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Re: TDS readings


On Thu, 28 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.

When did "Maybe" become an obvious disagreement?  Conductivity meters have
a number of uses.  "Maybe" other people will find the meter to be "most"
useful for something else.

> > > 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.

Pardon me?

First, the original question was over the use of TDS values from a TDS
meter.  The method you describe here is based on conductivity values.  I
don't know that K9AUB's meter even gives conductivity readings.

Second, a conductivity meter measures conductivity, not hardness.  The
method you describe below allows you to *estimate* hardness from a
conductivity reading and other data about the water sample from other
sources.  That is not a measurement of hardness and it isn't (in an
analytical chemistry sense) a determination of hardness.

> 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 5S->TDS and TDS->hardness in their quality reports. Is it then
> difficult to make the leap from 5S->hardness?)

Public water utilities in the US are required to provide their users with
some information about their water quality.  I don't think the conversion
between TDS and conductivity is among those things.  In fact (with some
state to state variation) I don't think that either TDS or conductivity
are commonly regulated under health standards, so the utility may not
have to provide either of those.

> 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 5S 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.

Do you grow plants in that water?

> 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.

You're asking for quite a bit of accuracy out of your meter.  I use a
rather expensive professional-quality conductivity meter in field work.
It's kept clean and its calibration is checked regularly.  Despite that I
don't expect it to produce results as accurate in the very low range as
you want out of yours.

I've used conductivity measurements (generally laboratory measurements) to
estimate a lot of things over the years and in many instances I've had the
opportunity to check the estimates against actual analyses.  Estimates
based on conductivity are *not* generally accurate.

I think hobbyists who need to know about their hardness will be much
better off with reagent test kits.  An inexpensive Tetra kit will tell you
whether the water is in the range of 0-9 ppm or 9-18 ppm (0.5 degree
increments).  If you use your own glassware you probably can get it to
read down to 0-4.5 ppm , 4.5-9, 9-13.5 and 13.5-18 ppm (quarter degree
increments).

> 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...

Conductivity meters are quite commonly used for monitoring gross changes
in water quality and I agree that this is a good use - certainly a better
use than for estimating hardness.  Still, you need to make sure the meter
is well calibrated or what looks like a change in your water quality may
just be a change in the meter calibration.  Or the temperature. Or
something else...


Roger Miller