On the DIY projects blog Mavromatic, Danny Mavromatis writes about a home water tester he just bought:

Recently my brother-in-law had a reverse osmosis system installed for their drinking water. The installer left him a water quality meter which is a digital TDS meter that measures the Total Dissolved Solids in the water. What this means is that it measures the total amount of substances in the water other than the pure H2O. He joked that this was the first time ever that he had a gadget that I didn’t… so to keep with my reputation, that night I ordered my very own TDS Meter.

DIY: Checking You Home Drinking Water Quality – Link