Not familiar with Arduino but it looks like a Raspberry Pi kinda deal. Hopefully, it works for you and you're able to calibrate it correctly. I personally use humidor humidity gauge that is calibrated with salt and distilled water.
In regards to humidity, one in the middle and inch or two above the substrate sgould be ok. It should give you a general idea.
I would caution against bioactive terrarium for a Ball Python though due to possibility of scale rot.
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