The magical 15 kg of "force/pressure" has often been used as a benchmark for tamping in industry. It gives baristas a baseline to work from – 15 kg of downward "force" is merely a trade off between the compactness of the bed of coffee and preventing RSI (repetitive strain injury) for baristas tamping hundreds of group handles a day. As long as you are consistent, it does not matter how hard you tamp.
The reasoning is this: tamping harder or softer will have negligible impact on an extraction flow rate once the coffee bed has been tamped with sufficient pressure to remove any air pockets between coffee grinds. 9 bar of extraction pressure is 16 times greater than the pressure of a 15 kg tamp. Tamping pressure is immediately alleviated during the period when the dry coffee is wetted and once the extraction is in full swing will always be much less than pump pressure.
So when someone says tamp harder to slow down an extraction, I call bs! :
Discuss, debate, theorise..... 8-)
The reasoning is this: tamping harder or softer will have negligible impact on an extraction flow rate once the coffee bed has been tamped with sufficient pressure to remove any air pockets between coffee grinds. 9 bar of extraction pressure is 16 times greater than the pressure of a 15 kg tamp. Tamping pressure is immediately alleviated during the period when the dry coffee is wetted and once the extraction is in full swing will always be much less than pump pressure.
So when someone says tamp harder to slow down an extraction, I call bs! :

Discuss, debate, theorise..... 8-)




Water delivery at the beginning of the extraction is not applied evenly, and water is a solvent. Depending on how the pressure is applied (slow infusion as from an E-61 or rudely as in a Silvia) can do all sorts of things to the coffee that make this comparison not very accurate at all.. IMO.
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