Socratic Coffee just published an interesting direct comparison of results obtained with the Atago and VST refractometers. They also examined the effects of zeroing with distilled vs. brew water as well as filtering samples with a syringe filter.

Measuring Total Dissolved Solids: A Refractometer Comparison

Overall, it appears the VST and Atago refractometers are very comparable for measuring the TDS of filter brewed coffee. While the second study examining the impact of the sample filter demonstrated a large amount of variability in the VST readings compared to the Atago, this was not seen in the first study (distilled/brew water; VST standard deviation 0.04; Atago 0.03).

Edit 5/23: Part 2 is now available.

Measuring Total Dissolved Solids: A Refractometer Comparison (Part II)

Overall, the VST III and Atago devices performed comparably, both demonstrating significantly less variability compared to the VST II. While a statistically significant difference was seen in the third brew between the VST III and Atago, the average difference in TDS measurement for that brew was 0.007. We leave it up to the reader to determine whether or not a statistically significant difference is meaningful in his/her application of refractometry (and whether or not it falls within the devices' stated measurement capabilities and tolerances).

Very methodical. Do note that the article compares the Atago vs. the Reichert based VST series II (and not the current model).

I was under the impression (based on a chewing out I got at Coffeegeek) that the new model of VST made by Misco has improved accuracy on the low end of the TDS scale that should improve the results for filter coffee (which has a lower TDS, and thus the Yield calculation is far more sensitive to absolute errors in TDS measurements). For filter coffees the new VST should indeed theoretically be better. They should be testing this soon according to their article.

I don't think it makes any difference for espresso which normally (I.e. Not in the Americano version of espresso) has a higher TDS and therefore far less sensitive Yield calculation. At least I am not convinced there is a difference (with the obvious exception that Atago are supposedly amateurs and VST are supposedly not).

Also note that in any case one would probably need the VST coffee tools application to calculate Yield. So the Atago would still need the VST app (albeit there is an Atago app out there on IOS).

So, the VST's performance isn't particularly impressive, and the Atago is better(!). It would be interesting to see performance vs. the newly-released Lab III. It'll also be interesting to see if the Lab II can be upgraded to the III via a firmware update - although I believe VST was charging several hundred dollars last time around.

AssafL wrote:Very methodical. Do note that the article compares the Atago one the Reichert based VST series II (and not the current model).

That's an oversight by the authors because of VST's somewhat confusing product revision history. The model they're comparing is the VST LAB II, which is a Misco Palm Abbe like all three revisions of the LAB refractometer. VST rates the accuracy in the low TDS range as +/- 0.03% vs. 0.05% for the LAB II. It's worth noting the first LAB model had a "warranted" accuracy of +/- 0.06% but a "typical" accuracy of +/- 0.03%, so I'm not sure how much emphasis to place on the exact numbers. The other significant changes between each revision were more to do with improving battery life, adding a combined scale, then eliminating separate coffee and espresso scales in favour of full auto-ranging.

I'm not sure I understand their methodology. Why are they testing the devices outside of their stated working temperature ranges and using those results to determine precision?

+1

I'm confused, too. I posted the following in their Reply section, but it hasn't appeared (maybe replies are moderated):

You say that the VST refractometer showed variability in the readings when used with a filter, but the variability decreased at lower temperatures. However, the temperature range in your graph is 63C to 30C. VST clearly states in its instruction manual that the sample must be cooled to 15C to 30C:

https://cdn.shopify.com/s/files/1/0092/ ... 96.pdf?265

(see Temperature Correction, page 13)

If I've read your chart and your sample protocol correctly, it would seem that the impact of the filter decreased as the temperature approached the top end of the range recommended by VST. If that's the case, it seems likely that the VST instrument would produce the expected results if it was used properly -- i.e., within the recommended temperature range.

Also, it appears that your protocol doesn't match the protocol recommended by VST. You allow the brew to cool for 1 minute in the carafe, but you don't specify whether the carafe is sealed to minimize evaporation (and it doesn't seem that the coffee would cool much in that short amount of time anyway.) Then you draw four samples with the syringe and place them in a ceramic cup. But you don't specify the size of the samples and the amount of time they are allowed to cool in the cup. Then you place individual samples in the refractometer wells and wait 20 seconds before taking a reading.

