From a few months back (for those who don't frequent Home-Barista) Interesting discussion on H-B here


First Post of discussion thread :
It's holy writ: espresso grinders need to be adjusted frequently and precisely. Holy writ is wrong; instead, you only need to adjust the grinder when you change the dose, the blend, or the style of shot (e.g. from ristretto to normale). All other grinder adjustments are a sign of sub-par technique.

How can this be? We all know that a grinder with too few grind settings will produce inconsistent shots, and we all know any cafe where grind adjustments are not allowed will have espresso that sucks. It turns out that frequent grinder adjustment is what economists and engineers call a 2nd best solution, something you do when the best solution is unavailable:

As the weather changes, as the coffee ages, as the static charges on the grinder wax and wane, the ground coffee becomes fluffier or less fluffy, and also more or less compressible.
These changes affect the density of the prepared puck.
So, if you dose by volume, it will vary the weight of coffee you use from shot to shot.
But the flow rate depends almost entirely on the weight of coffee.
So if you dose by volume, you will see frequent changes in the shot's flow, and have to make frequent grind adjustments.
Since these grind adjustments are retrospective and cannot anticipate how the ground coffee characteristics will change for the next shot; volume dosing will always be jittery, both in shot by shot flow rates, and in the compensating grind adjustments.

If you dose by weight, the jitters go away. Instead, the same dose from the same blend will always get you the same flow. Call this the principle of grind-weight invariance.

How do I know this? I first noticed it when I was doing the TGP. I could pencil in the grind settings on the grinders, come back to them four to five days later, and still be perfectly dialed in. You have to be more precise on the weight and the grind adjustment for smaller burrs or flats than for larger burrs or conical ones; but the principle of grind-weight invariance holds. Since then, for the past two years, I've been weighing doses. I've found, without exception, that when dosing by weight, once a blend is dialed in, it stays dialed in.

Consequently, I believe that the overwhelming reason for inconsistent shots in high end cafes is dosing by volume or grind time. Once they find a way to routinely dose by weight, the consistency of the shots will improve tremendously.
Jim Schulman

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Minor quibbles:

Shadow said:
never seen 7-8 seconds difference regardless of the bean/grind/etc), but in a true commercial setting eyeballing works just fine. No luxury of weighing the doses, so you just do the best you can to keep things consistent in order to achieve a certain standard and simply go with it.

"Can be". I used those terms because It's happened to me, and then suggested that four or five are far more common. In a true commercial setting, whatever technique you use should get you closer than half a gram, and four or five seconds away from your target.

Shadow said:
... there should be... ... an art to espresso ... if every single extraction were flawless the human element would be removed from the experience.

Art is a communicative form. You meant 'craft' Art is when you use your craft to tell people something.
And the human element is what makes a decent lever pull so much more 'flawless' than a super-auto. If every single pull were flawless, we'd spend a whole lot less time in these coffee community fora! ; >

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