that. However, stopping your shots manually has never been the best option (unless you really weigh the shot) - click here. Same with lower pressure. Getting ZERO chanelled shots is worth the game. We've gone far from tamping as hard as possible a while ago. Consistency is a major factor. And actually, you CAN tamp too hard - click here.

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