Three more tests now added, same protocol for both devices.
Stir coffee sample and place a teaspoonful into an espresso cup/shot glass to cool. For all devices this sample will need syringe filtering if espresso, French press, cupping, Aeropress, or pour over made with very fine grinds (e.g. any brews with significant suspended solids, or for absolute best results for all methods).
Calibrate with distilled water left on the lens for 1;00. Clean lens with lint free cloth/science wipe.
Add sample to refractometer (3 drops to VST, 6 drops to DiFluid R2), wait 1:00. Now would be a good time to start tasting your brew, if cool enough,
Start readings, I took 10 from each.
The averaged difference in TDS between the devices was +0.03%TDS higher with the DiFluid. Which is reasonable as both claim +/-0.03%TDS accuracy..
The stdevs in readings were:
DiFluid R2: 0.007, 0.010, 0.005 - 0.007 averaged and 0.016%TDS extrapolated to 95% confidence level.
VST Lab II: 0.007, 0.070, 0.070 - 0.007 averaged and 0.016%TDS extrapolated to 95% confidence level.
So, as far as this test can tell they are equivalent with this protocol. In terms of workflow, the VST Lab is quicker to stabilise and reached the average reading in 3-5 reads (I wouldn’t be too concerned about whether the reading recorded was 0.01%TDS out). The Lab II I used has been replaced by VST for the more accurate & precise Lab III.
This makes the DiFluid R2 quite a step up from the Atago & Amtast offerings in terms of performance. The workflow isn’t as smooth as the VST Lab series but I guess at the price difference, that’s a fair trade off. The Atago workflow is much slower due to the rolling display.