The most accuse way to measure ammonia is to use a photometer. Thanks to LEDs the cost of test are dropping. Hanna instruments as a good one for $50 . It will measure 0 to 3 ppm with a resolution of 0.01ppm.it iIt is easy to use and has a digital readout. Unfortunately they don't have a nitrate meter. http://hannainst.com/hi700-ammonia-lr.html