Measuring TDS

[David Leithauser]

I am trying to build a simple device to measure total dissolved solids (TDS) in water by measuring electric conductivity (EC). I have two metal plates in the water. I run a current through the water, and use a voltage divider to measure the resistance. I take the reciprocal of the resistance to be the EC. The EC goes up as the PPM TDS goes up. So far, so good. The problem is that everything I have read says that EC should go up linearly with TDS. The general figure is that TDS PPM = 500 times EC. The exact number is not important, because I am not calibrating the EC to standard units, but the point is that PPM is supposed to go up linearly with EC. The problem is that in my experiments, it is not linear. I get a ratio of 35 at 14 PPM, 25 at 32 PPM, 10 at 162 PPM, and 8 at 214 PPM. Can anyone point out the flaw in my system?

[Tom Oliphant]

Dear David,

I am confused as to the value and significance of TDS and EC and their relationship to aquaponics health. Generally what I have read on the internet is that they are general measures of water purity. Aquaponics water is never pure (meaning contaminant free like distilled water). It must have Ammonia, Nitrite, Nitrate, Iron, and dozens of trace minerals. So what are we really measuring?

TVO >:)

[David Leithauser]

EC is Electrical Conductivity, how well the solution conducts electricity. TDS is total dissolved solids, how much stuff is dissolved in the solution. EC goes up with increased TDS, because the dissolved material helps conduct electricity, so the easy to measure EC is a simple way to measure TDS. 

The idea is not to have totally pure water, but to determine how much TDS you have. Too little means you do not have enough nutrients for your plants. Too much means that you have too much waste from your fish for the plants to handle, or other problems. So, measuring TDS and making adjustments helps you keep your system in a healthy balance.

Regarding my question, I believe I have solved the problem, and the book I was doing the research for, "How to Cheaply Monitor and Automate Your Aquaponic/Hydroponic Garden with Arduino/ Genuino," is now on sale on Amazon.com.

[Tom Oliphant]

Dear David,

Thanks for your information. I was researching on line and found the attached document that gives me greater clarity of the significance of TDS and EC. They can be used to determine kH (Carbonate Hardness) water hardness and general pH buffering capability.

Let me know what you think of it.

TVO >:)