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GlennV
Senior Member


Joined: 27 Oct 2011
Posts: 29
Location: UK
Expertise: I love coffee

Posted Tue Feb 21, 2012, 5:08pm
Subject: Re: coffee refractometer
 

Kafeman Said:

On the brix correction, that's great to know.  I've been usuing 0.84 as the factor until I can make my calibration curve for my own refractometer ...

Posted February 21, 2012 link

Oops, semantics - I meant a difference in reported yield of about 1% (20% instead of 19%, for example) - not a 1% difference in yield - i.e. a massive difference.
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Kafeman
Senior Member


Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Tue Feb 21, 2012, 5:38pm
Subject: Re: coffee refractometer
 

LOL !  That difference is a perfect justification for dumping all of these one-size fits all things, no matter how clever, and instead taking an afternoon to calibrate one's own refractometer.  It is so much more work to make a rule/product that works for every situation vs. tailoring something to one's specific situation!  By making ones own standard curve, one doesn't have to worry as much about reproducing someone else's measuring technique as much as developing a consistency of their own measuring tecnnique, and what better way than by drying out some grinds or solutes.

Call me selfish, but it is hard enough to figure out for myself what the optimum conditions are, let alone worrying about sharing them with the world before I can crawl ;-) It will be some time to reach that point that I can specify the conditions for someone to put in a cup the same thing I do.

To boot, nature doesn't have a rule that TDS of one coffee is comparable to another coffee at the same TDS and whatever other parameter(s) one chooses, it is just a working assumption lacking anything better - until we learn more and become experts in our favorite coffees.  If only life were so simple, but then there would only be one coffee.

Cheers!
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GlennV
Senior Member


Joined: 27 Oct 2011
Posts: 29
Location: UK
Expertise: I love coffee

Posted Wed Feb 22, 2012, 5:56am
Subject: Re: coffee refractometer
 

Please, I don't want to be  misunderstood here. I'm not for a moment putting up 0.85*Bx as a standard to compare the VST calibration to - it was just an example to show what can go wrong if people start relying on anecdotal evidence from forums. The whole x*Bx thing is clearly flawed, for any x. The refractive index of sugar solutions exhibit significant nonlinearity wrt concentration in the range of interest, whereas coffee is much closer to linear. This is easily verified using a single clarified espresso sample and dilution.  

Personally I'm very happy with de facto standards. For example, AndyS says he likes likes his espressi at 19-20%, personally I prefer 18% when at 65% brew ratio. We might have different tastes or be drinking different coffees. More likely it's because he's got better kit - and I have to jump through too many hoops to get  yield up to 19% with that small amount of water. It's nice to know that it's not because we have a different notion of what 19% is though.
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Netphilosopher
Senior Member


Joined: 14 Jan 2011
Posts: 1,602
Location: USA
Expertise: Just starting

Posted Wed Feb 22, 2012, 8:43am
Subject: Re: coffee refractometer
 

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Kafeman
Senior Member


Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Wed Feb 22, 2012, 2:06pm
Subject: Re: coffee refractometer
 

Hi Glenn and Netphilosopher;

This is a quick thank you to both of you for the glorious 'geeky' detail you've been so kind to offer.  I'll probably be back to edit this post when I have more time to experiment with some of the material you've both given to me and other readers of the forum.  I am literally hanging on some of the wording in both posts and rereading it, so please not for a second think that I would misunderstand - I would definitely ask if something were unclear.  

Example, when Glenn helpfully suggests the quadratic fit to the brix conversion is better represented under the specific conditions of interest with (0.800 + 0.003*brix) instead of a constant 0.85, which has dubitable foundation behind it, I would say - hmmmm .... am I interpreting this properly in thinking for the 0.85 to be correct under these assumptions, that we would need to be in the range of 16% brix ;-) ?

Thanks again and I'll be in the kitchen ... errr kitchatory for a bit testing some of these ideas out and will hopefully come back to report what I was able to do.

On the comment of having a standard we can all reproduce - hopefully it was understood that I aspire to get there, admire that, but at the moment, have a lot of grounds to cover ;-0

Thanks again guys

--Dave
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 712
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Wed Feb 22, 2012, 3:21pm
Subject: Re: coffee refractometer
 

Netphilosopher Said:

I tried higher temps which evap quicker, but closer to 300F is getting close to the decomposition/melting point of some sugars, and the grounds and the solids from evap of the coffee beverage start to smell burnt-caramelly.  It's also starting to get into the temps for Malliard Rxns to start.

