# gh, Ca and Mg calculations...



## Bert H (Mar 2, 2004)

I just want to be sure that what I have been calculating in the past is correct. Recently some posts regarding Ca/Mg have made me wonder.

Using Edward's formulas: 
12(17.86) - 2.5 (80) /4.1 = 3.4ppm Mg

Therefore, I have believed I have essentially no Mg in the water then. Do the math whizzes out there agree or not?


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## Salt (Apr 5, 2005)

I'm not totally understanding that formula.

What are your test readings? For example, I use the Lamotte GH kit and the Lamotte Calcium kit. Both are almost the same, but the Calcium kit's reagent contains an inhibitor that stops magnesium from showing up on the test. Both kits' readings are in a CaCO3 equivalence.

If I take a GH reading and get 60 ppm, and a Calcium reading and get 40 ppm, then I conclude that my magnesium level is 20 ppm as CaCO3. To convert that to *just* magnesium, divide by 4.11796. So that would equal 4.86 ppm of magnesium.


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## Raul-7 (Feb 4, 2004)

Magnesium = dGH - Calcium

Mg ppm = ((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1

Going by your values-
dGH = 12
Ca = 80 ppm

Thus,
Mg = 3.5 ppm

Bert, going by the Fertilator 2-5 ppm is the recommended range.


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## Bert H (Mar 2, 2004)

> Bert, going by the Fertilator 2-5 ppm is the recommended range


So is the recommended range more important than the Ca:Mg ratio? I have been dosing based on the disparity of the ratio.


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## Salt (Apr 5, 2005)

Raul-7 said:


> Magnesium = dGH - Calcium
> 
> Mg ppm = ((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1
> 
> ...


That's only correct if his calcium reading is in a calcium equivalence. If his calcium reading is in a CaCO3 equivalence, then the 3.5 ppm result is WAY off.

The "2.5 x Ca ppm" (2.497303 to be precise) translates a calcium equivalence into a CaCO3 equivalence.


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## Bert H (Mar 2, 2004)

> The "2.5 x Ca ppm" (2.497303 to be precise) translates a calcium equivalence into a CaCO3 equivalence.


From what I read in the Hagen Ca test, that's what I am measuring. So the calculation should be correct.


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