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Netphilosopher
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Posted Mon Jun 17, 2013, 11:36am
Subject: .
 

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jpender
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jpender
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Posted Mon Jun 17, 2013, 6:41pm
Subject: Re: Comments on "Extraction" - It's just a guideline
 

Netphilosopher Said:

In the above example, I made the assumption that even the indirect measurement (refractometer) was a given - I haven't even applied typical error analysis to any of these parameters, just illustrated how many different interpretations in calculating or predicting "extraction" there are.

Posted June 17, 2013 link

That would be interesting to see you do. The accuracy of the measurement is what matters, not whether it is direct or indirect.


Netphilosopher Said:

Even with a good measurement of strength/TDS, there's still the variation in application of the measurement.

Posted June 17, 2013 link

I think the points you raised were mostly about how practical it is to get to the number, not what the number is.
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Netphilosopher
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Posted Tue Jun 18, 2013, 6:53am
Subject: .
 

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jpender
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jpender
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Posted Tue Jun 18, 2013, 12:01pm
Subject: Re: Comments on "Extraction" - It's just a guideline
 

Netphilosopher Said:

Indirect measurements are subject to more error.

Posted June 18, 2013 link

It depends.

I wanted to know the weight of a short strand of 4 lb flourocarbon monofilament line. So I measured it's length (4.80.1 cm) and determined the linear weight by measuring a 1 meter strand and weighing that on a milligram scale (0.450.03 g/cm). So, indirectly, I determined the weight to be 2.20.2 mg. I made an assumption that the diameter of the line was constant. But even with this consideration, if I had instead weighed it directly on my scale (which is only accurate to 2 mg) I would have obtained a much less useful number.

Many other examples like this exist. Even with coffee... suppose you brewed a thimbleful. I'm quite certain your refractometer would do better than my oven and milligram scale at determining the concentration. Of course I could buy a better scale, but then you could buy a better refractometer too. So, it depends.


Netphilosopher Said:

Lots of the sources of error in coffee concentration by refractometry are things I'm not privvy to.  We just trust it's irrefutably correct.

I believe that the translation from nD to coffee concentration, corrected by the compensated sample temperature, is considered a fixed calculation.  I'm sure that it's probably pretty accurate, but honestly I have no way of telling.

BUT my particular refractometer must do the following:

-measure the sample temperature (with error)
-measure the refractive index (with error)

Apply both of these to a formula and spit out the TDS%

but the translation formulae were constructed experimentally, to correlate to a super wazoo multi-kilobuck dehydration oven and scale.  I have absolutely no clue how well the correlated samples that were used to generate the translation were filtered, what coffee they used to correlate, the resulting expected variance in nD sampled to measured dehydrated sample.


So, all I can do is assess the measurement system for precision (not accuracy - I have to trust that my machine is accurate since there's no way to adjust it and every time I check with distilled water at proper temperature I get 0.0% like I'm supposed to).  The only way I know how to do that is to take a sample of coffee and test several batches with the same procedure.  Best I can tell, a sample of coffee seems to have a typical variance of around +/- 0.02%.  If I take the median value of a sub sample measured about 11 times, I get a repeatable measurement that is for all practical purposes stable and repeatable.

I consider that adequately precise.  I have only a general idea of the accuracy.

Posted June 18, 2013 link

VST states the "typical" and warrented accuracy for their Lab refractometers in their literature and on their website.
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coffeeguydenton
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Posted Sun Jun 23, 2013, 4:37am
Subject: Re: Comments on "Extraction" - It's just a guideline
 

jpender Said:

It depends.

I wanted to know the weight of a short strand of 4 lb flourocarbon monofilament line. So I measured it's length (4.80.1 cm) and determined the linear weight by measuring a 1 meter strand and weighing that on a milligram scale (0.450.03 g/cm). So, indirectly, I determined the weight to be 2.20.2 mg. I made an assumption that the diameter of the line was constant. But even with this consideration, if I had instead weighed it directly on my scale (which is only accurate to 2 mg) I would have obtained a much less useful number.

Posted June 18, 2013 link

Netphilosopher is mostly correct.  Indirect measurement always introduces more error.  An indirect measurement is one that is calculated using two or more measured values, each of which introduce their own error.  Sometimes, you are the one measuring both, sometimes the second is an accepted value based on previous measurements.  Either way, that second measurement introduces more error into the system.

