LMSC Iāve done 3 sets of 10 readings (V60 paper filtered drip, no syringe filtering of samples) with the VST LAB II and the DiFluid R2, Both zeroed at the same time & readings taken one device after the other in equivalent timeframe. I followed the VST protocol for the VST and the DiFluid protocol from their videos (the manual is not specific in this regard).
Test 1:
DiFluid average TDS after 10 reads 1.41, stdev. 0.007 (0.016 at 95% confidence).
VST Lab II average TDS after 10 reads 1.40, stdev. 0.006 (0.013 at 95% confidence).
Test 2:
DiFluid average TDS after 10 reads 1.42, stdev. 0.007 (0.016 at 95% confidence).
VST Lab II average TDS after 10 reads 1.38, stdev. 0.004 (0.010 at 95% confidence).
So far, so good, the difference in span between readings didnāt exceed 0.06%TDS, as both devices claim +/-0.03% accuracy, this seems fineā¦
Test 3 and the wheels seem to come off a littleā¦
DiFluid average TDS after 10 reads 1.48, stdev. 0.008 (0.018 at 95% confidence).
VST Lab II average TDS after 10 reads 1.40, stdev. 0.007 (0.016 at 95% confidence).
So the variation in readings between devices is not a constant interval, As both are quoted as accurate to +/-0.03%TDS accuracy something seems adrift here as the difference in readings differed by up to 0.09%TDS (I, and no influencers that I am aware of, have the facility to check refractometer accuracy.) , However, the precision of the readings for both seem acceptable.
I then wondered whether the difference in protocol was causing the differences in the averages, so I repeated Test 3, using the VST protocol for both devices. So rather than plonking the hot sample onto the DiFluid lens and taking readings, I cooled the sample in an espresso cup then placed it on the lens.
Test #3 repeated with VST protocol for both devices:
DiFluid average TDS after 10 reads 1.44, stdev. 0.031 (0.069 at 95% confidence).
VST Lab II average TDS after 10 reads 1.42, stdev. 0.005 (0.012 at 95% confidence).
The DiFluid readings started at 1.50%TDS, dropping to 1.41, the VST only drifted between 1.41 to 1.42. But the DiFluid settled to 1.41. 1.41, 1.41 for readings 8, 9 & 10 (vs 1.41, 1.41 & 1.41 for the Lab II).
So I feel I have to start again, it seems logical that the plastic casing of the DiFluid may not be allowing samples to reach a steady state as quickly, compared to the steel peltier dish on the VST.
The DiFluid spoon (volume 0.65g) is a bit daft/messy. Printed instructions are scant, YT videos donāt offer any further info on taking readings,
Iām a bit disappointed that Iām having to explore a previously unmentioned test protocol seeng that this has been in the field for some months, but Iāll see what occurs over the next few days.
Data is here:
VST Lab II vs DiFuid R2
