Professionally speaking, ORP probes need more frequent calibration than pH probes to give you good data, and unless you're running ozone, it isn't really a great indicator of water quality. Like the juice said, just watching rising and falling. As for salinity, it's another parameter that's just going to give you a failsafe (i.e., OH CRAP I DUMPED ALL MY ATO WATER!). IMO, neither are worth the expense and calibration duties.