So after a few weeks of single-dose grinding with the blow up hopper, I opened the machine up to clean it. To my surprise, I see a lot of the grinds inside the machine! Has anyone opened there's up?

Did you weigh em?

I think in all the eureka grinders you will have retention. Older Atom keeps about 3ish gms, specialita is around the same too. Its not a true zero retention unfortunately, all the reviews online and videos i am not sure how they claim its good for zero retention. I used to complain about the sette keeping grinds in its path, but seems like you are always going to have retention accumulated over time.

Grinds are going to build up no matter what. To my understanding, for single dosing you need to add ~2g of coffee to purge out old grinds, then add the dose. I'll have an Atom 75 with blow-up hopper shortly to test it out, but I plan on switching to a standard short hopper and just filling it rather than single dosing in the near future.

From my experience the blow up hopper has some flaws the way Eureka made it:

It doesn't sit flush with the grinder, so you have air escaping through the mount, not to mention the chute to close them off.
For this i used tape to better seal the hopper:



Another issue is that there is no valve on the lid. So what happens, if you use the hopper during grinding, you're actually sucking ground coffee back towards the hopper. Fine grinds will also clog the grinder because of this.

I added a one way valve from one of those rocket hand pumps used in cleaning camera lens.


After these two changes single dosing works much better (around 0.1-0.2 gram retention) and i can grind very fine if needed, also with zero clumping.
Popcorning is non existent as well.

In use i press the bellows multiple times while the grinder is running.