A £10 TDS pen does not measure dissolved solids — it measures electrical conductivity and multiplies by a guess. For tap water that is a useful guess; for near-pure blending water, it is a number that moves the moment the cap comes off. Here is what the meter can and cannot tell you, and why a trade buyer should ask for a conductivity figure at 25 °C instead.
The same water can be described as 150 mg/L, 15 French degrees, 8.4 German degrees or 8.8 grains per gallon — and be classified "hard" by an American agency and "moderately hard" by a British one on the same day. Hardness is not one measurement but a family of conventions, and a buyer comparing water reports across markets is usually comparing units before comparing water.
Drinking-water science says demineralised water is a bad idea — flat-tasting, mineral-poor, and corrosive to pipework. Regulators agree, and several set a minimum mineral content for treated drinking water. So why does every serious proofing specification run in the opposite direction, toward zero? Because the water in a whisky bottle is not the water in a glass, and the arithmetic that governs one has almost nothing to do with the other.
Every bottle of blending or finishing water carries two numbers on its spec sheet — pH and TDS. Here is what each one actually measures, what "good" looks like for drinking versus for whisky, and how a non-technical buyer can read a spec at a glance.
Total dissolved solids and hardness are the two numbers that decide whether water disappears into a dram or leaves a fingerprint. Here is what they mean, and why the water that tastes best in a glass is the wrong water for whisky.