WTB: PH Monitor

I would like to find a monitor that I can use for my Kalk reactor - I don't know if anyone has tried this, but I'm thinking that I will be able to tell when it's time to add more Kalk when the PH starts to drop. Anyone have an old one they want to get rid of????

[super7]

I have one that I could let go for $100, its brand new never touched water. Its a hannah grocheck.
Super7