Hi All.
"TDS" stands for "Total Dissolved Solids".
This is a new method of testing the quality of water, but is mainly European so far.
It works on the principle that "PURE WATER" has zero solids or impurities, which means that it is a very poor conductor of electricity, thereby giving you a reading of zero.
As it is an electrical measure, the results are in "MICROSIEMENS".
The measure is a lot more accurate: a bit like inches and millimeters, you get a finer reading.