Originally Posted by banderbe
But isn't dGH also equal to the Total Hardness in ppm / 17.8 ?
His formula is for readings which are not in a CaCO3 equivalence.
EDTA hardness chelation tests read in an equivalent of CaCO3. That means when you do a GH test and get an "as CaCO3" ppm reading, it means, "if the only thing dissolved in the water is CaCO3, this is how much CaCO3 there would be in ppm."
"Degrees of German Hardness" is a standard that is equal to 10 ppm of calcium oxide.
Calcium weighs 40.078 grams per mole. Oxygen weighs 15.9994 grams per mole. Therefore, a degree of German Hardness is 71.46908 % calcium and 28.53092 % oxygen, or 7.146908 ppm of calcium.
Since GH test readings are in CaCO3 equivalence, you have to account for the amount of carbonate that will be present along with the 7.146908 ppm of calcium. Carbonate weighs 60.0089 grams per mole. Therefore, calcium carbonate is 40.04320 % calcium, and you would need 2.497303 milligrams of calcium carbonate to raise the calcium content of the water by 1 ppm per liter. (1 / .4004230). Since we know a degree contains 7.146908 ppm of calcium, then 2.497303 x 7.146908 = 17.847993 ppm of calcium carbonate in 1 degree.
There are different ways to apply this to tiny's readings. If the readings are not
already in a CaCO3 equivalence, then we don't know exactly what all the calcium compounds are in his water. What Raul-7 posted is a way to translate everything over to calcium carbonate. (That's one of the things I like about using RO/DI water... you actually know and control what compounds the GH comes from.) If tiny's calcium levels are 77 ppm, then you would need 192.29233 ppm of CaCO3 to achieve that level of calcium.
Another thing about GH tests is that magnesium cations react the same on these tests as calcium cations do. The atomic weight of magnesium is 24.305 grams per mole. That's 60.64424 % of what calcium weighs. Since 1 degree of hardness for calcium is 7.146908 ppm, then 60.64424 % of that is 4.33419 ppm of magnesium in 1 degree of German hardness.
Again, applying this to tiny's magnesium reading of 29 ppm as a calcium carbonate equivalence is a bit of a conundrum, since we are assuming that the magnesium containing compound that is present is calcium carbonate, which of course contains no magnesium.
Since it takes 1.64896 times as much magnesium to make the GH test read 1 ppm as CaCO3 (40.078 / 24.305), we need to multiply the 2.497303 milligrams of CaCO3 it takes to raise calcium by 1 ppm by 1.64896, which equals 4.11796. So 29 ppm of magnesium as read as calcium carbonate on a GH test would equal to 119.42084 ppm.
The 119.42084 ppm + 192.29233 ppm = 311.71317 ppm of CaCO3, or 17.46489 degrees.
To get a much better idea of the actual "total hardness," you would really want to take a TDS (total dissolved solids) reading using a conductivity meter
If tiny's water report is like yours (and it probably is), then the readings are already in a CaCO3 equivalence, and the above does not apply. In that case, the GH would be 108 ppm as CaCO3, which is 6.05110 degrees of German hardness, with a 2.65517:1 calcium:magnesium ratio.