I got my probe from BRS the other day and I prototyped the circuit using the schematics posted on practical maker site. Unfurently I don't have any calibration packets so i cannot calibrate it at the moment.
It might be actually simpler to get one of these stamps, i was thinking to purchase AS EC kit, each seems to be priced fairly.... However i need to move in couple of week and i need money to pay the movers lol