i've used the pinpoint probe submerged before i'm not sure about Milwaukee, should be ok. regardless as to whether or not the probe dried out it still may be shot, if you can't calibrate it that's the indication that you need to replace it unfortunately.
till then, just try to set it at 6.7 and see if it works, when the pH drops to that point it should close...at least your controller will be operational.
as far as using the kh/ph relationship to get a known level of co2, there's a few things that can effect that relationship. it doesn't always reflect an accurate reading.
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