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Re: TDS readings



> Date: Wed, 27 Dec 2000 10:01:15 -0700 (MST)
> From: Roger S. Miller
>
> On Wed, 27 Dec 2000, David Youngker wrote:
>
> > A conductivity meter is _most_ useful to those breeding
> >and rearing the delicate soft water fishes.
>
> Maybe.

Obviously you disagree. And even as a single- word response, this is rather
terse.

Perhaps you could share a few thoughts here - at least give us an idea as to
where you're headed?

> > It's an extremely handy device for determining hardness values at
> > resolutions that a hobby- grade reagent kit simply can't handle.
>
> TDS meters don't measure hardness.

Au contraire, mes ami.

The key is in knowing the _makeup_ of the dissolved solids in the water. TDS
is a gravimetric measurement that not only includes truly "dissolved"
solutes such as salts, but those classes of suspended particles that meet a
"definitional" requirement of "dissolved" - i.e., those whose particle size
is small enough to _remain_ in suspension. Like turbidity agents.

I will obviously grant you that a conductivity meter will _not_ tell you the
hardness of an unknown source or testing sample. Some salts conduct better
than others, some particulates do not conduct at all, etc., etc.. It's just
a matter of doing the empirical work to establish the "local" conversion
factor. (BTW, your local water utility will list a local conversion factor
for both ÁS->TDS and TDS->hardness in their quality reports. Is it then
difficult to make the leap from ÁS->hardness?)

But, starting with water of _known_ purity, and adding chemicals of _known_
compositions in no way restricts the use of the meter for such
determinations.

So, when I set up something like a breeding tank for D. filamentosus, then
I'm shooting for mid-4 pH values and _total_ hardness values of less than 1
dGH. None of my hobby kits can give me this sort of resolution. A pH meter
gives me the acidity values, and a conductivity meter gages my "dilution
ratios". Starting with a premix of 3:1 Ca:Mg salts, I can dilute to conducti
vity readings in the 10-15 ÁS range. Using a conductivity conversion factor
of ~0.74 (the range of values is ~0.6 - 0.9, dependent on composition) for
the types of salts I'm using tells me that the water is about 7-12 ppm
hardness. Add to peat and I'm in business.

Hard to do with a kit whose resolution is a half- degree at best. And I get
the added bonus of having a clearer picture of the osmotics involved.

At that point, the meter becomes even _handier_, although admittedly _not_
for testing hardness. It then becomes my primary gage for water *quality*,
as DOCs and other organics and salts increase and conductivity rises with
them...

-Y-