A "Water Hardness Meter" ... :
I've spent days ... weeks .... it's probably months, figuring out how to wean people off "water hardness". An arcane description that predates any understanding of chemical elements. That's been updated to comply with modern knowledge, but I don't know why. WE DO NOT NEED IT, except perhaps some who've been bought up using it and understand its quirks. For everyone else it's a complete and utter waste of time.
You will not understand it and make the most ridiculous errors trying to use it.
Its greatest use for modern brewers ... is so some can prance about and show off how much they know and laugh at the hoards of underlings failing to grasp its basic principles.
Do yourself a favour, forget it. Most use American water calculators for judging water additions: Most of them (e.g. Bru'n Water) work without any "Hardness" inputs - they'll be present, but you can just ignore them. Along with its stupid hangers-on like "CaCO3" *** (if you think that means "calcium carbonate" you've already been overcome by the "water hardness" trite). "Water Hardness" is especially stupid in America because many supplies are "softened" which makes trying to do calculations based on water hardness 10x as baffling (and probably quite impossible to derive any useable information from).
I'm just one of a long line of homebrewers who have tried to wean people off "water hardness". I'll fail, making way for more to have a go. Meanwhile, I advise as many as who'll listen to step away from it and concentrate on brewing good beer.
*** Unfortunately, "Alkalinity", a useful subject to know a tiny scrap of, uses Hardness's "CaCO3" as a unit of measure. Although many sources are now changing to "bicarbonate" ("HCO3"); which is still daft, but nothing like as daft as "CaCO3". You can even treat "HCO3" as if it really is "bicarbonate". Eh ... okay ... hands up who thought you could do the same with "CaCO3" ... no ... put your hands down again, if those creeps who feed you all the junk about water analysis see you, they won't be able to stop sniggering.