Calibration solution is dirt cheap. Mine was accurate out of the box. I did find that my old one was off about 100 ppm's or so, not enough for me to worry about since it is now a back up.I'm using the exact same pH and TDS meters. Have you ever calibrated the TDS meter? I haven't and I'm a little worried it might be off point, although I don't think TDS meters need that much maintanence as pH pens.