SHOT MAKING CHARACTERISTICS OF THE BASKETS
It is impossible to gather data for these baskets on all machines, all coffees, and all grinders. This study looked at each basket using one grinder, one coffee and one machine. The guiding assumption is that the data gathered from this will indicate how the baskets perform in general.
The procedure was to make six shots with each basket, for a total of 84 shots, using the same grind, coffee, and machine. Each shot was timed to run for 25 seconds and the resulting shot was weighed. The average dose weight for each basket was designed to produce a shot that weighed about 1.5 times the dose (brew ratio of 0.67). The range of doses was roughly plus or minus 0.5 to 1 gram from the middle dose, depending on the sensitivity of the basket to dose changes. The idea was to get a range of shots that on the ristretto end weighed as much as the dose (brew ratio of 1.0) and at the lungo end, weighed about twice as much as the dose (brew ratio of 0.5). In some cases, with baskets that responded more strongly to dose changes, this range was exceeded.
This procedure generates a good deal of variation and noise. The flow is greatest at the end of the shot, so small changes in dwell time create fairly large changes in shot weight. All the shots in this data set looked OK, and would have passed muster on an automatic machine or if the barista ended shots when they paled. However, these much more reasonable procedures, used whenever shots are made for consumption, would not have yielded the directly comparable data required for this study.
There are alternative ways for gathering the same data. Instead of varying dose, one could vary grind settings. The reason this study went with dose is that it is easier to accurately record a weight than a grind setting. Another alternative is to make each shot to the same weight and record the time it took. This requires more elaborate synchronization, and either a fast responding, accurate scale, or a highly accurate flow meter. It is therefore a more expensive and error prone way of collecting these data.
The raw data is illustrated in the graph below. The squiggly lines show each of the fourteen baskets, the colors and numbers show the nine basket types. The legend is given in the graph.
The more the lines squiggle, the less regular the response of shot weight to dose weight. It should be noted that all these shots were visually OK, and that using fixed shot times strongly accentuates small differences in flow.
The data suggests that a separate straight line shout be drawn either for each individual basket or for each kind of basket. A line for each kind of basket explains about 78% of the variation, while one for each individual basket explains about 82%. The difference is too small to be statistically significant or to matter in everyday shot making.
The model with a straight line for each kind of basket is illustrated below.
The shallower slope (wider dosing range) of the LM baskets, and the steeper slope (narrower dosing range) of the VST baskets are massively significant. The models' accuracy drops precipitously if these effects are not included.
VST claims that its baskets are more consistent in commercial operation than regular baskets. The study does not support this claim. Random shot to shot variation is large enough that the differences in conventional baskets are only a minor exacerbating factor. Moreover, the narrower dosing range of the VST baskets means that it amplifies small random errors in grind and dose. In this study, this added error proneness offset any gain from tighter manufacturing tolerances.
The VST-18 and equivalent Strada basket have become popular among third wave professionals and serious amateurs. This section and the taste section suggest that more important for this acceptance than any consistency gain is that the 18 gram VST basket fills a niche in dose-to-grind relations, roughly a 2.5 shot, that was previously unoccupied. People wanting to make 18 gram shots would have to under-dose a triple or over-dose a double, with correspondingly extra fine or extra coarse grind settings. The relevance of avoiding this to making good shots will be discussed in the taste section.