eMoJo Senior Member Joined: 19 Sep 2010 Posts: 5 Location: Boston Expertise: I love coffee
Posted Sun Mar 25, 2012, 2:14pm Subject: Re: VST LAB Coffee Refractometer
Vince, I hate to be nuisance here but (from above) did you mean 2.00 +/- .05 like from 1.95 to 2.05? Or did you really mean 2.0 +/- .05% ?
Hi Gary:
No, I didn’t mean “2.0 +/- .05%”, because 0.05% of 2.00(%) would imply a range of value of +/- 0.001%, which exceeds the resolution of the refractometer.
I did mean, as stated, 2.00 +/- 0.05% TDS, ie: 1.95-2.05 % TDS coffee, also equivalent to Mass Fraction, as in 2.00g/100g x 100 expressed in percent as 2.00.
The notation used; “2.00 +/- 0.05 % at 20.0 +/- 1.0 Deg C” does not specify the unit/descriptor (% and ºC) until both the target and range of tolerance is specified, implying (therefore), that the "descriptor" applies to both terms, similarly at the specified target and range for Temperature (i.e., ºC).
Finally, the range of TDS outlined does not include stack up or accumulated error. For example, if the scale (or a calibration weight used to verify accuracy of a scale) used to make up the sucrose solution has a stated accuracy of +/- 0.1g, then this could add approx +/- 0.04% error to the measured result. In addition, temperatures outside of the range I suggested would be additive to those two previous offsets. So, as you can see, one has to be careful about making statements of “measured accuracy”. A fairly significant investment in equipment accompanied with the requisite analytical protocol must be used to end up with an “accurate” result.
That said, if you’re careful, you should be able to get an idea whether your refractometer is in w/in a reasonable range of expected accuracy. The final alternative, if one has good reason to suspect there might be a problem with accuracy is to buy the Reference Solutions, which are specified at +/- 0.02% and +/- 0.03% for the Low and High REF points, respectively, and within a temperature range of +/- 2.0 Deg C of 20.0ºC.
Posted Mon Mar 26, 2012, 5:36am Subject: Re: coffee refractometer
Agreed - the stackup in error is a factor, but can be mitigated by using larger amounts.
For example, you can cut the error of the reference fluid by a significant margin (assuming using pioneer sugar is acceptable) by quadrupling the batch (30g sugar and 1174.8g distilled water).
Question for you, Vince: for range checks, I assume since 100% distilled water is 0.00% TDS, then can any part of the range be checked by simply cutting the solution concentration? (I'm asking another question - is the relation of sugar solution refraction from 2.0% on the VSTcr a linear relationship?)
The sugar solution you suggest is 2.49%. If I make the solution and get a TDS measurement, then cut it to 1.245% concentration, I should measure 1.00% TDS on the VST, right? And if I double or quadruple the solution concentration, will I be able to do a quick check of the range?
Or am I missing something?
(edit: corrected the solution concentration due to a math error)
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
I propose that any posters willing to write this much useful information - and able to bring so much knowledge to the table - receive a free pass to frame their contributions however their mood strikes them.
IenBell Senior Member Joined: 17 Feb 2012 Posts: 25 Location: Uk Expertise: I like coffee
Posted Mon Mar 26, 2012, 10:39pm Subject: Re: coffee refractometer
I have been requested about this by a few of my visitors also. I am going to see if I can't get my arms on one of these. If I do I will be satisfied to reveal my ideas with you.
For quite a while I too used the Aeropress filters to prepare espresso samples for refractometer measurement. But when I compared the results to syringe filter measurements, the AP filtered measurements read high by an inconsistent margin.
The price is right when AP filtering, but unfortunately the results can't be trusted. IMO.
Do you still use the Aeropress filters for non-espresso brewing methods? I use them to re-filter cloudy Aeropress coffee for testing, but it doesn't seem to make much difference. Using the same method for French press, I often get wandering readings, which I assume indicates solids settling on the lens. I've never used my refractometer for espresso, which is something I should probably change, especially considering I ordered it with espresso filters.
I've never really liked transferring the coffee to a cup to cool. It always seems like an added chance of evaporation or other factors like differing initial sample size messing up the reading. I can get very stable readings putting a few drops directly on the lens well, though I think they tend to be slightly different from pre-cooled coffee. Going by videos online, it would seem quite a few people use the VST refractometers this way. I often wonder whether a lot of users follow a strict enough consistent methodology to get repeatable and useful results, though.
I am interested to know how you are supposed to determine the water loss ratio when brewing coffee. The 2.0g/g figure is a reasonable average, but a range of 1.6-2.4g/g affects the TDS reading by +/- 0.5% TDS. Factors like whether you rinse the filter and squeeze it after brewing pourover, and extremes of grind setting like Turkish powder or coarse boulders certainly seem to affect the WLR. It mostly comes up for me when doing cold brew experiments. Last time I mentioned this on HB, I was kind of brushed off without a useful answer.
