Review on ROOYLE Hydroponics Accuracy Measurement 0:00:14:00 by Brian Sage

out of the box, the device needs to be calibrated, which is not easy. First, you need an accurate thermometer to calibrate. Several glasses of pure distilled water must be heated to exactly 77 degrees and thoroughly mixed with the powder supplied. This gives you very little time to calibrate. Also, you only have one try. I've also found that repeated immersion in the same solution after calibration produces different results. After all, it's reasonably accurate compared to pool storage tests, and handy for other liquid/paper methods.