We've previously seen a transparent portafilter that revealed some enlightening properties of puck expansion and I'd been wondering what we might learn from a transparent basket... now I didn't machine one but I encourage anyone who can... I instead found some containers approaching the size of my caravel tamper and looked at what happened.
To me, the most interesting findings so far is the superficial nature of the grooming tool (idea from here and a local cafe owner/barista consultant) and the springly nature of coffee grounds - perhaps a reason to think that thought experiments on the "absolute" compression of tamping need to consider more factors.

Best shot of grooming tool:

Springy coffee:
After question about air excaping used dismantled milk frother with holes in both ends:

This effect is not just with light roasts!

While I like that your video gives evidence that top of the puck grooming tools are rather useless, the rest of your videos mainly demonstrate that compressed air re-expands, not that coffee has any significant degree of "springiness". I imagine if you had a container the same height as a standard basket you'd see it easily, but such a test isn't really necessary since you can do the same test with a regular basket you already have. If the coffee is springy, you'd see it re-expand right after removing the tamper. Which is something I've never seen occur.

TomC wrote:compressed air re-expands

Hi Tom, thanks for your response!
If compressed air is not involved in a regular basket can you help me understand why the air is compressed in the example with the milk frother - both ends have holes? I also note that a gap is present between the tamper and rearward wall of the containers - it's not a tight fit.
The other factor that might explain, if this is a real phenomena, that it might not be seen before is that the expansion happens quicker and more subtly than I can see it from the top when removing the tamper, I only saw it from the side when doing the video of the grooming tool and investigated further.

aecletec wrote:Hi Tom, thanks for your response!
If compressed air is not involved in a regular basket can you help me understand why the air is compressed in the example with the milk frother - both ends have holes?

You have a column that appears to be several inches high ( at least 7-8 inches), which would greatly amplify any minute phenomena, which is why I suggest you repeat your experiment with something the same height as a basket. That plus having only one hole would probably alter your findings. Even your very tall column in one of your videos doesn't re-expand much when it was done in a cylinder that has a hole to let the air out.

Have you ever seen a tamped basket expand?

As I mentioned before - no, I don't even see this phenomena when doing the test, only when viewed from the side. It's worth noting that it happens fast enough to occur before a tamper is slowly removed so I wouldn't expect to see it either.

The idea is for a test of concept so magnifying the effect to be caught on my woeful camera setup was the purpose!
If dose is the sole reason for dismissing the idea when both ends have gaps for air to escape I'll re-do it or see what else I can capture to confirm or refute and see what happens

You can see the effect also happens with about 10g of coffee in the darker grounds (again the tamper is not contacting all walls of the container).
I don't really have the capability to test much more rigorously, but the thought is that any expansion effect, even a very small one between grounds would minimise the impact of harder tamping.

As an alternative method to test the same hypothesis using all your standard equipment:
1. Measure the depth of your tamper and basket.
2. Fill a normal basket with coffee grounds.
3. Tamp as usual.
4. With the tamper still held at the compressed position, measure the thickness of the tamper protruding above the basket. Subtract this measurement from your total thickness in step one to get your compressed puck measurement.
5. Remove the tamper and measure the distance from the top of the uncompressed puck to the top of the basket. Subtract this from your basket measurement to get your uncompressed puck measurement.
6. If the grinds are springy then your compressed and uncompressed puck measurements should be different.

I guess I don't understand the issue.

Isn't sawdust springly? What about sugar powder?

Why would coffee be different?

Guess it is no different but it sure is not the same either so the question IMO is, does it affect anything at the scale of a filter basket and if so what and why and what can we do with that info.

I for one have never seen the puck come back up in a notable amount after tamping, not even while waiting a bit longer with a fresh roast. What I saw seems like there was air trapped inside the grounds that were partially tamped and thus restricting airflow which appears as springiness in the grounds where it is a system consisting of coffee and air.

mrjag wrote:As an alternative method to test the same hypothesis using all your standard equipment:
1. Measure the depth of your tamper and basket.
2. Fill a normal basket with coffee grounds.
3. Tamp as usual.
4. With the tamper still held at the compressed position, measure the thickness of the tamper protruding above the basket. Subtract this measurement from your total thickness in step one to get your compressed puck measurement.
5. Remove the tamper and measure the distance from the top of the uncompressed puck to the top of the basket. Subtract this from your basket measurement to get your uncompressed puck measurement.
6. If the grinds are springy then your compressed and uncompressed puck measurements should be different.

Excellent idea!
I don't have precise measuring equipment and wanted to pass on something I'd seen - this is a great process thanks