If you're buying your own minerals it might help to have your own TDS meter! TDS stands for Total Dissolvable Solids, the amount of minerals or aggregates found in the water chemistry. Some uses include:
Verify the water you are adding Third Wave Water to is truly 'empty' as recommended (distilled water, reverse osmosis, etc).
Test your current water to see how many minerals are present.
NOTE: this will not define which minerals are present in your water, only that something is there.
The cap can also be used to hold water to measure. Simply fill cap then insert meter and use as normal.