As for testers I use the Hanna 4 in 1 model. It cost a little more then some of the others but worth every penny. You need the full range of calibration solutions and if anyone tells you different (they are wrong) This is a scientific internment and should be treated like one. I calibrate mine once a month at least and it has lasted me for 3 years with no problems. There are others cheaper but you get what you pay for.