testing RO water

Hello everyone,

I'm not so rich so I need a way to test my ro water to see if it needs changing and when. I have a larger ro system that I use but not enough in my opinion to change the filters as they recommend (once every year). Any way to do this. Thanks

This model http://www.filtersfast.com/HM-DM-1-M...ered-Water.asp works very well, easy to install and use, and reasonably priced. Monitors BOTH the supply and product water a the slide of a switch.

http://www.filtersfast.com/ProdImages/DM-1.jpg