squaremile Senior Member Joined: 6 Jan 2011 Posts: 34 Location: Portlandia Expertise: Just starting
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.
Posted Thu Mar 7, 2013, 8:01am Subject: Re: TDS & Distilled Water
The difference in TDS (or the index of refraction) is extremely small.
Personally, I haven't been able to see it over noise, using very hard water (300ppm calcium equivalent) vs. distilled.
Furthermore, I found no measurable difference if I measure coffee with the refractometer zeroed using distilled water vs. zeroed using hard water. For consistency, however, I always zero with distilled water. That's considered standard, and under scrutiny people will point to the teeniest error and call your work crap if it doesn't match their picture of the world. <shrug>
The index of refraction for distilled water is ~1.333 at 15°C to 20°C (going by remembry).
There's obviously variation in the fourth-fifth decimal places with regard to temperature, and this or finer is the range of differences between hard water and distilled water.
A 2% sugar solution is a change in this index by a couple thousandths (something like 1.336). This is going to register around 1.7% (very rough approximation) on a coffee-calibrated refractometer. The effect of 300ppm (very hard water) is expected to be in the 5th-6th decimal place, and therefore really not measurable.
EtOH (ethanol) is way off the coffee scale, at 1.36, just for comparison.
You can see that there is no effect on coffee strength given the same brew parameters with a large variation in water hardness from distilled all the way to ~300ppm. (Very noticeable differences in TASTE - but it isn't an "extraction" effect, it's either chemical or flavor-mechanism effect).
Bottom line - don't sweat it, you likely won't be able to read the difference in the index of refraction for normal water hardness components.
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.
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.
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
jpender Senior Member Joined: 11 Jul 2011 Posts: 406 Location: California Expertise: I like coffee
Grinder: Kyocera CM-50 Vac Pot: S/S Moka Pot Drip: Aeropress
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.
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.
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.
Posted Thu Mar 7, 2013, 12:02pm Subject: Re: TDS & Distilled Water
jpender Said:
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%.....
Theoretically, it's the difference between 1.17 and 1.185%TDS (for 150PPM TDS). But again, water hardness and TDS is not as large an effect on index of refraction as salt, sugar, EtOH, or coffee solids. It is possible to see this in dehydration measurements, though. We're really talking some pretty minor things.
Example: 150ppm water hardness, 1.20% strength coffee. Assume it is extremely well-filtered (15nanometers! LOL)
1000g of this coffee is made up of: 12g Dissolved Coffee Solids 998g of water, which itself is made up of 987.852g of pure water and 0.148g of dissolved water solids.
Considering we went decades without properly recognizing the difference between immersion and percolation brewing, this is some pretty minor stuff.
What is amazing to me is the effect of the absence of those water dissolved solids - no changes to the calculated extractions and such, but big differences in taste.
jpender Said:
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.
Careful... you might overturn all of the traditional thinking of how to determine extraction. ;-D
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
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.
Posted Fri Mar 8, 2013, 11:57am Subject: Re: TDS & Distilled Water
jpender Said:
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.
Physics lab, nearby universities. Friends in university places.
It is a byproduct of when I was researching the refractometer I was going to buy.
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
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