Hi Rob
Thanks for your diagram. Both the PM25 and the scanners will be used in differential mode. I know what you mean about which 2 end pins come together and go to ground but glad you pointed that out. If you are looking at a dual gang pot from the back the 2 pins on the right need to be connected together and taken to ground. This will make the pot increase voltage when turned clockwise.
As neather the PM25 or the scanners have actualy landed yet theirs not a lot I can do but I will keep you posted.
Carl