squaremile Senior Member Joined: 6 Jan 2011 Posts: 71 Location: Portlandia Expertise: Just starting
Posted Wed Mar 6, 2013, 10:32pm 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.
jpender Senior Member Joined: 11 Jul 2011 Posts: 535 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Thu Mar 7, 2013, 9:56am Subject: Re: TDS & Distilled Water
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.
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.
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