I worked with the one located here,
http://www.octiva.net/projects/ppm/ but it was hard to understand. I tried to reach out to the person, but never heard back from them. I also wanted to include the - 12 VAC conversion in the board so I only needed to suppy 12 VAC instead of + and - 12 VAC.
Unfortunately I do not have a scope.
The probes I purchased from China. The range should work for saltwater, but I will not know until I get a working prototype.
PM me on the PCB if you are interested. I think I have 6 or so extra ones.