I recently purchase a digital refractometer and use an online calculator to convert the ºBrix value to SG.
My question is, at what temperature is the resultant SG calibrated?
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Both Brix and specific gravity are temperature sensitive but the conversion seems to be independent of that fact. That is, if you start with the correct value in Brix, your converted SG value will be correct.
So you will want to make sure that your Brix value is temperature adjusted before you do the conversion. From what I've heard, refractometers usually adjust for temperature automatically. So provided yours does as well, you should be fine with just using the straight converted value.
If you were using different methods for measuring your OG and FG, I suppose it's possible that you would need to take this into account. For example if you used a refractometer for your OG that was normalizing the Brix value to 70ºF, and a hydrometer for your FG that was calibrated at 60ºF, I suppose some error could be introduced that might be worth accounting for. But in practice this should be pretty small. And generally people will use the same method to obtain their OG and FG so any small differences that might arise between methods should balance out if you're being consistent.
Conversion from Brix to SG doesn't require temp adjustment.
Only the reading you measure needs to be adjusted for temp. Or better yet zero out your refractometer with distilled water that's the same temp as your wort. Do this easily by letting both the wort and distilled water sit at room temp until even, then calibrate, then take reading.
Edit: SG scale is based on water of 1.000 @ 4°C or 39.2°F, but this is just where they choose to call 1.0, it could have easily been 100°F to call 1.0. So knowing what the scale is based on has no relevance in gravity calculations. So a converters calculation of SG isn't based on any calibration temp at all, it's not an issue of the scale, only measuring devices.
I believe the answers from @Evil Zymurgist and @thesquaregroot have missed the point. Nowadays, for brewers at least, the SG of a sample is conventionally reported as its density relative to water at 20 degrees Centigrade. If the standard was defined at a different temperature then the specific gravity value would most likely be different since the thermal coefficient of expansion of the sample and water are usually different.
However, you should realise that Brix is a measure of the concentration of a sucrose solution and is inferred by the refractometer by the measurement of refractive index. Since you're measuring an unknown mixture of sugars converting refractive index to Brix to SG is an approximate measure in anycase.
The short answer is that the calibration temperature is whatever is used by the online convertor. However it would be suprising if it was anything other than 20ºC since the ASBC, EBC and IoB all use a definition of specific gravity that has a reference temperature and measurement temperature of 20ºC.