Re: [Killietalk] TDS vs Conductivity

[LeeH920226 at aol_com]

In a message dated 2/2/06 1:14:25 AM, ron_schulz at gmail.com writes:

<< What is the relationship between conductivity and TDS?. I have a Hach
conductivity meter which consistently measures my well water at 270uS/cm.
The hardness of my water is consistently around 130ppm as CaCO3  measured
with a Lamotte test kit.  >>

With my water which contains (according to the water company) sodium 14 ppm, 
potassium 3.7 ppm and calcium 24 ppm I calibrated my TDS meter (Hanna) against 
my conductivity meter (Pinpoint) and the relationship is consistently 0.67 
TDS/conductivity or 1.5 Conductivity/TDS. It will vary somewhat depending on 
what cations are present. Potassium chloride is what they use for calibration and 
that ratio is 0.5 or 2.0 depending on which fraction you use. Now, you might 
wonder, if my TDS is about 200 ppm what are the other ions? I have no idea. I 
also think the divalent ions like calcium contribute twice as much to 
conductivity. Also they do not test for iron (trivalent) but I don't have any evidence 
for iron in my water supply. The anions add up to more than the cations but 
still not 200 ppm. There are other anions possible, like ammonium which they do 
not test for. I don't find any either. As Wright says, it should not be a 
worry, just pay attention to differences between your own operations -- like 
adding water to tanks or wadding fish to tanks and also to whether it is high, 
medium or low. I classify my own water supply as medium and doctor it for those 
SAA that need low TDS and for those Tanganyikan fish that need higher.

Lee Harper
Media, PA
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