Coffee Data Science
WDT in Basket vs Dosing Cup for Espresso
Adding graphs to raw data
Recently, a channeled called Tech Dregs made a video on espresso puck preparation, particularly WDT. Weiss Distribution Technique (WDT) has been around for a few years, with the aim to improve even espresso extraction. Many tools have been developed to make WDT a better user experience as well as more repeatable.
They collected 90 data points, but the video was presented with a lot of raw data and not many graphical representations of the data. So I took their data and produced some graphs so it is easier to see their conclusion.
The experiment compared no puck preparation with the WDT. The author compared doing WDT in the dosing cup and in the basket to understand these two questions:
- Does WDT improve shot performance?
- Does WDT improve shot performance because it breaks up clumps or because of how it distributes the coffee in the basket?
#2 is done using WDT in the dosing cup to isolate the clump breaking.
The second question is very important in recent times because grinders are starting to have declumping mechanisms using electrical charge. This could potentially make doing WDT obsolete.
Performance Metrics
These metrics didn’t include a taste metric, only measurable metrics.
Total Dissolved Solids (TDS) is measured using a refractometer, and this number combined with the output weight of the shot and the input weight of the coffee is used to determine the percentage of coffee extracted into the cup, called Extraction Yield (EY).
Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.
Data
The raw data came in a table, and the author of the video guided people through that data and the flow profile data.
The author looked from high level statistics on the distribution of sample points. I like to compared paired data because it makes for a stronger t-test. The author did take back to back shots while changing variables.
The data is broken down to 9 sessions with a few shots per each. So I sorted the data by IR within each session and puck preparation. This allows comparing the best shot for the three metrics.
Results
I presented this data in a scatter plot where the y=x is drawn to show where the performance is equal. TDS and EY show WDT is advantageous.
This is the same story in IR, and this metric covers the variance in output yield.
I also sorted the data overall by IR to compare the best with the best. The story here is very clear.
This data shows a very close trend no matter where you do WDT, cup or basket.
In the Basket vs In the Cup
Let’s focus just on WDT in the basket vs the dosing cup.
This suggests where you do WDT does not make a difference.
Statistical Level
The original author looked at a few statistical intervals on the distribution. I looked just at the difference of the paired data based on IR sorting within session. I then did a two-tailed paired t-test on the pairs showing the p-values in the table; below 0.05 means the difference between the two distributions is statistically significant.
There is not a difference between in the dosing cup and in the basket which means WDT is most helpful for breaking clumps.
Output, temperature, and shot time did not see a significant difference. Shot time means water contact time, so the effect of WDT is not simply a function of flow increase EY.
This data on WDT showed WDT statistically improved espresso extraction. We can draw a few conclusions and answer the questions asked:
- WDT does improve shot performance.
- The main effect on the preparation of WDT is declumping.
- WDT does not affect shot time which suggests using WDT doesn’t change the water contact time.
If your grinder has a declumper, you probably don’t need to spend the time or money on WDT.
If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.
