From: fernando@aplcomm.jhuapl.edu (Fernando Pineda)
Subject: Re: dKH to ppm CaCO3 conversion
Reply-To: fernando@aplcomm.jhuapl.edu (Fernando Pineda)
Organization: Johns Hopkins University/Applied Physics Labs
Date: Mon, 12 Oct 1992 19:34:09 GMT

In article <1992Oct12.164047.4357@javelin.sim.es.com>, 
krogers@javelin.sim.es.com (K. Rogers) writes:

> 
> fernando@aplcomm.jhuapl.edu (Fernando Pineda) writes:
> 
> >I was looking at Martin Moe's Marine Reference book and I'm puzzled by
> >his dKH conversion (cf. p. 36) . Sea water should have ~400 ppm Ca+2 ions or 
> >equivalently, ~1000 ppm CaCO3. Now if I take his conversion to dKH I either 
> >divide ppm CaCO2 by 17.9 or multiply by 0.05 (I'll forgive his approximation 
> >0.05 ~1/17.9).
> 
> [calculations deleted]

> krogers@javelin.sim.es.com replies
> 
> You're confused on the various hardnesses.  You're trying to equate
> calcium hardness with carbonate hardness.  Both are measured as CaCO3
> by the test kits available.  The 17.9 conversion is for carbonate
> hardness.  To confuse things even more, total hardness is also
> expressed as equivalent amounts of CaCO3.
> 
> Generally speaking, when "hardness" is mentioned with respect to
> seawater, they're refering to carbonate hardness.  That hardness is
> some 7 german degrees or 3.5 meq/l in "standard" seawater.
> 
> Calcium hardness is measured to insure sufficient calcium for inverts
> to use.  As you cited, a value of 1000 mg/l CaCO3 = 400 mg/l Ca ion
> which is a good value to keep.  I keep my tank between 400-500 mg/l.
> The Halimeda seems to do better when things are closer to 500 but I
> also find more limestone deposits at those levels as well.
> 
> It took me a long time to get the hardnesses straight in my mind, and
> I'm still not sure I have a complete understanding of them.
> -- 


You bet I'm confused and I'm still confused. Let me pose the question another 
way:

 The La Motte test kit gives me a reading of ~400 ppm CaC03 for freshly mixed 
Instant Ocean. This is what I expect from Moe and Spotte and all the other 
references. Now I want to convert this to a dKH so that I can compare to 
comments made in Thiel's books. I won't prejudice the answer by suggesting a 
way to do it.  Just show me a calculation that gives me an answer that's 
consistent with Thiel and other sources, or give me a reason why it can't be 
done. Then I'll go away enlightened and happy.


P.S. 

Here is what the FAQ has to say about it:

The hardness of the water refers to the amount of dissolved
"hard" metallic ions such as calcium and magnesium.  This is
measured in dH. and GH. ????

dH - degree of hardness.  Usually denotes general hardness.  It is
     equivalent to 10 mg oxide of calcium or magnesium in 1 liter
     of water or 1dH = 17.9 ppm.
KH - carbonate (and bicarbonate) hardness.
GH - total (general) hardness.  This is not always equal to the sum
     of the carbonate and non-carbonate hardness because of the
     differences in the way all of these things are measured.

In general, what you need to know about hardness is the following:
     0-4 dH  -- very soft
     4-8 dH  -- soft
     8-12 dH -- medium hard
     12-20 dH -- hard
     above 20 -- liquid rock (Lake Malawi and Los Angeles, Ca.)



Fernando Pineda (fernando@aplcomm.jhuapl.edu)
Newsgroups: sci.aquaria