gcrest01 Senior Member Joined: 7 Dec 2012 Posts: 2 Location: NY Expertise: I love coffee
Espresso: La Nuova Era Cuadra Grinder: Baratza Vario Drip: Chemex Roaster: stumptown, metropoliscoffee,...
Posted Fri Dec 7, 2012, 3:51pm Subject: Re: coffee refractometer
I was looking at PA202 model http://www.misco.com/products/PA202.html and it outputs result in both units: Brix and nD(Refractive Index). The range is 0.0 to 85.0 Brix or 1.3330 to 1.5040 nD.
The Temp. Comp. Basis is: Sucrose.
I did call the company and asked them if it outputs the temperature that was used in the measurement along with the nD(refractive Index). They said no. That means that one needs to measure the temperature of the coffee sample as well with the IR thermometer to solve the formula:
Z = a + b*X + C*Y +d*X^2 + e* Y^2 ( taken from the VST patent document)
Z = -7.27705E+02 - 8.72678E-01 * TEMPERATURE + 5.46065E+02 * nD + 9.210185E-04 * TEMPERATURE - 4.68197E-01 * nD^2 + 6.648126E-01 * TEMPERATURE * nD
where Z is %TDS.
I did the Z(% TDS) interval bracket estimation for the minimum and max value of the refractometer (min nD: 1.3329 / max nD: 1.5040) and the temperature of 22c:
min Z = 0.424% max Z = 93.58%
My question is when the PA202 measures the nD(Refractive Index) versus measuring it in the Brix does it apply the temperature calibration or not ? If it does how much different the nD reading would be using the Sucrose for the Temp. Comp. Basis if the actual fluid was coffee.
Keep in mind the d nD/dT of brewed coffee and espresso is different from that sucrose solutions (or brine solutions, for that matter). You'll have to get every coffee sample at the same correlated temperature or apply yet ANOTHER temperature compensation factor if the MISCO unit is temp calibrated for sucrose solution (since it probably reads out in °Brix). This is ON TOP of the transfer function of nD to coffee concentration.
The VST folks are very sensitive about using VST proprietary information (such as the contents of the patent document) for publicly posting theories or suggestions on ways to circumvent or copy the VST product and/or methods. The patent is public, and still protected IP.
Here's a recent conversation that references the sort of meter you're looking at. CLICK for LINK It seems that the consensus is that VST is worth the extra pennies.
RandallSluder Senior Member Joined: 5 May 2012 Posts: 5 Location: Asheville, NC Expertise: Pro Roaster
Posted Fri Dec 7, 2012, 10:37pm Subject: Re: coffee refractometer
The VST folks are very sensitive about using VST proprietary information (such as the contents of the patent document) for publicly posting theories or suggestions on ways to circumvent or copy the VST product and/or methods. The patent is public, and still protected IP.
The folks at VST may be sensitive types, but information disclosed in a patent is not proprietary. The word patent itself is from the Latin meaning open or accessible. For information to be proprietary, it must be kept secret or confidential in some way.
I'm not going to debate patent law with you, except to say that you seem to be misinformed. Patented material is still protected, even after it is disclosed for us to read.
RandallSluder Senior Member Joined: 5 May 2012 Posts: 5 Location: Asheville, NC Expertise: Pro Roaster
Posted Sat Dec 8, 2012, 10:26am Subject: Re: coffee refractometer
I'm not going to debate patent law with you.
Me neither.
Patented material is still protected, even after it is disclosed for us to read.
While US law gives patent owners certain rights and remedies, my point is simply that the information disclosed in VST's patent, in any patent, can not be "proprietary information", another term for "trade secrets".
VST may in fact have some trade secrets but the equation and coefficients gcrest01 references are not among them. They are stated explicitly in VST's US Patent 8239144 (http://www.google.com/patents/US8239144), and as far as I know, you and gcrest01 and I are free to discuss their validity, applicability, verisimilitude, or anything else about them without prior restraint.
gcrest01 Senior Member Joined: 7 Dec 2012 Posts: 2 Location: NY Expertise: I love coffee
Espresso: La Nuova Era Cuadra Grinder: Baratza Vario Drip: Chemex Roaster: stumptown, metropoliscoffee,...