Posted February 22, 2012 link

Did you measure how much less the dried coffee grounds weighed compared to dehydrating at 215F?
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OwnCoffeeTech
Senior Member


Joined: 9 Feb 2012
Posts: 11
Location: San Francisco
Expertise: I love coffee

Posted Wed Feb 22, 2012, 3:58pm
Subject: Re: coffee refractometer
 

I've visited several awesome coffee shops that utilize this technology. It seems to help deliver quality cup after quality cup. Not to say this is the key to success but it does marry chemistry and coffee, which I think is pretty cool!






www.thinkown.com
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andys
Senior Member
andys
Joined: 10 May 2003
Posts: 857
Location: NY
Expertise: Just starting

Espresso: Speedster, Londinium 1
Grinder: EK-43,Robur, HG One, M3
Vac Pot: Yama
Drip: various
Roaster: PIDed Popper
Posted Wed Feb 22, 2012, 8:47pm
Subject: Re: coffee refractometer
 

Kafeman Said:

As for the refractometer, since I have a +/- 0.0001 uncertainty in my refractive index measurement, it would be better to just go directly from refractive index directly to TDS.  The extra effort wasn't worth it to find a reliable TDS vs. nD conversion (if it is published!) and within a few weeks it will be all moot since I'll make my own calibration curve directly, and if I do it right it will be more accurate for the ranges I calibrate than instruments in the class of the VST, since they typically are limited with detectors which have twice the uncertainty

Posted February 20, 2012 link

Your instrument's uncertainty of +/- 0.0001 in refractive index corresponds to about +\- 0.06% TDS. Let's consider a typical coffee target extraction of 1.40% TDS and 19% extraction yield. If you measured a TDS of 1.40%, you could be 1.34% or 1.46%. This corresponds to extraction yields of 18.2% to 19.8%. Your instrument is marginal in this application.

The VST refractometer I own (LAB model) resolves to 0.00001 in refractive index.


Kafeman Said:

if someone wants really to use a refractometer as an R&D tool, the best approach is to get a more precise instrument (for $1000 or more) and just make your own calibration chart.  It is really not hard if you've ever done intro to analytical chemistry as a college freshman or AP high school, to get a fine curve that is specific to your problem if you're ibnto the wet aspects of coffee.

The issue with that is when people use an instrument and don't report what the instrument measures (in this case refractive index), but rather report TDS or so other measure derived on a primary metric, but don't tell you that metric.  For example, if a lot of people start using the VST, it could become a defacto standard, even if the TDS it reports is not accurate, it would be standard (precise) for sharing data, especially casually.

Posted February 21, 2012 link

I totally disagree. You have zero evidence that the VST refractometers are not accurate. Earlier in this thread Vince (VST's owner) listed various independent coffee associations that had tested and approved the accuracy of his instruments. If you don't believe him, contact the associations yourself.

Since you already own a refractometer, I fully understand how you'd want to use it. And I understand how, as a fellow hobbyist, you'd get a kick out of spending the time adapting it for coffee. But considering you've been on this forum for a grand total of four days, don't reveal a real name or who you are, and haven't yet done this yourself, I don't think too many people are going to take your advice, drop $1000 or more on a refractometer, and then try and follow your instructions in calibrating it themselves. Not when proven instruments are available, ready to go, for $399 or $599 dollars.....

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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Netphilosopher
Senior Member


Joined: 14 Jan 2011
Posts: 1,602
Location: USA
Expertise: Just starting

Posted Wed Feb 22, 2012, 9:49pm
Subject: Re: coffee refractometer
 

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Kafeman
Senior Member


Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Wed Feb 22, 2012, 10:23pm
Subject: Re: coffee refractometer
 

Hi Andys

Ouch!  Real name?  You know I'm Dave - The fact out of the blue that my last name isn't here is personal, well, I feel you are crossing the 'nasty' line.  I assume many people's aren't either that I've seen and at one point you were a newbie and whatever issue you have with last names, are comments way off topic, disrespectful, and unwelcomed IMO, if you see something you don't agree with, there is no reason to take that tone with me instead of discussing things in a stimulating environment, so things don't go south.  Perhaps I'm disappointed in this - only because I respect you and wanted to get off on the right foot.  

I am now very confused.  Three days ago I asked the nD resolution of the VST refractometer:

I'm wondering btw, what the VST coffee refractometer instrument accuracy is on the raw refractive index

To which you said:

Although VST warrants accuracy in coffee TDS, I'm not sure if they quote an nD spec.

but now you actually tell it (for the VST model you own):

The VST refractometer I own (LAB model) resolves to 0.00001 in refractive index.

If the last name thing is a problem, and we were forum friends, you know - all you have to do is ask.  I have 4 days here, give me a break.  In those 4 days I've learned a great deal from posters including you, one of which in reply to:

If you measured a TDS of 1.40%, you could be 1.34% or 1.46%

let me just say I have actually figured out on my own, in part thanks to looking at what others are doing here, that 3-4 times the concentration helps tighten the uncertainty, so this is now moot for me, though of course a very real consideration for the knowledgeable analyst working on tuning his/her methods whose project forces them to deal in that range.
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