I thought this might be of interest. It is a standard 2015 manufacture Robur with nothing altered or any mods of any sort other than a aluminium chute guide.

Everyone knows the bigger the burrs, roughly more retention results often up to about 6 grams.

The Robur has 71mm conical burrs and is driven by a 900 Watt motor and weighs 28kg so not your average home espresso machine although a lot are in homes like mine.

I cleaned it by brushing then blowing with a compressor and recovered 5.7 grams which is about what others have found.

Importantly this is AFTER TWO WEEKS of normal use. I labour this point as most measurements are taken after one grind on a clean grinder.

I then put through three weighed amounts of 18, 18.1, 18.1 grams.

I then recovered 13.7 giving 4.3grams retained for the first grind.

I then recovered 19.2 grams the second grind so some of the previous shot carried over, +1.1 grams.

The third grind I recovered 15.8 grams or -2.3 grams was retained.

The total retention in the grinder over three shots is 5.5 grams almost matching what was recovered after two weeks by cleaning the grinder. Close.

This means grinders quickly pick up coffee and the more places it can hide, the more is gathered.

It also means any carry over is generally only a few grams.

I find I can't notice variations through the day or the first shot of the day. I can't pick up any difference in either pour or taste.

A smaller grinder such as a Mini or similar will retain about 3 grams so 5.7 grams for a massive grinder like the Robur is not too bad and is in line with what other people have found. The entire retention is distributed and so a portion may flow into the next shot but often it just hangs somewhere and stays there until cleaned.