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Re: [Killietalk] Water
Duane Wake wrote:
Hi all, I know this has been done before but I need a refresher, what
is the difference between conductivity and total dissolved solids
TDS? Any help will be gratefully appreciated. Duane
You received several excellent answers, Duane, so maybe it is
presumptious of me to try to add anything.
Unfortunately, they got a bit complicated without directly answering
your original question.
If you boil away (in a vacuum, for example) the liquids and weigh the
remaining residue, the ratio of that dry weight to the original
solution's weight is the total dissolved solids expressed as a ratio
(often in ppm or ppt).
If you measure the conductivity of the solution, it can be expressed as
inverse Meg-Ohms or micro-Seimens (both are electrical terms meaning the
The two, tds and uS, tend to track pretty well, and in most solutions
the uS is about twice the tds as measured with a meter (normal hard
water). Those readings are actually just 2 different ways of calibrating
a conductivity meter.
The importance to us is that conductivity (hence tds as measured with a
meter) have a profound impact on the osmotic pressure across cell walls.
Large, sudden changes may cause severe cell damage or dehydration, and
are the most common cause for the "pH shock" myth. This is a parameter
we should watch when changing water or introducing fish to new
surroundings. High accuracy isn't very important, as most fish tolerate
a sudden change of tds that is within a factor of 2. It's the
*difference* in tds (uS) that is important.
The meters are really cheap and much more useful than a lot of the
titration tests sold by the LFS.
Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514
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