Which would you believe?

[naesco]

I think the whole discussion of salinity and it measurement is academic. Nobody is going to go to the trouble of calibrating in the manner Farley suggests. The important things are.
1. That salinity is within an acceptable range.
2. That is doing water changes you check the salinity of your tank, the salinity of your new water, and the salinity after you do the water change.
Whether you use a hydrometer or refractometer that is off target makes no difference because the off target reading is consistent for all 3 tests.
The important thing is that there is not a sudden change of salinity.

In the same way that all reefers would question a unusual water test that reads too high or low Ca or anything else and seek another test kit, if you tested an unusual salinity it would be time to buy a new hydrometer for 15 bucks or calibrate your refractometer.

This article in advanced aquarist (look under salinity) measures the changes in salinity in one day at a reef. You can see that in nature salinity drops and falls during the day. So measuring to a precise reading is irrelavent as long as you are within the acceptable range.
http://www.advancedaquarist.com/2009/6/aafeature