# Re: Conductivity (was Measuring hardness)

```>>1) What unit does uS symbolize?

>>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.

...<some of this excellent answer snipped>...

>>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++.)

Given this thorough explanation of the relationship (or lack thereof)
between conductivity measurements and total hardness, I was wondering if
the following would work:

Would it be possible for Frank to calibrate his conductivity probe/meter
to HIS water conditions?  i.e. it seems to me that he could
simultaneously measure his water hardness for several different hardness
levels using an off the shelf test kit AND his shiny new conductivity
meter.  He could then set up a table showing what hardness level
corresponds to a particular conductivity reading.

I suppose this would require that the factors contributing to
conductivity in the water supply would maintain fairly constant levels
with respect to the Ca++ and Mg++.  If that were true, then a simple
calibration table could be generated to tell what the Ca++ and Mg++
levels were for a given conductivity reading.

just my \$.02.

Steve in Ann Arbor, where summer is beginning to fade....
```