Re: [APD] GH versus PPM

[Liz Wilhite <satirica at gmail_com>]

On 9/26/05, Tom Wood <tomwood2 at flash_net> wrote:
> I bought a TDS meter with my new RO system. Input is 470ppm and output is
> about 10ppm. I understand that the TDS meter is just a conductivity
> measuring device with a built-in conversion factor. However, my GH test kit
> is saying that the water is two degrees 'hard' or about 38ppm when the TDS
> meter says it's 220ppm. I've read the posts about GH being only 60% of TDS
> but this is way off. My KH test versus pH test is also way off, my fish
> should be officially dead from CO2 poisoning. I use Kent RO Right to get
> the RO water's TDS ppm to where it is. Something is messing with the
> chemistry in a way that affects the titration (Aquarium Pharm.) test kits.
> Any ideas?

Yeah.  You are correct that a TDS meter is a conductivity meter. 
There is a pretty reasonable discussion (that you've probably already
read but someone else might not have read) here: 
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.php .

Using a TDS meter is not necessarily a trivial procedure as the kind
of ions (as well as the concentration) present have a strong influence
on the conductance you measure.  The upshot of that is that the built
in conversion factors may have little meaning (note the use of the
word "may" as the seem to vary from manufacturer to manufacturer). 
There are also calibration issues.

If you want to know where the problem is you can test the RO water
with both the probe (it should be about 0) and the KH kit, add some
baking soda and test with both.  If you get the results you would
expect then its likely that problem is all the other ions in the Kent
solution.  Just an FYI but the local fish store here strongly
recommends that well water people who use RO water just add back in
some well water rather than using RO Right.  The owner claims there
are less problems with both fish and plants that way. YMMV.

Liz

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