MediumRoastSteam ppm stands for parts per million, which is the same as mg/L as ion (a million milligrams in a litre/kg). There are multiplication factors for the calcium carbonate scale, so things tend to drift there. Plus, if you’re using a conductive TDS tester, it’s not measuring hardness, nor alkalinity, it’s measuring everything in the water organic & non organic, converted from a conductivity scale…of which there are three different ones in common use.
For example my tap water measures 312ppm (or mg/L) TDS using my conductive TDS meter.
It has 104mg/L Calcium as ion, almost no magnesium, giving 260mg/L total hardness as CaCO3.
230mg/L bicarbonate, which converts to 190mg/L alkalinity CaCO3.
Water bottle labels don’t ever seem to be ‘as CaCO3’, mains supply reports usually are, but you can sometimes get the ‘as ion’ values from an in depth report. Mains suppliers almost never give TDS in ppm/mg/L, more commonly giving conductivity in ‘us/cm’ (whereas water bottles state TDS as ‘dry residue’ after dehydration, in mg/L - Chase Spring is 220mg/L, or ppm).
So it’s simple…once you know which scale and which parameter people are talking about. :-)
I didn’t even get into degrees Clarke, German, or French…maybe another time, after people’s eyes have stopped glazing over…:-)