If I read OG and FG at the same temperature, do I have to correct for the variation from 60F? Or will they remain accurate relative to each other?
(And why, exactly, is the temperature for this not the standard 20C?)
The relative offsets will be the same, so if all you want is to determine ABV, and you’re using a simplified formula like 131(OG-FG), you’ll be fine.
Calibration temperature depends on the instrument. I have one that’s 20°C.
Probably because American manufacturing mostly uses english, not metric, measurements and hydrometers were designed for commercial brewing.
Some are calibrated at 60F and others at 68F. It should be recorded on your hydrometer.