TDS means Total dissolved solids, on behalf of impurities dissolved in water, the larger the value of TDS, indicating the content of impurities in the water, on the other hand, low impurity content.
The general conversion ratio between GH and TDS value of water hardness:
0-89ppm (s), strong soft water 0-4dh;
90-159ppm, soft 5-8dh;
160-229ppm 9-12dh, moderately hard water;
230-339ppm, moderate hard 13-18dh;
340-534ppm, water 19-30dh;
More than 535ppm, strong hard water above 30dh.