TDS or Total Dissolved Solids is a measure that is sometimes used in hydroponics to indicate the strength of a nutrient solution.
Total Dissolved Solids
The presence of TDS in a nutrient solution is detected by measuring the electrical conductivity (EC) of the solution. There are different conversion factors used by different metering systems to calculate the TDS level. Although the EC will be the same the results for TDS will be different depending on the conversion factor used.
TDS is a Guideline
The calculated TDS level is a guideline for you as far as your nutrient solution is concerned. Provided you have mixed your solution according to the manufacturer’s instructions you will have the correct TDS level.
There are two conversion factors that are both claimed to be the closest thing to a hydroponics solution. The 442 conversion is approximately 700 x EC in miliSiemens (mS). The NaCl conversion is approximately 500 x EC in miliSiemens (mS). As you can see, these two results are 40% apart.
The differences that the conversion factors for TDS create can be dangerous to your plants if you act on the wrong one. If you used a meter converting on the NaCl basis but the levels suggested were based on the 442 factor, you would create a TDS level at least 40% higher than it should be.