TDS is not actually a proper unit (like kilogrammes, litres or volts) but an indicative value based on conductivity (which is a proper SI unit, measured in micro Siemens per cm ie. uS/cm).
They base the TDS value on a solution of NaCl (sodium chloride or salt). However, this may not be the only mineral in the water in your area, in fact it is quite unlikely.
The TDS value is calculated by multiplying the conductivity by a scaling factor. This factor can be from 0.4-0.9 depending on the impurities (minerals) in the water in your area. Really to get the correct TDS you would need to take a water sample, evaporate off the liquid and weigh the remaining solids. This will tell you the scaling factor for that sample. To make matters worse the relationship between conductivity and TDS is not always linear (see
https://iopscience.iop.org/article/10.1088/1755-1315/118/1/012019/pdf) and is likely to change from day to day as different impurities find there way into the mains water. TDS (and conductivity) also changes with temperature, the amount of change is again dependant on the impurity.
To get around these limitations meter makers calibrate to a known chemical (often NaCl) at a known PPM and temperature (often 20 degrees C). By taking a temperature reading at the probe the we can compensate for a temperature difference between the calibrated temperature and the actual temperature, based on the assumption of NaCl impurities. This is how the Spring (Europe) Limited V16 TDS works.
However it all comes out in the wash because....
Since pure water has very few minerals in it, the conductivity is very, very low and approaches zero. Because of this the error in the TDS value also reduces, no matter what scaling factor the meter has been calibrated to. TDS meters tend to get more accurate as you approach a PPM of zero.
I hope this helps.
