Didn't need to calibrate as mine came out just perfect, seems to come out of the box. Tested with a known volume of water for PH and TDS. I've been using it in aquarium water for about a month now and so far both have worked with no problems. The PH is always between 7.1 and 7.25, which is consistent with where he used the API test kit or test strips. TDS rises slowly for a week until water change, then falls slightly and repeats. definitely worth the money.
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