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Conductivity (was Measuring hardness)

>Date: Fri, 1 Aug 1997 19:34:18 -0700
>From: "Frank I. Reiter" <FIR at istar_ca>
>Subject: Measuring hardness
>I have near my a shiny new conductivity meter, which I purchased for the
>purpose of quickly and easily (and therefor frequently) measuring the
>hardness of my freshwater aquariums.
>The instructions which came with it were disappointing - not a mention of
>hardness or TDS anywhere.  The unit measures conductivity in units of uS. 
>I turn again to the collective knowledge of the list:
>1) What unit does uS symbolize?

uS = microsiemen, which is a unit of conductance, *not* conductivity.  Are
you absolutely sure this is what your meter reports?

Conductivity, what you want, is expressed in uS/cm.  The difference between
conductance and conductivity is that conductance depends on the geometry of
the cell being used to take the measurement, while conductivity does not.
Therefore, conductance is useless for comparative measurements unless all
measurements are made with the same cell.  Not likely!

If your meter is truly reporting conductance (uS), and not conductivity
(uS/cm), then before you can get anything resembling useful data from it
you will need to use a standard KCl solution to determine the cell constant
of your meter.  (Unless the manufacturer supplies this info.)  Given the
cell constant, you can convert conductance (uS) to conductivity (uS/cm).

>2) How can I convert that unit to ppm of total dissolved solids?

In general, you can't.  There is no direct relationship between
conductivity and TDS.  I could easily give you a water sample with a very
high conductivity which also has near-zero TDS.  Conversely, I could just
as easily give you a water sample with near-zero conductivity, but which
has very high TDS.  

Remember that Total Dissolved Solids means just what it sounds like.  It is
the TOTAL amount of ALL SOLIDS dissolved in a sample.  There are
potentially many substances in the aquarium which contribute to TDS but do
not affect conductivity, and vice-versa.

Now, there are tables, and even pre-calibrated meters, available which
purport to give you TDS from conductivity.  However, you should keep in
mind that these calibrations are performed using a particular set of
standard solutions.  Often just a set of KCl solutions.  So, unless the
sample you are measuring has exactly the same composition as the
calibration standards, any result you get is at best an estimate of unknown

>3) How can I convert either of the above numbers to degrees of hardness?

For the most part, the answer to this is the same as it was for #2.  That
is, in general you can't.  There are many substances in the aquarium which
will affect your conductivity, but which have no effect on conductivity.
Again, there simply is no direct relationship between conductivity and
hardness.  (Note: I'm assuming here that by "hardness" you mean the total
concentration of Ca++ and Mg++.)


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