Re: TDS meter: which one ?

[George Slusarczuk <yurko at warwick_net>]

Hello Wolfgang,

ALL conductivity meters measure the same thing -- conductivity. The
difference between the different models is
a) range
b) units in which the meter is calibrated.

The so-called TDS meter measures conductivity and uses a conversion
factor for a "typical" water to express the result as Total Dissolved
Solids.

Water hardness has almost no relationship to conductivity, exept that
hard waters have a rather high conductivity. BUT water softened by ion
exchange (the regular household water softener) will have high
conductivity and practically no hardness! A conductivity meter will give
you absolutely wrong "results".

The situation is not hopeless, because the relationship between water
hardness of a particular water and conductivity is usually linear. So,
once you measure water hardness by titration (or get that value from
your water supplier) and then measure conductivity, you obtain that
"fudge factor" -- water hardness vs. conductivity.

Because measuring conductivity is fast and simple, it is practical to
measure it instead of water hardness: If the conductivity of your
prepared water is half that of raw water, the hardness is also one half
of the value you measured. If conductivity is 1/10th, the hardness is
1/10th. 

THIS IS TRUE IF you are using either RO water or distilled water for
dilution. If you are using ion-exchange softened water to dilute your
tap water, a conductivity meter will not help you! You just have to keep
the dilution factor straight. (A conductivity meter is also VERY useful
in transferring fish to a different water/tank, so you don't give them
osmotic shock!)

So, determine the range in which your tap water falls and buy that
meter. If you are on a municipal system, then ask the supplier about the
range of water hardness during the year -- it can change substantially!
I prefer calibration in µS/mS, because then your know WHAT you are
measuring!

Best,

George



Wolfgang Schickler wrote:
> 
> I am thinking about buying a TDS meter to monitor
> my RO unit and to take the guesswork out of
> preparing/mixing water for my tanks.  After looking
> what is available, I am somewhat confused....
> 
> There seems to be different units out there
> measuring different things. For example here is a
> list of TDS tester from OAKTRON:
> Choose TDS 1 & 3 for testing water hardness,
> hydroponic fertilizer concentrations and chemical
> concentrations. TDS 2 & 4 are perfect for measuring
> low range-salinity in koi ponds, aquariums,
> recirculating systems and anywhere conductivity
> measurements are required. TDS10 measures dual range
> ppm/ppt. TDS20 measures dual range µS/mS.
> 
> Can anybody shed some light on this? Can anybody
> recommend a good TDS tester that is appropriate for
> breeding killies? It is my understanding that
> hardness (Kh and GH) is most important.
> So the question is, do I really want a TDS tester,
> or is a hardness test kit good enough?
> 
> Any help is greatly appreciated. Thanks.
> Wolfgang
> 
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References:

• TDS meter: which one ?
• From: Wolfgang Schickler <killiwolf80908 at yahoo_com>

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