TDS Reading Conundrum - The Planted Tank Forum
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post #1 of 9 (permalink) Old 11-12-2018, 01:57 AM Thread Starter
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TDS Reading Conundrum

I have a TDS meter (HM Digital TDS-4), which compensates for temperature, and have used it for many years, running along happily on the assumption that it was correct. Why? Because it reads 0 ppm with my RODI water and calibrates just about perfectly with the 342 NaCL calibration solution.

So, on a whim, I decided to mix up some KNO3 in a gallon of RODI to see if it would match. I add one gram of KNO3 to the RODI gallon. Reading should be ~264 ppm (162 NO3 + 102 K), right? Wrong! I get ~180 ppm. Did the same thing with some CaSO4, K2SO4 and MGSO4. TDS meter under-reports by similar significant percentages.

Can anyone explain this and have you performed similar tests with your own TDS meter?
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post #2 of 9 (permalink) Old 11-12-2018, 02:32 PM
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There is nothing wrong with your meter because not all elements have the same ability to conduct electricity.


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post #3 of 9 (permalink) Old 11-12-2018, 04:14 PM Thread Starter
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There is nothing wrong with your meter because not all elements have the same ability to conduct electricity.
Thanks for the link. I guess I can measure table salt [somewhat] accurately if itís a neat mixture in RODI water, but everything else is going to be a broadly relative guess. The inexpensive science for it just isn't there, yet.
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post #4 of 9 (permalink) Old 11-12-2018, 04:33 PM
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Deanna,
Measuring conductivity of other ions than NaCl is not a relative guess. It is as accurate as measuring NaCl. TDS testers can be calibrated for any ion. It is simply a different number. Also, the ppm can be calculated from the ratio difference. It is like dealing with gallons versus liters.


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post #5 of 9 (permalink) Old 11-12-2018, 04:59 PM Thread Starter
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OK, but the problem would come in when all solids are involved regardless of which ion is the base for calibration. It seems that there is no way to measure what the total ppm of all dissolved solids actually is, with these inexpensive meters. I only used the KNO3 example as way to try to see if actual "total" (total being NO3 + K, in this case) dissolved solids can be measured. If I calibrated it to nitrate, e.g.; it would not capture the correct ppm of K in the summed NO3 and K reading. Are these statements correct?
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post #6 of 9 (permalink) Old 11-12-2018, 05:22 PM
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It doesnít matter if the tester is cheap or expensive as it is the way electrical conductivity works. For example, if calibrated to display K ppm in only KNO3 solution then it would display exactly that. Or, if calibrated to display K ppm in KNO3 with K2SO4 solution then it would display exactly that again, as long as the two compounds ratio is the same.

More ions dissolved in solution -> lower electrical resistance -> higher electrical conductivity. In principal the TDS measurement does not distinguish between varying ions. It is a summary of all varying ion properties combined.


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post #7 of 9 (permalink) Old 11-13-2018, 07:20 PM Thread Starter
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Thanks, @Edward. I've read the recommended links - and more - and have a better understanding. It's really a conductivity meter, not able to measure total ppm of our tanks and, therefore, more relational in terms of a number displayed. For example: I have a known, approximate, total ppm of 264 from the KNO3 added to RODI, as above. The TDS meter displays ~180. Without doing some math and multiple calibrations, the number displayed as "ppm" is not the "ppm" of the water. That's fine, so long as I know that it is only useful as a rough relative guide concerning monitoring changes to the ions in the water.
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post #8 of 9 (permalink) Old 11-13-2018, 07:43 PM
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That's fine, so long as I know that it is only useful as a rough relative guide concerning monitoring changes to the ions in the water.
Rough, quick, reproducible and cheap.
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post #9 of 9 (permalink) Old 11-13-2018, 08:36 PM
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If we dissolve 0.264 g KNO3 in 1 L that is 264 ppm of KNO3. Then we find in this post that 300 ppm KNO3 reads as 220 ppm on NaCl calibrated TDS tester.

220 x 264 / 300 = 194

The TDS tester should display ~ 194 ppm.


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