Kaffebox APAX LAB TDS Meter
Produsent: Kaffebox
What it actually measures A TDS meter passes a small current between two electrodes and measures how easily the water conducts it. Dissolved minerals carry charge, so more minerals means higher conductivity, and the meter converts that into a parts per million figure. The important consequence is that it tells you how much is dissolved, not what. It cannot distinguish calcium from sodium, or hardness from alkalinity. Two waters reading 80 ppm can taste completely different in coffee if one is mostly bicarbonate and the other mostly magnesium. This is a quantity check, not a composition analysis. Where it earns its place in a water workflow If you build brew water from mineral concentrates, the meter answers the two questions that actually cause problems. First, is my base water really at zero? Filtered or distilled water drifts as a filter approaches the end of its life, and a base sitting at 15 ppm instead of 000 quietly shifts every recipe you mix on top of it. Second, did I dose what I think I dosed? A recipe targeting a known ppm gives you a number to check against, so a mis-measured dose shows up before you brew rather than after. It is also the simplest way to find out what your tap water is doing, which is worth knowing even if you have no intention of building water from scratch. Not a refractometer This is a common mix-up worth being clear about. A TDS meter measures dissolved solids in water, before brewing. A refractometer measures dissolved coffee solids in a finished brew, which is what you use to calculate extraction yield. They sound similar and do entirely different jobs. If you are trying to work out your extraction percentage, this is not the tool. Why won't this work to measure disolved coffee solids? Coffee's dissolved solids are mostly uncharged. What's actually in the cup is largely melanoidins, polysaccharides, caffeine, lipids and other organic compounds, none of which carry a charge in solution. They contribute essentially nothing to conductivity. The parts of coffee that do conduct are the mineral ions, potassium above all, plus the partially dissociated organic acids like chlorogenic and citric. That's a real but small and variable fraction of the total. So the meter is blind to most of the mass it's supposed to be measuring, and the ratio between the ionic fraction and the total shifts with roast level, origin, processing and brew method. There's no stable conversion to fall back on. The conversion factor is calibrated for something else entirely. TDS pens derive ppm from conductivity using a factor based on a reference salt solution, typically sodium chloride or the "442" mix. That's a reasonable model for tap water. It bears no relationship to the chemistry of brewed coffee, so even the ionic portion gets converted using the wrong assumption. The range problem is more immediate. Filter coffee lands around 1.35 percent TDS, which is roughly 13,500 ppm. The Apax meter tops out at 9,990. Espresso runs somewhere around 8...
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