Lets get one thing straight first of all:
All these tests for hardness (KH or GH) are measuring the EQUIVALENT of calciumcarbonate or calciumoxide or something like. Maybe we should call this "idealized measurements"[1]. They don't actually differentiate the different types of actual salts, so for example magnesium carbonate will be equivalent to calcium carbonate [and technically over-read in a sense of weight of compound per volume water, since magnesium atoms is lighter than calcium atoms - not that it makes a whole lot of difference, since one magnesium is as bad (or good) as one calcium from a conductivity/hardness point of view - it just weighs less when you have it on a scale].
By the way, the measurement of calcium carbonate/calcium oxide is based on converting the calcium carbonate to calcium oxide - all carbonates break down at 180'C into oxide and carbon dioxide, so CaCO3 will turn into CaO and CO2 at 180'C. [This is part of the production of cement, and old-fashioned lime-based mortar/plaster is simply calcium oxide that when mixed with water form calcium hydroxide - and the formation of calcium carbonate by taking up carbon dioxide from the air to form a solid compound].
GH is a total hardness measure - and the measurement is converted into equivalent terms of either calcium carbonate or calcium oxide (or perhaps something else), KH is a "buffering" measurement, which is how much (bi)carbonates you have in the water.
Today, the measurment is pretty much standardized for ppm or German scale (D(g)H).
So, what I'm trying to say is that the test will "idealize" the measurment - and the scale of the measurement is different depending on the country of origin of the scale (and what it actually tries to measure).
There are similar discussions when measuring nitrous waste in water - if you actually want to know how much NITROGEN the water contains, rather than nitrate, nitrite and ammonia - as they are all nitrous waste-products, and the measure is more accurate of how much waste you have if you only count the nitrogen part, not the hydrogen and/or oxygen that is tied to it. And some test-kits will give nitrogen, others will give nitrate in it's reading of ppm (althrough the latter is the by far most common). This can make a big difference, as one N is tied to three O's in nitrate, and they are of almost equivalent weight (N is 14, O is 16 in atomic weight). So for example 14 ppm NITROGEN would be 14 + 16 * 3 => 62 ppm NITRATE...
[1] Based on the case of "In an ideal world, all hardness is X", where as "in the real world, hardness consists of X, Y, Z, W, etc."
I hope this helps, and that you don't have to think about it for too long...
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Mats
:
Maybe have both? Any Aussies out there reading this? Could you confirm what you guys would say?
