Re: [APD] GH vs. PPM

[Roger Miller <roger at spinn_net>]

On Monday 26 September 2005 21:03,  Tom Wood wrote:

First, make sure that your TDS meter is calibrated.  Inexpensive meters 
require frequent calibration and without it they can produce useless results.

There is no meaningful relationship between GH and TDS.  It is even possible 
for the GH (measured as ppm CaCO3) to be higher than the TDS.  That is an 
extreme case.  More commonly the GH can be quite low while the TDS is high.  
Up until a couple years ago my tap water flowed with about 24 ppm of GH and 
500 ppm of TDS.

I can't comment on your KH/pH problem because you didn't describe it.

With all due respect to the manufacturers, it is a waste of money to use a 
chemical product to reconstitute RO water unless your tap water is *really* 
out of whack.  Just mix your RO back into your unsoftened tap water to get 
what you want.

If there's really something messing with the results of your titrations then 
it is most likely to be either the quality of the test kit or an inattention 
to detail offered by the analyst.


Roger Miller


> 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?
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