Posted Sat Dec 8, 2012, 12:33pm Subject: Re: coffee refractometer
I don't want to get anyone in trouble for any legal issues that I'm not qualified to judge especially in my first post on this forum.
I've just had a simple question can anyone estimate how much off will I be in the Refractive Index measurement if the Temp. Comp. Basis is: Sucrose where as the actual fluid is coffee.
I did ask the manufacturer (MISCO) if I can use the PA202 on the coffee and they said yes. I was told that I can use the PA202 to get the baseline on coffee.
All I really need to have is some kind of number that I can refer to get the consistency in my coffee made in Chemex and my espresso. However I want this number to be precise and repeatable.
Once I get the taste that I like I can remember the refractive index and use it as a reference. Also that way I can experiment with the brewing variables ( water temperature, grind, dose, basket selection, tamp...etc) to get various tastes and see how it relates to the refractive index. That way I can associate tastes with the refractive index. I like measurable output parameters.
I think sensitivity wise the PA202 model http://www.misco.com/products/PA202.html will be enough for be for regular coffee and espresso. Obviously I need some kind of paper filter to filter the espresso before taking the measurement.
The reason for me is simple $425 fits my budget much easier than $599 for VST LAB Coffee II Coffee & Espresso refractometer. Alternatively I can get the VST Standard Coffee/Espresso Refractometer for $399 but I'm afraid that it will not be sensitive enough for the regular coffee and repeatable( it only takes a single measurement ) instead of averaging a 30 measurements.
Please let me know your thoughts and if you see anything wrong with my idea.
Posted Sat Dec 8, 2012, 7:19pm Subject: Re: coffee refractometer
gcrest01 Said:
All I really need to have is some kind of number that I can refer to get the consistency in my coffee made in Chemex and my espresso. However I want this number to be precise and repeatable.
Once I get the taste that I like I can remember the refractive index and use it as a reference. Also that way I can experiment with the brewing variables ( water temperature, grind, dose, basket selection, tamp...etc) to get various tastes and see how it relates to the refractive index. That way I can associate tastes with the refractive index.
The reality of coffee is way more complicated than this.
Coffee strength is correlated to refractive index and temperature.
Coffee taste is correlated with numerous factors, including strength, temperature, and (crucially) extraction yield, which is the fraction of the original dry grounds that are extracted into your cup.
Extraction yield is typically calculated after weighing the dose, weighing the brew water (or beverage amount), and accurately measuring the beverage strength. You cannot simply measure refractive index and expect it to correlate to taste, particularly as you experiment with the standard brewing variables.
gcrest01 Said:
I think sensitivity wise the PA202 model http://www.misco.com/products/PA202.html will be enough for be for regular coffee and espresso. Obviously I need some kind of paper filter to filter the espresso before taking the measurement.
The reason for me is simple $425 fits my budget much easier than $599 for VST LAB Coffee II Coffee & Espresso refractometer. Alternatively I can get the VST Standard Coffee/Espresso Refractometer for $399 but I'm afraid that it will not be sensitive enough for the regular coffee and repeatable( it only takes a single measurement ) instead of averaging a 30 measurements.
Paper filters produce inconsistent results with espresso. The quantity is very small, the concentrated liquid passes through the filter slowly, and significant evaporation occurs. Unfortunately the recommended syringe filters are expensive, but they give accurate results.
IMO the $599 VST LAB II is a much better value for you than the $425 PA202. A huge amount of work and expense went into developing both the software and hardware. It will likely be more accurate and repeatable than anything you can develop on your own. If money is tight the VST Standard works fine, but I've used both and the LAB is a big step up in usability and (presumably) accuracy.
RandallSluder Senior Member Joined: 5 May 2012 Posts: 5 Location: Asheville, NC Expertise: Pro Roaster
Posted Sat Dec 8, 2012, 9:30pm Subject: Re: coffee refractometer
I would generally second what Andy says. It's important to be able to measure coffee strength when you're dialling in a brewer or brewing system, but even if you get the strength and extraction "right" for the Golden Cup or one of the European standards, the beverage may still not taste "right". If you can get up to Boston in the spring for the SCAA convention, take the brewing lab class. One of the demonstrations they do (or at least used to do) is systematically changing the brew parameters one at a time: coffee-to-water ratio, contact time, brew water temp, fineness of the grind. Or try it yourself.
