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As I shop for a new espresso tamper, I see that there are a dizzying array of options for the shape of the base (i.e., the shape of the part that contacts the grounds). Some shapes include convex ("Euro" more curved than "American"), flat, rippled, or hybrids of those -- see some examples at Reg Barber. (I'm also aware that there are many shapes and sizes of the handle, and differing sizes for the base; I'm not asking about those.)

In part spurred by this recent question on the essentials of tamping, and other questions tagged (which are mostly about pressure), I got to thinking that the shape of the tamper could have a significant impact on the result.

There also seems to be a fairly large amount of personal preference or opinion in this realm. I can't find any proper comparisons or trials; at best I can find conversations about various preferences (e.g., home-barista including this poll, coffeegeek, coffee forums, barista exchange). A few patterns emerge:

  • curved bottom provides a greater margin of error for avoiding the "donut" effect of water channeling around the perimeter of the portafilter;
  • curved bottom could over-compact the center;
  • ripple shape is pretty, is used by some pros including winner of a competition in 2009, but has no demonstrable benefit, and is marginally harder to clean.

I hope I'm not spurring a sectarian flame-war by asking this; so I try to ask as objectively as possible:
Is there any objective benefit for one tamper base shape over another?

For example, side-by-side trials; experienced use beyond preferences; ...

3

If you only use a double (14g) portafilter basket then you're best to use a 58mm convex tamper. This is because if you look at the basket, in most cases, the sides will slope in. Therefore, if you had a flat 58mm tamper then the you are compressing to outer more than the inner. Obviously the whole point of tamping is to compress the grounds consistently throughout the whole puck, so this would achieve this the best. If you want to use a flat tamper for any reason in this situation, you would be best using a 57mm flat tamper and tap the edge of the portafilter to knock any loose grounds back down into the basket and then rotate the tamper over to produce a flat surface for the water to pass through.

If, however, you are using a single (7g) portafilter basket then the 58mm tamper would probably be too wide to compress the grounds evenly as it may not go far enough down in the portafilter basket to tamp it at all. Therefore, you would be best using a 57mm tamper.

If you're using a triple basket then a 58mm convex rules would apply the same as the double basket.

This is all based on the assumption that the basket is 58mm. This is the most common size but other sizes do crop up, commonly on older machines which may use a 53mm basket. In this case the flat/convex rules still apply, just with the smaller diameter.

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I would never use anything but a flat tamper. It seems silly to think that adding a convex would somehow help even out your extraction when water takes the path of least resistance. Tamp light, even, and as simple as possible.

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