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Re: TDS readings

On Tue, 26 Dec 2000, K9AUB wrote:

> I recently had the opportunity to pick up a TDS meter at a bargain price, and
> have searched the web to see if I could learn enough about Total Dissolved
> Solids to make the readings useful.  Frankly, I'm confused.  I understand I
> am coming up with a number that corresponds to all the chemicals dissolved in
> water.  However, I'm not sure what a "good" TDS reading would be, and when it
> gets so high that it's a "bad" reading.  My tapwater tests at TDS=400.  Now
> that I have that number, what does it mean, and what (if anything) should I
> do about it?   Do I even need to be concerned about TDS at all?

Water with a TDS of 400 mg/l is OK.  Various states have drinking water
standards for TDS that range from 500 to 1000 mg/l.  I think average river
water is about 280 mg/l and the average US water supply is about the same.

If you're keeping fresh water aquariums there's not much value to the
measure.  If you're keeping brackish or salt water tanks it may be useful.
A fairly normal breakdown is:

Fresh water        1 - 1,000 mg/l TDS
Brackish water 1,000 - 10,000
Saline water  10,000 - 100,000
Brine        More than 100,000

Most freshwater organisms can live in water that exceeds 1,000 mg/l TDS.
If I remember correctly from past work in estuaries, freshwater plants and
animals generally don't extend their range into areas where the water
regularly (e.g., due to normal tidal fluctuation) exceeds a TDS of about
3,000 mg/l.

You might find it interesting to compare the TDS of your aquarium to the
TDS of your tap water.  TDS should build up in your aquarium over time.
You can pick some level (say, something like 150% of your tapwater value)
as a maximum amount of buildup you want in your tank. Do a fairly complete
water change when your meter shows that the tank water reaches that point.
That procedure is fairly arbitrary, but it would help keep the conditions
in your tank from drifting too much over time.

TDS meters actually measure the electrical conductivity of the water, but
they're calibrated so that you can read the results in concentration units
(usually mg/l or per mil) instead of conductivity units.  The conversion
is approximate.  Also, the meter calibration can drift over time, so it
will probably need to be recalibrated periodically.  The manufacturer
should be able to provide you with information about how and how often
you need to recalibrate the meter.

Roger Miller