Misusing a TDS Meter to evaluate anything about Espresso or Coffee quality
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Over the past few years I have often read about people buying/using TDS meter to evaluate the quality of their espresso, or to "tell them something" about repeatability etc. This little chestnut seems to do the rounds of the various coffee forums on an almost annual basis. I don't quite know where or why all this started, but it's time to put the myths to rest. Firstly, don't waste your money buying a TDS meter to evaluate anything other than what it's designed for, this might be your aquarium, for an RO system etc..It's definitely not for evaluation a product like espresso coffee. You would actually need some quite complicated pieces of equipment (not suitable for home use) to do this and even them I am not sure the results would be that meaningful.
TDS stands for Total Disolved Solids, unfortunately the name is a little misleading. A TDS meter measures conductivity of "dissolved" solids, but only certain types of dissolved solids.
- Sugars (not measured)
- Suspensions (not measured)
- Colloids/Lipids (not measured)
- Organics (not measured)
- Alcohols (not measured)
- Charged Ions in general metallic salts(measured) e.g. NaCl, KCl
Unfortunately coffee is a combination of lipids, sugars, complex organic compounds and very fine suspended particles, in addition to any charged ions. The meter really isn't going to measure any of this at all and the charged ion ratio is unlikely to change much, perhaps a little with volume and temperature. Even worse, the main seed for these charged ions will be the copper ions from inside the boiler of the machine and any existing hardness of the water.. also response to charged ions is not linear for most meters.
Buying a TDS meter to analyse your espresso or americano is a complete waste of money and will tell you nothing useful no matter how much you spend on the TDS meter.