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Discussions > Coffee > Machines > TDS & Distilled...  
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squaremile
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Posted Wed Mar 6, 2013, 11:32pm
Subject: TDS & Distilled Water
 

I had always believed that to get an accurate measure of coffee extraction, I should calibrate my refractometer with my brew water, rather than distilled water. However, everything I read about these things says calibrate with distilled, then take the coffee sample to get your reading. If that is correct, then the TDS reading for my coffee will include any dissolved solids that were already present in the brew water, and that this will be factored into the reading. Can anyone provide guidance on which way this should be since it will massively impact the calculations either way? Thanks in advance.
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NobbyR
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NobbyR
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Posted Thu Mar 7, 2013, 3:50am
Subject: Re: TDS & Distilled Water
 

Is there a significant difference in your readings when you use distilled in contrast to regular brew water?

 
***
"This drink of the Satan is so delicious that it would be a shame to leave it to the infidels." (Pope Clement VIII on coffee, when he was urged to ban the beverage)
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Netphilosopher
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Posted Thu Mar 7, 2013, 8:01am
Subject: .
 

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jpender
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jpender
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Posted Thu Mar 7, 2013, 10:56am
Subject: Re: TDS & Distilled Water
 

Netphilosopher Said:

More importantly, (talk about burying the lead) the system is calibrated (I assume) for coffee made with standard SCAA water (TDS: 150ppm/Hardness: 60 - 80ppm/Alkalinity: 40 - 80ppm), so this is already accounted for in the calibration curves for index of refraction and coffee concentration with a lab oven dehydrator (the gold standard for determining coffee concentration and TDS).  In other words, it's already baked in, and probably the most important reason to zero with distilled water.

Posted March 7, 2013 link

That's a good point. It wouldn't make any sense to use coffee made with distilled water for calibration purposes as it might differ in important ways other than the water TDS.

Netphilosopher Said:

Keep in mind that IF calcium carbonates/magnesium carbonates/iron/etc. were as responsive to changes of index of refraction as say sugar of coffee (or salt, for that matter), 300ppm implies a variation of 0.03%TDS.

Posted March 7, 2013 link

That should be detectable with a VST Lab refractometer. It would be on the order of a 1% error for normal strength coffee or a calculated extraction of, say, 20.2% instead of 20.0%.

I've wondered how much the index of refraction varies with extraction (for a given temperature and concentration). Since extraction is a proxy for the relative amounts of chemicals in solution, the index of refraction might vary measureably, at least for really high/low extractions. It's possible that different types of coffee and roasts have an effect as well.
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Netphilosopher
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Posted Thu Mar 7, 2013, 12:02pm
Subject: .
 

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jpender
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jpender
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Posted Fri Mar 8, 2013, 10:48am
Subject: Re: TDS & Distilled Water
 

Netphilosopher Said:

But again, water hardness and TDS is not as large an effect on index of refraction as salt, sugar, EtOH, or coffee solids.

Posted March 7, 2013 link

Oh, okay. It wasn't clear to me since you said 5th (which is true for coffee) to 6th (which would be less) decimal place for nD. I wasn't sure if you were saying it was in between or you didn't know.

By the way, how do you know?
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Netphilosopher
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Posted Fri Mar 8, 2013, 11:57am
Subject: .
 

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jonr
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Posted Tue Dec 24, 2013, 4:09pm
Subject: Re: .
 

So many deleted postings.  Let me guess - VST has once again been threatening people who simply discuss TDS?
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squaremile
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Posted Tue Dec 24, 2013, 6:08pm
Subject: Re: .
 

The answer is that the TDS in your brew water is so minimally different that it would barely effect TDS in the cup at the end if at all. Furthermore, calibrating on distilled water allows for a uniform comparison of numbers across cups as well.
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andys
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andys
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Posted Tue Dec 24, 2013, 9:33pm
Subject: Re: .
 

jonr Said:

So many deleted postings.  Let me guess - VST has once again been threatening people who simply discuss TDS?

Posted December 24, 2013 link

Here's an alternate explanation: so many of Steve's posts violated intellectual property rights that he found it an overwhelming task to edit them all one by one. It was far easier for him to delete them all.

I believe this is closer to what actually happened than what you propose. But obviously it's a free country so you can believe whatever explanation floats your boat.  :)

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