I usually measure it on Sun afternoon right before my water change. I measure with three devices, a reftracto, a floating and swing-arm. They all indicate a rise in Salinity. It is weird as I don't do anything unusual to the tank, just osmo kalk and that's it. I've been able to keep it under control by using slightly lower salinity in the water change water. It's not a big deal, but thought someone else may have an answer.