It won't matter what temperature your water is at.
1 drop of water will become the temperature of the instrument in a few seconds anyways.
Put the ATC refractometer in the fridge, or out in the sun, and that 's what it will compensate temperature for.

Sorry for getting back to this thread after a long time.

Anyway, my question was that in various forums, I always see people telling to measure PPT instead of SG and that's where I am scratching my head. Like does SG scale move up or down and ppt remains fixed? Because as far as I can see, the scale is fixed while for ATC, the metal strip expands or contracts for temperature compensation to show 1.026 - 35ppt ratio. Is measuring SG and PPT the same in a ATC refrac then?

I'm at a loss. I have never taken one apart. There is a metal strip inside?
I have always believed (but not known) that the scale had something to do with how the light refracted off, who knows what. The metal strip?
I'll have to do some research when not so busy.

Yah ATCs do have a metal strip which contracts/retracts with temperature to compensate the temperature effect on the scale. That's why its suggested to take reading after keeping the water for some time (30 secs is a commonly practiced one) so that the strip can adjust the temperature accordingly.

Didn't know that
Thanks

A refractometer measures reflective index. Manufactures stick a couple scales(specific gravity/SG or salinity) for you to see the levels easily. Reflective index, SG and salinity can be converted from one to other (with a small error that can be ignored for our aquarium use). If your refractometer says automatic temperature compensate (ATC), it means the reflective index level is being compensated.
I usually look at salinity because it's easy for me to see 35 ppt (salinity) than 1.0264(SG) on my refractometer.