OPR controller probe--Sandpoint

[wooden_reefer]

Guess what, an old seller gave me an old SandPoint OPR ph controller.

I don't know if the thing works but it is missing the probes.

What is an ORP probe? How does it work?

Is there a way to know if the unit works before buying a probe, if still available. Would other manufacturers probe work, if it fits physically.

Thanks