We talk a lot about puck prep and WDT and avoiding the dreaded clumps but it struck me that 9 bar of pressure is quite a lot. Physics isn’t my strongest subject but a quick Google revealed that 9 bar is equivalent to about 130lbs per square inch. Or a 60 kg person standing on your portafilter.

Given that most clumps dissipate with nothing more than a tap on the counter, this got me thinking about how clumps are able to survive 130lbs or pressure.

I get how an uneven bed, sloping puck or inconsistent grind could affect extraction and cause channeling but I don’t understand how clumps aren’t just smashed by that much force from the water.

Hoping someone who knows more about science can help explain!

    Ernie1 The only clumps I might worry about are a few large ones on the top when I’m tamping. I don’t RDT, WDT. I just invert grind cup on portafilter, few taps to side of portafilter to level…then my standard 3 point tamp. Mostly shots come out fine.

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      Nice, thanks. That’s exactly my workflow too, works a treat.

      Ernie1 Or a 60 kg person standing on your portafilter.

      It’s quite a bit more than that - it’s 240 kg of force, more or less, on a 58 mm portafilter. And even slightly more for 10.5 bar on a 54 mm.

      FWIW, I distribute (WDT) a bit, but that’s because with a 54 mm portafilter the grounds fall off-centre in my grinder, and that causes an unequal extraction if not corrected. Not spending more than 10-15 seconds on that, and the pucks are nice and compact after extraction (unless I got the grind setting completely wrong).

        Clearly maths isn’t my strong suit either 😅

        Levelling definitely seems the crucial thing. I have a levelling tool which I use if it’s clearly uneven, although I find using a funnel and pouring from the cup makes it easy to get a fairly level starting point.

        What we can probably agree is that if there is uneveness in the puck water will flow through paths of least resistance, eroding more content there and leave other spots dry (especially at high pressure). In my experience I saw this behaviour with almost 100% consistency when I don’t wdt.

        An argument against clumps would be that they can create uneven density of grounds after tamping. You could say that you nuke them with 20kg of tamping but what could result is only squashing them and form spots where grounds are highly compacted and other that are not. If you don’t agree try tamp over a mountain of grounds and see the bottomless, it’s the same thing only at different scale, grounds will not flow and distribute themself quite evenly just because they are tamped.