Re: [Killietalk] TDS vs Conductivity

[Wright Huntley <whuntley at inkmkr_com>]

Unless your roof is contributing something to the water (or steel mills 
up wind) that rain conductivity sounds a tad high.

Conductivity and tds usually correlate well, with uS about 2X the ppm of 
tds. [Depends a bit on how the tds meter was calibrated, but we don't 
care about great precision.] In your case, it sounds like most of it is 
hardness, with little salinity or other factors that also add to 
tds/conductivity. Hardness can be from any divalent ions, so Mg++ or 
Fe++ could well be in the mix.

In most of the US, where ancient seabeds provide the hardness, I think 
that Ca++ is usually about 5X higher than Mg++, but Fe++ must be below 
0.3 ppm or EPA goes non-linear. [Iron is a taste amplifier, and a few 
ppb can make chlorine or other tasty additives pretty obnoxious.]

With regard to stresses from osmotic shock, I tend to use a factor of 2 
as my limit, in either direction. Some can stand a lot more, and a guppy 
can jump from 100 ppm water directly into a full-strength sea-water tank 
with little harm. I somehow doubt if most rain-forest species would fare 
as well.

Brian Watters has pointed out that going from low tds water to higher 
tends to be less stressful than sudden changes the other way. That 
salmon hover for a day or more before going upstream to spawn suggests 
they take that long for their cell barriers to adjust to maintain the 
body-fluid electrolyte concentration when going into fresh water. 
Smaller fish adjust much quicker, but it does take some time. Grown 
salmon die before going the other way, so we don't know how they react, 
there. I don't think their fry take very long when going out to sea, tho.

If you suddenly dunk a small fish from hard water (say 400 ppm tds) into 
RO water at 20 ppm, The lower tds outside water rushes into the cells to 
try to dilute the more conductive body fluids. This can actually damage 
and even burst tender cells in gills and skin. Given an hour or so to 
adjust, the fish changes the three-level cell-wall barrier system that 
keeps body fluids at the right concentration and that stress is avoided.

Going the other way, from pure to salty water is still stressful, but 
most fish cells seem to tolerate dehydration better than over pressure. 
I'll dunk immediately, a fish from 100 ppm tds to 200 ppm water, but for 
any greater than a 2X change, I usually will drip acclimate them over a 
period of an hour or two. YMMV.

Wright

Ron Schulz wrote:
> What is the relationship between conductivity and TDS?. I have a Hach
> conductivity meter which consistently measures my well water at 270uS/cm.
> The hardness of my water is consistently around 130ppm as CaCO3  measured
> with a Lamotte test kit. When I measure the conductivity of rain water off
> my roof, I get around 35-40uS/cm. The hardness of the rain water measures 0.
> Are these measurements  consistent with the assumption that  the
> overwhelming cation species is Ca++ with little  other contributing species?
> .......................Ron Schulz....... Dutchess County NY
> Join the AKA at http://aka.org/modules/tinycontent0/index.php?id=9
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-- 
Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net 
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Unless you’re crazy” -- Chad Carter.

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