Coffee Data Science
DiFluid vs Atago for Total Dissolved Solids (TDS) in Espresso
Qualifying refractometers
The digital refractometer market has grown a bit in the past few years. At first, there was only VST. When the Atago came on the market, some experimental results suggested that the two produced similar results that did not differ statistically. Now, a cheaper, smaller DiFluid refractometer is available so let’s collect data and take a look.
A refractometer, in coffee, is used to measure the refractive index which correlates to the Total Dissolved Solids (TDS), and this can be used to calculate the amount of coffee extracted. A refractometer measures refractive index, and with data, companies have made equations using refractive index and temperature to measure TDS. Otherwise, one would have to dry spent coffee pucks to determine how much coffee was extracted.
In full disclosure, DiFluid sent me this refractometer, and they haven’t had any communication with me since. They are assuming that I’m doing an independent analysis, and we don’t have a business relationship. My review concerns using the meter for espresso readings with little comment on user experience. The meter reads samples faster than the Atago, which is advantageous at times, but it requires that you cool a sample.
I only compared to the Atago, of which I have previously done multiple characterizations:
Measuring Refractometer Accuracy to Groundtruth for Coffee
Inside and Out Coffee Refractometer Measurements
Refractometer Accuracy for Coffee and Espresso
Affordable Coffee Solubility Tools (TDS) for Espresso: Brix vs Atago
Total Dissolved Solids Measurement in Coffee is Slightly Affected by Sample Temperature
Filter Syringes for TDS Coffee Measurements seem to be Unnecessary
I used this meter side by side with my Atago over the course of a few weeks to capture readings. I put a sample in each, and I didn’t pull a reading from the DiFluid until the Atago registered a reading. This allowed the sample to cool in the same manner as in the Atago. The Atago usually took a minute or so.
I have previously found a slight, statistical shift in TDS when cooling samples, but I have continued to use hot samples to stay consistent in my measurement technique. I also did not filter the samples as I have found that to be unnecessary.
Data
Let’s start with hot samples. I took 45 samples, and of those 45, 25 samples had an additional sample after I cooled the same sample in cool water (20C) using the top of the pipette.
There is a strong trend as the TDS for the Atago increases that the DiFluid TDS increases at a higher rate.
In comparing these to the cool samples, there is a different trend.
Looking at cooled vs hot samples, both meters follow a similar trend, but the offset for DiFluid is larger meaning that it is more affected by temperature when measuring the TDS than the Atago.
When comparing the cooled samples, the DiFluid and the Atago perform almost the same. In a two-tailed t-test, the p-value is 0.46, which means the difference in their distributions is not statistically significant (p>0.05).
DiFluid also displays the refractive index (nD), so I plotted nD to show how it changes with hot and cooled samples. The trend is very linear, and this suggests that the difference in TDS measurement for the two devices is due to the equation DiFluid uses, not an inherent difference in hardware.
Single Sample over Time
I looked at a single sample over time for the DiFluid. Once the temperature stabilized, the refractive index did as well.
Overall, I really enjoyed the DiFluid, and for espresso, it proved to be a good companion. For someone looking to not break the bank, I think it is a good option. More data should be produced for lower TDS coffee, but there seems to be a lack of data for refractometers in general. I am hopeful that more data will be shared so as a community, we can better understand how refractometers function with respect to coffee.
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