We know that the SG of a salt solution will change with temperature and so it is important to take this into account when taking a measurement.
All D-D refractometers have auto temperature compensation which will adjust the reading depending on the room temperature or instrument temperature.
However - some people assume that because the refractometer has ATC that they can use it in any situation and it will sort itself out but this is not the case.
Refractometers are designed to measure the salinity at 20C where as most hydrometers are calibrated at 25C. You will therefore get a higher SG with a hydrometer as the water is cooler and therefore denser.
The calibration process should be carried out with both the room and the instrument at 20C. Once calibrated the refractometer will compensate for shifts in temperature between 10 and 30C without affecting accuracy.
It is no good having a room temperature of 20C and keeping your refractometer in the shed outside where it is 10C as it is the refractometer temperature that is important.
The temperature of the liquid quickly equilibrates with the temperature of the Refractometer.