So as an addendum to the previous post regarding spent puck temperature variances and their potential causes, another dimension comes to mind as a possible culprit for temp variance–the way grounds settle or are settled in the portafilter basket. Every grinder will do this differently, and to the naked eye a pile of grounds looks like a pile of grounds. But on a microscopic level there have to be huge differences in the way grinders debit coffee grounds into the basket. For instance, a doserless model will lay grounds into the basket in a more or less even manner, all at once and, generally (at least in the higher end models) into a nice centered cone. A dosered grinder is at the mercy of several steps that could cause variance.
First in these steps is what is often a two stage dosing process. If the grinder has no timer, the barista will turn on the grinder for a set amount of time and then physically stop it. Not all the coffee has been dosed into the basket at this time and the rate/volume of dosing is different than once the grinder has been turned off and the barista given a chance to finish dosing. A timered, dosered grinder will eliminate that middle pause, but will still likely show variance shot to shot because it grinds and doses inconsistenly into the dosing chamber and then on into the portafilter basket. Tamping, no matter how consistently we think we may be doing it, should therefore, necessarily mean some uneven pockets of tamped coffee: areas that are prime targets for channeling, sinkholes and the like.
So just another quick thought around temp variance as it pertains to uneven pucks.
Can you think of any other variables that would/could cause temp variance?