I was pissed that it would not read epsom salt so I called the place in New Zealand to find out what was up. They told me they sell them as a promotion for there really accuarte testers for 19 bucks in bulk to the US. They are usually 120 in the stores. Some one is banking. I brought the ppm up to 1000 and then started dumping in epsom salt on both of them and the thing accually dropped one time. I like the way they light up when I stir the nutes and casting spells on the plants is cool but I just had a bad experience with them myself.