Just curious if I can utilize a ORP probe without a ORP board, is it possible to use a ORP probe with Arduino without the use of a ORP sensor board? I am curious because I see a ton of sellers selling the probes some very generic but very few people selling the actual boards. I imagine that the board has a chip that interprets the read on the probe and puts it into something an MC can read?
Thanks.
ORP Probe = Oxidation-Reduction Potential Sensor
Boards:
The analog input has a high impedance, that is good. The voltages are in the hundred millivolt range, that should work. But the negative voltages is a problem.
Suppose 2.5V is made with two resistors and that is connected to the ground of the probe. That means the ground of the water should be lifted to 2.5V. That messes up the very thing that needs to be measured.
Reading a negative voltage with a high impedance requires a specific opamp and a specific circuit.
The solution being measured needs to be isolated anyway so lifting the ground isn't an issue I think. The extremely high impedances though are a problem, the Arduino boards are not made of teflon and may leak too many electrons. Some probes are 10^12 ohms or so impedance, so the sense amp has to be above this, which requires very careful design and materials. Some opamps are designed specifically for this use, the MAX406 being one I found.
How many JigoOhms is that ? 
The input resistance of a analog input is very high, but it needs a little charge for the ADC. If you can charge a capacitor and don't use the ADC too often, then it might work. A capacitor of 1nF is more than enough. I have not tested how low that value can be.
Do you know the MyoWare muscle sensors ? They also use a very high impedance input, 110GΩ with a AD8221.
Lifting the water to 2.5V to allow that the Arduino can read a negative voltage feels very wrong, even when it is isolated. It is against everything I have ever learned.
Huh? You learned wrong then, I guess, all voltages in real life are differences in potential. The datasheet for the MAX406 for instance shows exactly this sort of divider circuit.
Either the solution or the meter or both have to be isolated to a high standard for electrochemical probes like these.
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