A while back during my big conicals researching ran across someone posting that the M7D retained 14-20g single dosing. Now I thought that seemed strange, retaining virtually 100% of the dose? I honestly gave the statement little credence.

Just got our 3rd M7D in today and while breaking in the burrs decided to test single dose retention for myself since considering it for my home too now. 5 tests of 20.0g in yielded 17.2-17.6 out, between 2.4-2.8g retention for me. I vacuumed burrs and snout between single doses.

This also confirms the ~1/4 double shot time I've been using for the single dose to clear path when it's been idle at the coffeehouse(s) is more than enough.

All espresso grinders with the conventional Italian design have a lot of powder compacted in the chamber below the burrs. The weight can run from 7 to 20 grams, and the large estimates are based on this. Most of this material stays put, and doesn't end up in the basket. However, nobody knows for sure how much of it is exchanged with fresh grinds each time one grinds; is it just a few grains, or is it enough to make a taste difference? A lab could tag the mass with dyes or radioactive isotopes; but that's not a good option if you want to use the grinder again*.

If you exclude this mass, you get a retention of a few grams with most commercial grinders, conical or flat.

• For sale: Macap 7D, almost new, with traces of florescent dyes and radioactive isotopes. Best offer

Even more ludicrous retention numbers were posted for the Robur. I did my best to dispel them in this thread, but dang! what an uphill battle that was.

If you sweep the chute clean, and purge a couple of grams at the start of a session, any of the big conicals should be good to go.

That thread was epic.

Yeah my hat goes off to you for that one!

I couldn't believe how some people just jumped all over you and would try at every turn to disprove what you had obviously proven hand down.

Rob

TomC wrote:That thread was epic.

Yeah, I think that was the one that quickly mecame a battle over what "retention" means then devolved further into a war over definitions of "clean," "empty," and finally "on" versus "off."

All that thread taught me is that people have such divided opinions about what counts as retained coffee and whether their personal grinding protocol sets the standard that any further discussion is almost certainly pointless. The more work you are willing to put into brushing the accessible grind path after every shot, and the less you care about how much partially pre-ground coffee is in every shot, the lower the "retention" of your grinder. Almost all large commercial grinders will perform similarly given the same testing protocol, and claims about huge retention numbers are silly.

The test described in the OP only measures a meaningless single dosing "retention", which doesn't actually tell you anything about cafe use with a hopper or single dosing (for which all that matters is whether you can easily remove that 2.4-2.8 g with a brush). Since you are running a cafe and have access to a set of identical grinders, it would be easy to test whether there is any significant difference in shot flow time or taste when purging 10 g, 5 g, or even 0 g after your typical maximum idle time. Knowing that is probably a lot more useful than figuring out how much is technically "retained" by any arbitrary measuring method.

At most there is a 2-3sec longer flow time first shot of the morning if not purging a few grams first.

I did not intend to re-open a can or worms merely reporting what I observed and weighed.