A coffee strength meter is just a tool, like a thermometer or a scale. Before spending hundreds on a refractometer, I would suggest getting a $30 TDS meter and some calibration solution, and experimenting with your Chemex.
A TDS meter won't work with espresso but then I'm old-fashioned and think that you can dial in espresso with only a scale to measure the shot and beverage weights, and simply tasting the shots. I suspect the espresso aficionados will disagree, but I think setting up a commercial drip brewer is much more difficult than an espresso machine.
If you do spring for a refractometer, I would go with the VST product, but just the coffee-only LAB model and that will save you $100.
I agree with the other posters that it really doesn't make sense to get anything other than a proper coffee refractometer. The instrument you reference is stated to have a resolution of 0.1Brix or 0.0001nD so, even if it were perfectly accurate and repeatable, and even if there were no added error due to the incorrect temperature compensation for sucrose instead of coffee being used, it can never possibly display coffee %TDS in better than 0.06% increments. Adding in the minimum practical errors in accuracy and repeatability of 0.06% TDS, you could easily be off the mark by 0.12%TDS even without the temperature correction issue. Both the standard and lab VST units give you roughly an extra decimal place, reading coffee %TDS to 0.01%, and this is not just noise. As I understand it, significant improvements were made to both the hardware and firmware which justify this. I've not used the standard refractometer, but can vouch for the fact that my Lab II Coffee & Espresso refractometer meets (and usually exceeds) its claimed typical precision of +/- 0.02%TDS coffee (this is equivalent to approximately +/- 0.00003 nD). In fact, if successive readings on brewed coffee fluctuate more than 0.01%TDS then I take this as a signal that either the sample and refractometer have not yet reached thermal equilibrium or the sample has not been filtered well enough. The claimed real life typical precision of the standard VST refractometer (+/- 0.06%TDS coffee) is already equivalent to better than that 0.1Brix, and roughly equivalent to 0.0001nD. I think most people find that it does even better than this in practice. This has to be a better bet than the Brix unit you refer to, even without the issue of how to implement a coffee scale without infringing on the VST patent.
This is what 0.02g (approx) of granulated sugar looks like on a teaspoon. This, in 100ml of water, is about 0.02Brix. I compared this with another glass of 100ml water without the sugar, and it was easy to pick out the one with the sugar using the Lab II unit. I did it as a blind test, to keep me honest, but needn't have bothered - the difference was absolutely clear. It really is a remarkable instrument and (I know this sounds ridiculous, but) a delight to use. I couldn't taste the difference between the two glasses. It's worth contacting Vince at VST to check what current offers they have and whether they have any reconditioned units for sale.
Posted Mon Dec 10, 2012, 12:54pm Subject: Re: coffee refractometer
I've had very good luck with 4 minute centrifuge on espresso and press pot. Identical to syringe filtered measurements, by drawing off the supernatant after centrifuging.
Basically spins the particles that are prone to settling. Use capped tubes to prevent evaporation, and this doubles as cooling.
Of course, you'd need access to a centrifuge - but it works.
I also got a deal on a bunch of 0.45micron glass matrix syringe filters (about half price), but dropping nearly a buck each time you take an espresso reading, I'd rather load up six samples in the centrifuge, wait my 3 minutes...
All you're looking for is a stable reading. If you want to toss away some stuff, I've had moderate luck (only moderate) by allowing a test tube sample to settle for 10 minutes, then draw off the top. More variance, but still definitely within a decent accuracy. Sometimes it works - but sometimes it doesn't because of the suspension of small solids that can continue to settle out of espresso for hours. I've found the particles that settle against the lens tend to make the strength reading drop. I assume there's some local index of refraction interference with small particles at the lens/fluid boundary.
Also, another thing to be careful of - you can get a quick reading with some particles present, but if you're trying to get dynamic measurements (like sampling at 30 seconds, 1 minute, etc. during an immersion brew), I've found that early on in the extraction process for immersion, if you suck up even a couple-three grounds particles and don't filter them, they will extract in the syringe or pipette and the strength reading will go very high for those early samples.
The good news is that the really fine particles extract in about 30-60 seconds, and then reach their own equilibrium. Fun fun fun.
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"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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