Do you still use the Aeropress filters for non-espresso brewing methods? I use them to re-filter cloudy Aeropress coffee for testing, but it doesn't seem to make much difference. Using the same method for French press, I often get wandering readings, which I assume indicates solids settling on the lens.
I believe you're correct, when the solids aren't filtered out they gradually settle and the readings keep changing. Also, there are some soluble compounds in the suspended solids that slowly come into solution. So if you let an unfiltered sample sit around, the TDS will increase slightly.
Bob_McBob Said:
I've never used my refractometer for espresso, which is something I should probably change, especially considering I ordered it with espresso filters.
I'm sort of the opposite, I've used it 99% of the time for espresso. I find it endlessly fascinating how grind, preinfusion, shot volume, baskets, different coffees, different grinders, etc, affect extraction yield and, of course, flavor. I also find it unfortunate that some of the "golden tongue" pundits reject the technology without giving it a fair trial. There are so many things that refractometry can clearly objectify: when a grinder is out of alignment, when burrs are defective, when a roaster screwed up a batch (even though the previous lot was fine), differences between dark and light roasts, differences between baskets, etc. Of course taste is paramount, but there are numerous instances where people posture and prattle with their subjective observations when a few refractometer readings could have helped organize their research and eliminated a lot of unnecessary effort. IMHO. :-)
Bob_McBob Said:
I've never really liked transferring the coffee to a cup to cool. It always seems like an added chance of evaporation or other factors like differing initial sample size messing up the reading. I can get very stable readings putting a few drops directly on the lens well, though I think they tend to be slightly different from pre-cooled coffee. Going by videos online, it would seem quite a few people use the VST refractometers this way. I often wonder whether a lot of users follow a strict enough consistent methodology to get repeatable and useful results, though.
I'm not the expert on this, but I believe you will get more accurate readings if the measurements are made with the sample and the prism at the same equilibrium temperature that you used for the refractometer calibration. IOW, zero the refract with distilled water at your current room temperature, cool the sample to room temperature, place it on the room temperature prism, wait a minute or so for the sample and prism to equilibrate, then read.
Bob_McBob Said:
I am interested to know how you are supposed to determine the water loss ratio when brewing coffee. The 2.0g/g figure is a reasonable average, but a range of 1.6-2.4g/g affects the TDS reading by +/- 0.5% TDS. Factors like whether you rinse the filter and squeeze it after brewing pourover, and extremes of grind setting like Turkish powder or coarse boulders certainly seem to affect the WLR. It mostly comes up for me when doing cold brew experiments. Last time I mentioned this on HB, I was kind of brushed off without a useful answer.
In my mind, the "retained water factor" is a convenience or a workaround for people who can't measure (or don't wish to measure) the final beverage mass. For best results it is safer to measure beverage mass, thereby eliminating the uncertainty that you mention above.
Still in process of refining some procedure adjustments.
The coffee absorption varies greatly, and is highly dependent on grind level and contact time, as well as the brewing method. For example, I can vary the absorption from 1.05 to 2.20 using an Aeropress. Short contact times and larger grind levels trend toward less absorption. The amount of coffee you extract is also a fairly significant effect - gravity filtering on a drip with a small sized grind can have absorption ratios approaching 3.
One thing that helps for cooling samples and getting decent consistency with Aeropress brews (Vince is right, the Aeropress samples need to be treated like a metal filtered or coarse filtered coffee) is a test tube. Take a decent 8-10 ml sample and put it in a test tube, and cap it to prevent evaporation. Allow to cool, and even with the Aeropress brews you'll notice a bit of settling - when you pull your sample with the pipette, pull it from the upper zone of the tube. I've found this nearly eliminates the fluctuations in readings. I simply can't afford to run every sample through a $1.30 disposable filter.
I also find that it is better to allow the sample in the well to acclimate for 60-90 seconds rather than just the suggested 30 seconds. If you're putting room-temperature sample on the lens, evaporation is minimal. Of course, YMMV.
I'm zoning in on my individual refractometer offset. Best I've been able to figure, it reads about 3-5% low at 2.0% TDS, about 2% low at 8.0 TDS, and about 6-9% low at 1.0% TDS (based on a couple similar estimates for brix vs. coffee). All work done at 20°C(68°F).
To rule out initial moisture content of my sugar, I've dried 30g samples of sugar in my oven (I know it sounds weird, but I actually stick a silica dessicant pack in my sugar storage bag, which is a Ziplock freezer vacuum bag, so I was pretty sure the initial moisture content was pretty low) and the mass doesn't change in the 0.1g range (put in 30g, get out 30g). I'm still replacing my 0.01 scale, but I'm pretty confident that the moisture content of my sugar solution is low enough to ignore (less than 0.5%).
BTW, as a total non-sequitur, anyone ever tasted nearly-pure coffee oil? LOL! Ah, the serendipitous things one encounters when puttering around.
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
Thanks again to the individual who loaned/gave me some filters!
If anyone else isn't using their syringe filters, I would consider taking them off your hands. I don't need 100 of them...
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
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