Do yourself a Huge favor--get a TDS meter when you buy the unit that you are looking for. It measures the impurities in the water. I got one and absolutely love it. They are usually battery operated, I just had to replace the batteries after 10 months. My unit is an Aqua-Safe Systems unit 100gpd 6 stage RO/DI unit that I purchased on an internet auction house for substantially less than anywhere else (even with shipping).