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  • Writer's pictureNasko Panov

Testing Manual Grinders – Flair Royal Part 2 Introduction

Different effects influencing Particle Size Distribution in manual grinders

Let’s continue exploring the science behind coffee grinding. This is my third article on this topic and you can find my previous ones on my Patreon page on, and/or following the links HERE and HERE. The first article is showing the effect of coffee addition speed on the electrical grinder performance and the distribution of the particles of ground coffee. This is in strong relation with the coffee extraction efficiency and coffee taste. In the second article, I transferred the findings in my first experiment on manual grinders. The main idea of these experiments is to find dependencies, effects, and correlations that are influencing the consistency and performance of the grinders. Such knowledge will lead the coffee world to a better understanding of the multistep process of preparation for our beloved drink.

I started this test with Flair Royal manual grinder and will continue with this grinder for the current experiment. Once again, I have to say a big thank you to Flair Espresso for supporting me in this project. The main goals of this experiment are the following:

1. How is the particle size distribution influenced by the coffee addition speed in terms of Skewness and Kurtosis?

2. Does the speed of rotation influence the PSD, grinding performance, and consistency?

It is really hard to explain what PSD is in short. It consists of so many values, calculations, and variables, thus, it is hard to even describe it to someone with a proper scientific background. Some of the main terms used to describe PSD were mentioned in my previous article, so it will be advisable to go back and read them carefully. One of the most information-providing values from the distribution is the Span and COV – a representation of the distribution width. I like to use COV because it is normalized, meaning that can be used for distributions with different Mean values – often seen in coffee grinding. The coefficient of variation (COV) is defined as the ratio of the standard deviation to the mean. Sometimes the ratio is multiplied by 100 to report the coefficient as a percent. In any case, the value is always dimensionless.

Once again, here are the results for COV and Span on Flair Royal manual grinder. The blue bar is the distribution width achieved with a standard grinding; the orange is when the coffee is added on small portions. The three graphs are for Fine (Pic. 1), Medium (Pic. 2), and Coarse (Pic. 3) grind settings.

Pic 1: COV and Span result for Flair Royal manual grinder at fine grind settings.

Pic 2: COV and Span result for Flair Royal manual grinder at medium grind settings.

Pic 3: COV and Span result for Flair Royal manual grinder at coarse grind settings.

As seen from the particle size distributions width represented as COV and Span, the coffee addition speed has small to no effect on the distribution width when using the Flair Royal manual grinder. This is in contrast when comparing the same experiment conducted with electrical grinders. In electrical grinders, the influence from the coffee addition speed is much greater. The distribution width is in close relation with the grinder consistency and can show how well the grinder is performing in different conditions.

However, the width is only one element from the complex representation of the particle size distribution. There are two other variables I want to investigate which have a connection with the grinder performance. These are Skewness and Kurtosis coefficients.

You can read the full article on my Patreon page HERE, visit or click the red button below. At the price of a single espresso per month, you can read all my experiments, work-in-progress projects, future ideas, and allow me to continue my work. Thank you!

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