Worst experience, bought SX620 pentester on 16th November 2019 and it stopped working in April 2020 (I've only had it 5- used 6 times !). I followed all the instructions, stored the pen with storage solution, cleaned it with distilled water, etc. I bought a replacement sensor thinking it was just bad luck (the product had good reviews) but it had the same problem, it didn't work at all. Same problem, tried to calibrate it and the reading goes from -1 to 15 and error. I also tried calibrating with new solutions, didn't work either. The funny thing is the $15 tester, 2 yrs, saved my brew day.
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