Standard pH meters and testers are not designed to test pure water, so the answer depends on what kind of water you want to test. If you are trying to test water that is very pure such as distilled water or reverse osmosis water, you will need a pH meter that has a specially designed probe in order to get accurate, repeatable results. (Pike Agri-Lab Supplies can get these specially designed meters for you, if you need one.) If, on the other hand, you need to test water that is not very pure, such as most tap water, well water, or water that has had substances added to it (such as the water from your fertilizer or foliar feed spray tank), most of the pH testers we sell will do nicely for you. You might want to look for meters that are waterproof or even one that is buoyant (if you drop it in the tank of water, it will float).