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Netphilosopher
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Posted Mon Apr 30, 2012, 12:27pm
Subject: Re: coffee refractometer
 

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RandallSluder
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Posted Sat May 5, 2012, 12:03pm
Subject: Re: coffee refractometer
 

A note for Kafeman (if you're still reading this forum) and others interested in the accuracy in TDS and index of refraction of refractometers for measuring the strength of drip brewed coffee.   A few months back, I looked at getting one of the VST refractometers -- I have a long and troubled history with the conductivity type of TDS meters -- and being both the frugal sort and always wanting to understand things for myself, I wondered if a “generic” handheld digital refractometer could do the job and at a lower price.

The VST Standard model looks to be based on the Reichert BRIX/RI-Chek, and the Lab model on one of the MISCO PA200 series. These refractometers don't read out in TDS, but in index of refraction (nD) or Brix. As AndyS points out, VST specifies accuracy only in TDS, not nD, but Reichert and MISCO do:  +/- 0.0002 and +/- 0.0001 for these two refractometers, respectively.

The first problem though, was how to convert from nD to TDS.  At the time, I was unaware of the VST patent, so I simply looked at VST's literature and at on-line articles that had images of the MoJoToGo software display, and used the TDS and nD numbers from them. I found several that were all conveniently at the same 22C temp, and when I plotted them, I found that the points can be fit neatly with a straight line:  

       TDS = 545 * nD - 726

This equation gives not only a simple nD-to-TDS conversion (at 22C), but the slope of the line (the 545 number) is how TDS changes with changes in nD.   To see how this compared to the "real" conversion,  I checked the VST patent  (referenced by GlennV in this thread). The explicit expression for TDS published there is a quadratic that includes terms that correct for changes in temperature, but at constant temperature, TDS vs nD is quite linear, and at 22C can be approximated closely by the line:

       TDS = 560 * nD - 746

This equation is reassuringly similar to the "iPhone" equation above.  If we then use the manufacturers' accuracies for nD and the slope from the patent, we can relate the accuracies in nD and TDS units:

       Reichert/Standard accuracy = +/- 0.0002 * 560 = +/- 0.11% TDS, and

       MISCO/Lab accuracy = +/- 0.0001 * 560 = 0.056% TDS

So that's good news:  the accuracies published by the refractometer manufacturers are consistent with the warranted accuracies from VST (VST Refractometer Specifications Overview; the Lab brochure gives slightly better specs), and consistent with the figure AndyS gave of +/- 0.0001 nD corresponding to +/- 0.06% TDS.  

Kafeman, I believe you said your refractometer was accurate to +/- 0.0001 nD -- that's comparable to the MISCO unit underlying the VST Lab model, and I think that's Good Enough for measuring the strength of brewed coffee, especially so if you can verify its results with an independent calibration. I now don't think the VST Standard sort of refractometer is quite accurate enough for drip coffee -- not enough pixels in its detector I suspect -- , but should be fine for espresso.

An aside about accuracy, precision, and resolution:  accuracy can be thought of as how close you are to the right answer, precision is how tightly the answers you get cluster together, and resolution is the smallest change in a measurement that can be detected.  One (of many) illustrated examples is here.  MISCO (who are a little loose with the terminology on their website) and others make refractometers with both accuracy and resolution to 0.00001 nD, but they cost $10-15K, and won't make your coffee taste any better anyway.

-- Randall
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Netphilosopher
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Posted Sun May 6, 2012, 2:01pm
Subject: Re: coffee refractometer
 

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RandallSluder
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Posted Mon May 7, 2012, 8:24am
Subject: Re: coffee refractometer
 

Netphilosopher Said:

Interesting info.  

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.

Posted May 6, 2012 link

I think that's right.  I looked at Brix very briefly, and doing the Brix-to-nD correctly while possible, is confusing and quite a bother. There are many MISCO, Reichert, et al refractometers that read directly in nD, though in unit quantities at not much of a cost savings over the VST.  And I wouldn't be surprised if VST were having the manufacturers select their units for tighter-than-warranted specs out of normal production lots.
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JonR10
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Posted Tue May 8, 2012, 12:50pm
Subject: Re: coffee refractometer
 

Mod note: Posts deleted and thread locked pending review.

 
Jon Rosenthal
Houston, TX
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JonR10
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JonR10
Joined: 26 Apr 2004
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Location: Houston, Texas
Expertise: I love coffee

Espresso: E61 Legend, Livietta,...
Grinder: Robur, B-Vario-W
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Drip: Technivorm
Roaster: 1-lb US Roaster, Behmor 1600
Posted Mon May 14, 2012, 9:40am
Subject: Re: coffee refractometer
 

Thread unlocked.  
Thanks to all who contacted me about this for your concern.  

Best regards,
Jon

 
Jon Rosenthal
Houston, TX
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GlennV
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Posted Tue May 15, 2012, 2:54pm
Subject: Re: coffee refractometer
 

Netphilosopher has taken the interesting discussion onto another thread, but I just wanted to round off what I was saying about espresso. I mentioned several pages back that my espresso was tasting best around 17 or 18% - I could easily get extraction up to 19% using more water, time or higher temperatures, but it didn't taste as good (and if I used more water, time AND higher temperatures it could taste quite unpleasant without exceeding 20%). I was beginning to wonder whether this 19% business was really true (might people be exaggerating their yield? surely not …). Seriously though, there don't seem to many references online to concrete examples of people getting 19% extraction yields with espresso from lighter roasted coffees. Anyway, that was with a large flat burr grinder, single dosed (which I think was the problem). I've since picked up a large conical and the coffee is so obviously sweeter and more viscous. I hadn't actually bothered to do any measurements until the other day, I'd just been enjoying the coffee. When I did, the first one came out at around 19.4% (cue much rejoicing) at about 60% brew ratio, 30sec, 93C (so totally standard) and was tasty. So yes, it does indeed seem to be true that the best espresso is around 19-20% yield. However, (and this is obvious, but I think still needs emphasising) just hitting 19% yield, even with top quality beans, does not on its own guarantee good espresso. It does seem to me that the key is being able to get there without jumping through hoops.
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Netphilosopher
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Posted Wed May 16, 2012, 4:26am
Subject: Re: coffee refractometer
 

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Buckley
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Posted Sat Jun 9, 2012, 11:55am
Subject: Re: coffee refractometer
 

Dear TimothyH,

This whole thread begs the question: why do you want a refractometer and what do you plan to do with the numbers that it provides you!!??

As my meditation teacher tells me: when it comes to soup, you can either be a tongue or a spoon.

You already have a tongue.
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Netphilosopher
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Location: USA
Expertise: Just starting

Posted Sun Jun 10, 2012, 9:46am
Subject: Re: coffee refractometer
 

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