# Re: Aquatic Plants Digest V2 #870

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> 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.
>

Its a pretty expensive purchase for that purpose.  You may want to take
it back after you read on.

> 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 (mu S) is microsiemens, which is the same thing as a micromho per
centimeter (you will sometimes see those units used, rather than
microsiemens), which are both the inverse of ohm-centimeters.
Ohm-centimeters measure resistivity.  Its inverse is the conductivity or
specific conductance.

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

For most water, you can mulitply the conductivity in microsiemens by 0.60
to get a good estimated of TDS in ppm or mg/l.  To do that, the
conductivity measurement must be made at 25 degrees C, or corrected to
that number.  Many conductivity meters automatically correct to that
value, some do not.  If yours doesn't include a temperature sensor, then
it doesn't automatically correct for temperature.  If it does include a
temperature sensor, then it may still not actually do the correction.  The
correction is about +2 % per degree C.

If your aquarium is at or near 25 C (77 F) don't worry about temperature
corrections.  Suppose you measure the conductivity at 200 uS, and your
temperature is 85 F (29 C, 4 degrees C above the 25C standard value):

the conductivity at 25 degrees C is 200+0.02*200*(-4) = 184 uS

the 0.02 is the 2% per degree correction, the -4 is the temperature
difference (negative because 25 is lower than 29).

Your TDS is then given by

TDS = 0.60*184 = 110 ppm.

Without the temperature correction your TDS would be estimated at 120
ppm.  The difference is usually not a big deal.

Both the 0.60 factor and the temperature correction vary somewhat with
salinity and details of the water composition.  For fresh water (say with
a conductivity below 1000 uS) the 0.60 factor is very good.

The 2% temperature correction is fairly constant.  It does vary some with
composition, but for small corrections, the variations just aren't
important.

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

Here's the big hit.  You can't.  "Hardness" is an often misused term.  If
someone told you that you could use it to measure hardness, then they
were probably using the word to describe mineral content - same thing as
TDS.  Conductivity responds mostly to the amount of the salts in the water,
with very little sensitivity to the kind of salts present.  Ca and Mg
(hardness) create about the same effect as Na or K.

If you want a fast electrical measure of hardness, you need an ion selective
probe, and lots of \$\$\$ to pay for it.