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?