In all honesty, the conductivity probe is problematic at best. It isn't a good tool for accurate measurements of salinity, due to its extremely sensitive and fussy nature: even when calibrated.
That doesn't mean that it can't be useful, though. It is very good at illustrating big swings in conductivity, and thus salinity.
Here's my chart for the last seven days:
As you can tell by looking at the chart (it is too bad it doesn't go back farther than seven days), something went wrong on the 13th of this month: which resulted in a significant dip in conductivity.
I got home from work on the 13th to discover that for some reason (as yet undertermined), my ATO over did it. It didn't over pump enough to trigger the emergency float switch, but it definitely pumped too much water in. I let evaporation return the salinity to "normal."
You can also see that something kind of funny happened on the 17th (I was what happened
).
Relative consistency and patterns in the graph is what really matters. I wouldn't get too concerned about the actual numbers.
With that in mind, I think it is perfectly okay to use it uncalibrated for now. You are always going to need your refractometer to get a true salinity/specific gravity reading. I've ordered my calibration solution from J&L and GoReef in the past.