This pocket pen tester will give you an accurate reading of TDS (Total Dissolved Solids) in your hot tub water. Ideal levels for TDS in hot tubs should be less than 1500 ppm over startup TDS. If your reading is greater than 1500 ppm, it’s time to replace the water. This simple battery operated function makes it totally portable and easy to use (batteries are not included). The TDS Pocket Tester Pen works with our TDS Pen Calibration Solution.