A Closer Look at Tamping

For some time, I’ve assumed that the specific force you tamp with has little effect, as long as you tamp hard enough that the puck stops compressing. However, the results of a recent experiment led me to believe this might not be the case.

To check this, I purchased a Normcore spring loaded tamper, then devised an experiment to measure the effect of tamping force on the density of the tamped puck. To save coffee, I decided to tamp 20 g of coffee first with the lightest spring, then with progressively heavier springs, measuring puck depth in between tamps.

When I was done with the heaviest spring, I thought I’d check if the puck continued to compress with additional tamps using the same spring. Much to my surprise, it did.

So I devised a new experiment. In this experiment, I use 20 g of donation beans from Level Ground, and a grind setting of 2.0 on my Eureka Mignon Specialita, which corresponds to about 100 μm burr spacing. The dose was chosen so that the Normcore tamper would be in the middle of its calibrated range when pushed all the way down.

For each trial, I tamped 10 times with the tamper, holding it down for about 1 second each time. After each tamp, I measured the depth of the puck. I repeated the test three times with each spring installed. The results are summarized in the following plot:

To me, it looks like the first trial—the blue curve at 30 lb tamping force—is a little denser than it should be. This could be because I had used a grind setting of 1.65 for my morning coffee. Before this experiment, I flushed the grinder with 20 g of beans, and blew it out with bellows, but it could be that some of the grounds remained in the grinder.

Otherwise, there are a few things I see here:

• Density varies by about 0.005 g cm^-3 between trials, with the same tamping force and number of tamps.
• Density increases with additional tamps.
• Density is more consistent within each trial when using a weaker spring compared with a stronger spring.
• Density is about 0.03 g cm^-3 higher when using the strongest spring than when using the weakest spring.

I realized in the course of these experiments that soil engineers might have a thing or two to say about this. As it turns out, what we’re doing here is essentially a soil consolidation test.

Engineers model soil consolidation using a damped spring analogy, which gives an exponential return to equilibrium in the over-damped case. This means we should expect our depth measurements to approach their final value exponentially with respect to time under load, and this is exactly what we see in the experimental results above.

I’ve already built a homemade oedometer to make a more rigorous measurement of consolidation in ground coffee.

One thing we haven’t touched on here is what effect, if any, this has on the taste of the espresso. This is something I would like to come back to in a future experiment.

Update: In a recent post on Instagram, Samo Smrke outlined the results of an experiment measuring the effect of tamping force on espresso extraction dynamics and yield. For tamping forces greater than 10 kgf (22 lbf) tamping force, the researchers were unable to find any measurable difference in the extraction.