I use a hanah,the old style full sized low range colorimeter 713.No calibration. I use it almost every day( compulsive about PO4) and have for about 3 yrs. All of the colorimeters and test kits have an accuracy standard of arguably around .04 +/- .1 . Never could distinguish shades of pink well enough to be confident with a Salifert test kit. Love the digital readout on the colorimeter. I think the new hanah checker may rate slightly better. I haven't used the new less expensive checkers but many do.