[This was meant to be an edit to another post ... don't know what I did to make it turn up here!* Ah well, same subject, now I'll need to find the one I meant to edit!]
[EDIT2: Well, I sort of know how it might have messed up: I originally edited here, but moved it to a better place, then edited the post but was actually editing what I didn't post ... Eh? ... You lot are so lucky you don't have to live in a head like mine!]
I've been working hard on my "water mineral" pre-processor ... the "Defuddler" (hey ... I can hear the groaning from here you know). Expanding it, making it simpler, making it more accurate, etc. I've been doing much of the work on the sister site to this one "HomeBrewTalk.com" 'cos there's more of them (it's an American site, they're a bit more "critical" (so I learn more about what I'm doing) and the have some "difficult" waters (often very "Hard" and also with a lot of Sodium which may be due to the ion exchange plants they operate out there.
So, this is a quick catch up. It has not outwardly changed much...
But it now relies totally on "calculated" values for "Alkalinity". The published figures would never be significantly different.
The "engine" that drives it is very simple, and I'm still trying to figure out why it's not so prominent in home-brewing It is based on the formula:
Total Alkalinity = āconservative-cations - āconservative-anions
And you can read about (in mind-numbing detail) here in
Wikipedia. It might be all the talk about "conservative" ions, combined electrical charge balancing and proton balancing, and so-on. Don't know about anyone else, but it used to scare the pants off me. It's all jargon, you only need to know a snatch, and it's all so simple. Anyway, a diagram should illustrate what the "Defuddler" is handling:
The "Conservative Anions" plus the "Alkalinity Anions" will equal the "Conservative Cations". The ions will need to be converted to "equivalents" (such as milli-Equivalents per Litre - mEq/l) to make this easy (all done by the sreadsheet, so notthing to learn). There will be numerous other ions in treated drinking water, but the main players are listed in the diagram (quantities less than 1mEq/l can be safely ignored). "Nitrate" at about 1-6mg/l is probably the smallest ion quantity to be counted. Potassium may make an appearance but is of no great importance and lost amongst the sodium
This diagram illustrates how the calculations can do what they do ... make very tight estimates of Alkalinity. Which in this environment will be "Carbonate" Alkalinity (hence represented "as" bicarbonate ions).
Now this diagram might be parallelled by a "Hardness" version. I must make it clear so this following diagram doesn't trigger any mis-comprehensions comparing it with the "Alkalinity" version:
IT IS NOT CORRECT!!!
This suggests "Temporary Hardness" is the same as Alkalinity. IT IS NOT! But you often see them quoted (incorrectly!) with the same concentrations in water reports. What is obviously missing is the Sodium (and Magnesium but that can be roughly extracted because it is in there). Hardness has no concept of "Sodium" and assuming "Temporary Hardness" is the same as "Alkalinity" can cause problems, especially when the water harbours sizeable quantities of Sodium (I always used to think of them as the same too). But if careful we can extract useful information from "Hardness". As will be seen!
[EDIT: Changes to diagrams to make them clearer.]