When I first got my Herpstat, I noticed they offer a humidity probe so I called to order it. I spoke with Dion and he told me to just buy a combo unit from Xikar as that is what they use to calibrate theirs. After installing it I noticed a 5-10% difference from the accurite model. This is what he recommended to me.
https://www.xikar.com/popup_pix.asp?...l%20Hygrometer
Since I have multiple enclosures I've purchased this unit and couldn't be happier...
https://www.xikar.com/products/humid...-hygro-system/
I figure I'm using the industry leaders product to control my RHP's, why not use the industry leader in humidification monitoring too.