I have a couple of the cheap PPM pens I use, neither can be calibrated so I keep 2 base lines written down, one is my well water which fluctuates between 50-100 depending on the time of the year and I know how much my base grow mix is so I kind of use that as the scale on when to toss the meters. About a month ago one of the meters had over a 4000ppm reading so its history, and it wasnt gradual either one day 1000ppm's the next 4000ppms.