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gt
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Joined: 29 Jan 2007
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Posted Sat Mar 24, 2012, 2:40pm
Subject: Re: VST LAB Coffee Refractometer
 

eMoJo Said:

Final comments and suggestions
This should read 2.00 +/- 0.05% on the VST Coffee Refractometer at 20.0 +/- 1.0 Deg C.  

~Vince@VST                                                               Edit 3/19: corrected typo on temp

Posted March 18, 2012 link

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% ?

Thanks Gary
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eMoJo
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eMoJo
Joined: 19 Sep 2010
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Location: Boston
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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.

~Vince@VST
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jbviau
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jbviau
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Posted Sun Mar 25, 2012, 4:52pm
Subject: Re: VST LAB Coffee Refractometer
 

^^^ See also the definition of pedantic... ;)
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Netphilosopher
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Posted Mon Mar 26, 2012, 5:36am
Subject: Re: coffee refractometer
 

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TonyVan
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Posted Mon Mar 26, 2012, 10:39pm
Subject: Re: VST LAB Coffee Refractometer
 

jbviau Said:

^^^ See also the definition of pedantic... ;)

Posted March 25, 2012 link

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.
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IenBell
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Joined: 17 Feb 2012
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Location: Uk
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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.
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Bob_McBob
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Bob_McBob
Joined: 30 Jan 2006
Posts: 456
Location: Waterloo, ON
Expertise: I live coffee

Posted Fri Apr 6, 2012, 2:23am
Subject: Re: coffee refractometer
 

andys Said:

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.

Posted March 18, 2012 link

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.

 
Chris
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andys
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andys
Joined: 10 May 2003
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Posted Fri Apr 6, 2012, 9:50am
Subject: Re: coffee refractometer
 

Bob_McBob Said:

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.

Posted April 6, 2012 link

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.

Posted April 6, 2012 link

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.

Posted April 6, 2012 link

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.

Posted April 6, 2012 link

If it was me that brushed you off, I apologize.

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.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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Netphilosopher
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Joined: 14 Jan 2011
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Location: USA
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Posted Mon Apr 9, 2012, 6:44am
Subject: Re: coffee refractometer
 

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

Posted Tue Apr 17, 2012, 11:01am
Subject: Re: coffee refractometer
 

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