I would think that some evaporation of the second two samples occurs in the ceramic cup as the first two samples are drawn, placed in the refractometer wells and allowed to cool for 20 seconds. That will affect the TDS readings. Also, the temperatures of the two sets of samples will be different. Finally, VST recommends that the sample be allowed to cool for 30 seconds after being placed in the well, not 20 seconds. The time isn't so much the issue as ensuring that the sample is well-cooled before being placed in the well and that the temperature of the sample and the well have time to equalize within VST's recommended range.

For those who haven't read the VST instructions, the protocol goes something like this:

1. Brew coffee
2. Pour coffee into a small cup and allow to cool for 1-2 minutes
3. Draw 3ml-5ml of coffee with the syringe and place in a heavy-bottomed shot glass
4. Allow the coffee to cool for 30 seconds in the shot glass (swirling helps)
5. Use the pipette to transfer a few drops of coffee to the sample well (just enough to cover the glass -- a large sample will take longer to cool)
6. Allow the sample to cool for 30 seconds
7. Take the reading

The objective is to get the sample cooled to the 15C-30C operating range of the refractometer with minimal evaporation.

As for the filters, VST doesn't recommend using filters for brewed coffee. It should be harmless to do so, but again you have to observe the recommended temperature range of 15C-30C.

If I haven't misunderstood your charts and protocol, I believe you should run your tests again using the recommended VST protocol and temperature ranges. If the results differ, please correct your post.

Actually it was my oversight- not the authors. They specifically state the Misco.

BTW - an advantage of the Misco is that Misco should be able to reprogram the unit to whatever units you want for a service charge. So if one wants to replace the coffee TDS with Baume and Brix (replace the coffee shop with Pattisserie?) or sports Urine and Sugar (please no) it should be possible. That is assuming the VST model doesn't have this capability locked.

The Atago already comes with Brix range so QA for the sugar
Syrup or ice cream base is built in. But one cannot have it reprogrammed.

jpender wrote:I'm not sure I understand their methodology. Why are they testing the devices outside of their stated working temperature ranges and using those results to determine precision?

Good point.

What datum was used with respect to accuracy, dehydration, reference solution? Precision needs to be assessed in the context of accuracy.

What was the point in deliberately zero-ing with tap water, contravening standard practice?

I thought their findings were as to be expected from these mid range electro optic devices. If they weren't this close I'd be concerned.

As for variances in results: These devices (whomever makes them) measure a physical property that is extremely dependent on temperature. There is temperature compensation, but to stabilize the readings the solution temperature must stay relatively constant. If it keeps dropping, and the well temp as well, as will the thermistor for the temp compensation readings will be all over the place.

I have a dip in refractometer for Brix (Atago PenPal) which works great since it reads continuously even hot liquids. By dipping and stirring (e.g. Ice cream base, fruit pulps, etc) the prism and lens and liquid temperatures all stabilize and the temp compensation can do its job. The Atago shows ttt as the temperature changes fast, but as it slows down the measurement on the LCD appears and keeps changing as thermal equilibrium is achieved.

Coffee placed in a well needs to be at a stable temp as well, especially since there is not enough thermal mass to bring the refractometer temp up to equilibrium with the liquid.
Having a continuous read mode would have been a great help as one would wait until the readings stabilize. ( but those models tend to be more expensive than single read mode devices. )

Marketing wise, there are greater differences between these devices. VST markets the devices to help one make better coffee. Atago markets the devices for coffee QA (if it was good at a specific reading yesterday - try to get the same reading tomorrow).

AssafL wrote: Coffee placed in a well needs to be at a stable temp as well, especially since there is not enough thermal mass to bring the refractometer temp up to equilibrium with the liquid.

Isn't this the whole point of the zeroing at consistent room temp, cooling the sample & letting it reach equilibrium in the refractometer? All points that were seemingly ignore in the "detailed test".