Ok smart people, I know you are out there! I am interested in assessing the purity of the water (RO) that is available to me for almost free. I'm certain that it is probably sufficient for my b/w photochemisty purposes, but it is still something that I am interested in. I believe that testing resistance is a good method for determining the relative amount of dissolved solids. Here are my questions: if I use a multimeter to test the resistance of the water, how far apart should the probes be? (Please note: I have not yet tried this at home, so maybe it's not important - I was just thinking about it today at work.) What are "decent" figures? 8-10 M Ohms? Like I said, this probably isn't super important. Just curious.