I recently acquired a Viponel S15 Darkroom timer to attach to my grinder as a non-permanant timer mod. My reasoning for this may be explained at a different point.
There has been a serious take up on the timer modded grinders both in competition use and in shops looking for consistency alongside freshness.
I became curious as to how dependable the timer mod is in terms of ground coffee ending up in the portafilter. In order to test I measured the grind over 50 timed grinds of 2.5secs each weighed to an accuracy of 1/100th of a gram. I used a single origin coffee of a relatively dark roast that I use for seasoning and such with an espresso grind setting and the coffee loaded into the hopper in one large portion. The use of a single origin is to negate any possibility of having a variance in the load upon the motor from different density varietals.[1]
The grinder used is a Wega 2.6k which is a rebranded Compak K6.[2] 
The results were an average grind of 6.83g of coffee reaching the portafilter. Exactly half (25) of the doses were within 1/4 gram of the average and 37 of the doses (74%) within 1/2 gram of the average.
An interesting pattern of weight distribution also occurs. Almost uniformly a portion is above average and then below with few outliers from the pattern.
I would suspect that the main variable that is affecting the result is clumping. I noticed during the experiment that at times there were more grounds being held at the chute rather than going into the dosing chamber.
I suspect that using a courser ground there would be a lot more consistency within the dosing and perhaps using a lighter roast of coffee may have a different result being that there would be less surface oils to adhere the coffee particles.
In the end a relatively interesting set of results. I would be interested to conduct further experiments to see what differences may be found within different fineness settings, roasts and varietals. Though a full range of doses extended from 5.90 grammes to 8.71 (an extreme outlier,the next largest dose being 8.10) I would suspect the consistency is much higher that of a full, calibrated dosing chamber due to weight of the coffee above it etc. in addition to the fact that coffee in a dosing chamber is sitting going stale. In my opinion timer-dosed coffee, even with a margin of inconsistency is undoubtedly preferable in terms of the resulting cup.
If anyone has any insight into the results or would like to see my (slightly messy) spreadsheet for the verbose results then e-mail/twitter me and I’ll forward them on.
1. It should be noted that the roast looked slightly inconsistent so that may have affected the ground dose by adding load to the grinder, even if only momentarily.
2. Another point of note with the grinder is that the exit chute from the grinding chamber holds approximately 4-5g of coffee before it reaches the chamber, so there’s an amount of room for compression and clumping within the chute. Also the dosing chamber in this grinder has a few points where coffee can remain.
Awlred 