Hi everyone. I have a petfran ph pen that I've been using for a few years on and off since I've been shrimp keeping. I've long suspected the pen of being abit off and wanted to re-calibrate it. I got ph4 and ph7 buffer solutions.



The pen has 2 screws to adjust it. Which should I be turning? I can't find the box and I've not been able to find any instructions online either.


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