However, indirect measurement is sometimes better.  In your example, the available equipment is not accurate enough for a direct measurement.  But given the appropriate tools, direct is always better.  Either way, I think netphilosopher's point that the system by which extraction is measured has too much error in it too be useful is valid.  Rather, it gives a nice starting point and then it is up to us to tweak our methods to fit our taste.  That and it gives us something to debate and use to put lesser mortals in their place :p.
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jpender
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Posted Mon Jun 24, 2013, 10:46am
Subject: Re: Comments on "Extraction" - It's just a guideline
 

coffeeguydenton Said:

Netphilosopher is mostly correct.  Indirect measurement always introduces more error.  An indirect measurement is one that is calculated using two or more measured values, each of which introduce their own error.  Sometimes, you are the one measuring both, sometimes the second is an accepted value based on previous measurements.  Either way, that second measurement introduces more error into the system.

However, indirect measurement is sometimes better.  In your example, the available equipment is not accurate enough for a direct measurement.  But given the appropriate tools, direct is always better.

Posted June 23, 2013 link

I don't know if that argument holds water or not, but it's beside the point here. We don't have access to the ultimate tools. And it is indisputible that in many real life cases an indirect approach is more accurate. Sometimes it is the only way.

Netphilosopher himself encountered the differences between direct and indirect measurement when he first obtained his refractometer. Measuring indirectly the error was dominated by the limits of the refractometer itself. Measuring directly error came not only from his pocket scale but also due to undissolved solids in the coffee. To seperate all of those solids is not trivial and introduces an additional error as some dissolved solids are inevitably lost in the process. Which method is more accurate?


coffeeguydenton Said:

Either way, I think netphilosopher's point that the system by which extraction is measured has too much error in it too be useful is valid.

Posted June 23, 2013 link

Valid? It has been demonstrated that there is a good correlation between extraction and taste.

The issue is not the error in the extraction calculation or whether it is achieved by indirect measure, it is that extraction itself is an indirect and imperfect measure for what we really want to know about coffee, namely the relative proportions of the many dissolved compounds.
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Netphilosopher
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Posted Mon Jun 24, 2013, 11:47am
Subject: .
 

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jpender
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jpender
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Posted Mon Jun 24, 2013, 3:09pm
Subject: Re: Comments on "Extraction" - It's just a guideline
 

Netphilosopher Said:

My original point is that it isn't the exact number we all think it is.  If I give you the same brew parameters but do not tell you the method - then you may be correlating taste and extraction incorrectly.

Posted June 24, 2013 link

I don't think extraction is that complicated.

For percolation methods it is the ratio of dissolved solids in the cup to initial dry coffee mass.
For infusion methods it is the ratio of dissolved solids in the brew mass to initial dry coffee mass.


Netphilosopher Said:

Recipe:

400g brew water
25g brew coffee,

340g of produced coffee at 1.35% strength.


I haven't told you the method by which it is brewed.

What's the extraction?  

I also haven't mentioned that I think I know how much moisture is in my brew coffee - does it matter?

Posted June 24, 2013 link

Yes, you have to describe the brew method. And to be precise you need to specify the dry coffee mass. The dry mass is not easily obtainable but fortunately it does not affect the extraction calculation by too much. If the dry mass is 2% less than the measured dose the extraction will calculate low by about 0.4%, which is about the same as the typical error contribution from your refractometer. Since the correlation with taste isn't all that precise anyways it's not worth getting worked up about. But just because we routinely accept this error doesn't mean it isn't an error.

Admittedly, if everybody uses a slightly different formula it does get a little confusing.
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Netphilosopher
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Posted Tue Jun 25, 2013, 5:17am
Subject: .
 

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jpender
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jpender
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Posted Tue Jun 25, 2013, 10:15am
Subject: Re: Comments on "Extraction" - It's just a guideline
 

Netphilosopher Said:

Really?  I haven't seen any publication from SCAA, SCAE, or other governing body that mentions this. :D

Posted June 25, 2013 link

I appeal to the logic of these definitions rather than an industry organization or some governing body.


Netphilosopher Said:

And what happens if you read a paper written in the 1980's and conclude that all coffee has 2% CO2?  Now the calculation is off by ANOTHER 0.4%.

Posted June 25, 2013 link

The same CO2 content for all coffee, all the time? If that were true then, yes, you'd always be off by that amount. But if the offset were constant then it would be meaningless. It is only because the dry weight to dose weight ratio varies from coffee to coffee and day to day that it matters, albeit in a minor way.
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