While similar in some ways, there is not a directly proportional relationship between Conductivity and TDS. Conductivity is a measurement that represents how much resistance the solution has. TDS on the other hand, is a measurement of the dissolved solids that are present in the solution. Conductivity is just a straight resistance reading where TDS need to be determined by not only knowing the resistance of the solution being measured, but one must also know what the solids are that are dissolved in the solution.
There is no ‘universal’ conversion between Conductivity and TDS, although the Hydra Meter, Mesa’s ‘water quality Meter’ provides a TDS reading derived from the Conductivity measurement. The Conductivity reading is converted using a mathematical formula based on what is known as 442, which is a mixture of sodium sulfate (40%), sodium bicarbonate (40%) and sodium chloride (20%). The 442 mixture is designed to mimic the ions often present in natural fresh water systems.
While the Hydra programming does allows it to be put into the TDS calibration mode, the varying electrical properties of the different TDS standard solutions available on the market make it extremely difficult to calibrate while in this mode. There is not a TDS standard specified for use in calibrating the Hydra so this unit must be calibrated in Conductivity mode using the specified Conductivity solution.
For more information about the Hydra Meter, Mesa’s NIST traceable solutions or Conductivity vs. TDS, visit the Dialysate Meters page.