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Re: Measuring TDS level with Ohm meter

On Sat, 27 Mar 1999, Andrew wrote:

>      I have read that it is possible to measure the amount of Total Dissolved
> Solids
> (TDS) in your tank using a DVM or analog VOM meter.  There is some fairly
> simple math involved to convert the ohms reading over to a TDS reading.
> Does anyone know how to do this?

Under the right conditions you would be able to measure the water
conductivity.  See my post earlier this month on determining salt levels
from conductivity measurements.  If you think that part is easy, then you
might be ready for the rest.

The approach is to build a cell with two electrodes spaced a small
distance apart.  You put that in the water, measure the resistance between
the electrodes and calculate the conductivity.  Ideally, I think the
resistivity in ohm-cm is equal to the resistance measured between two
electrodes each with a surface area of 1 cm^2, separated by 1 cm.
conductivity in microSiemens/cm (the standard reporting units) is
1,000,000/resistivity in ohm-cm.  

Nice ideal.  In reality, you have to come up with a cell size and shape
that produces very consistent potentiometric fields in the water
over the entire range or conductivity that your interested in, and the
resistance measured in the cell has to be calibrated against results from
standard solutions.

You have to make the electrodes from something that will not react with
the water, otherwise the patina that builds up on the electrode will ruin
the cell.  Older commercial cells use metal electrodes coated with
platinum that require replatinizing at regular intervals. New ones seem to
use some kind of ceramic that needs no maintenance.

The voltage you use when you measure the cell resistance has to be very
low. I don't know how low for sure, but it has to be low enough that
there will be no oxidation or reduction reactions at the electrodes and I think
that puts it down into the low fractions of a millivolt.  Normally the
cell is wired into a resistance bridge that lets you make precise
measurements at low voltage.

Roger Miller