Old thread, but I wanted to point out that the calibration is not necessarily linear. Yesterday, I mixed solutions of known specific gravity using sugar and water. At low gravities (1-1.020) my hydrometer reads 0.004 too high. But at a gravity of 1.083, my hydrometer reads exactly 1.083. I suppose this can happen if the marking on the paper were printed a hair too far apart from each other.