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Re: measuring the amount of salt

On Wed, 17 Mar 1999, Hugo Hoekstra wrote:

> What is the best way to measure the amount of salt in water? I haven't seen
> test kits for it here yet.

I don't think there is a test kit for salt content.  That's what
conductivity (aka specific conductance) meters are used for.  I've always
thought that a conductivity meter would be a great DIY project for an
electronically handy type.

The ability of water to conduct an electrical current improves as the salt
content increases; in fresh water that increase is very nearly linear with
increasing salt content.  The ability to conduct current is measured by a
conductivity meter.  The step from measured conductivity to salt content
is a little difficult, as this also varies with temperature and somewhat
with the kind of salts dissolved in the water.

Very basic conductivity meters can be had for $200 US, maybe less.  Meters
that also measure temperature cost slightly more; meters that
automatically correct the conductivity for the temperature cost a little
more than that.  The nice, idiot-proof, German-made meter I use at work
cost about $850.  Some are also set up to read in salinity units (ppm or
ppt dissolved solids), usually assuming a seawater composition.  The
latter is generally not correct for fresh water, but the result is still
qualitatively useful.

In order to make a conductivity reading quantitatively useful for
determining salt content the conductivity in microSiemens per centimeter
must first be corrected for temperature then the corrected reading must be
multiplied by a factor appropriate to the kind of salts dissolved in the

The temperature correction is close to +2% per degree C, and the
correction is from the temperature of the water the reading is made in to
a standard temperature of 25 degrees.  The correction varies somewhat with
the kind of salts in the water, but tropical aquariums are usually kept
near enough to 25 degrees C that it doesn't make much difference.

In the second case, the multiplying factor is usually near 0.6 for
fresh water (increasing as the water becomes brackish) but it can
vary somewhat depending on the kind of salts in the water.  My tap water
is an extreme case because it contains rather a lot of silica, which
increases the dissolved solids ("salt") content but has no effect on
conductivity.  In my case the multiplying factor is about 0.7.  Your water
will of course be different.

Roger Miller