ATC is more of a marketing ploy than anything else. If you look at these refractometers, they work in a relatively narrow band of temperature (50F/10C - 86F/30C). I did a google search and found that the density of water at various temperatures:
Temp. Density (kg/m^3) Specific Volume (m^3/kg) (calculated)
10C 50F 999.8 0.0010002
15C 59F 999.2 0.0010008
20C 68F 998.3 0.0010017
25C 77F 997.1 0.0010029
30C 86F 995.7 0.0010043
If my calculation is correct, kg/m3 is the same as g/l. Anyway - salinity of ocean is 35 psu - which by definition means exactly 35 grams of salt per liter of water. So let's see what happens when we go from from 10C to 30C in solution. Lets assume we have 35 PSU at 10C and heat it up to 30C.
Assuming we have exactly 1 L of water and exactly 35g of salt, we would have a reading of exactly 35PSU. (I think PSU is similar to PPT) Increase the water to 30C, the volume of the water would increase by 0.23% [ (0.0010043-0.0010002)/ 0.0010002 x 100% = 0.23% ] or 1.0023L. Since the amount of salt has not changed, the new salinity is 35g/1.0023L = 34.92PSU . The water temperature change caused the salinity to change 0.23% [ (35-34.92)/35 * 100% = 0.23%).
Just looking at some of the refractometer out there, they have an accuracy of +/- 1ppt or accuracy of 0.2%. Remember - this is when the refractometer is calibrated at 20C, and I calculate the error over the whole range. So even with the changes in salinity caused by the temperature change within the refractometer temperature range - the refractometer is still reading within it's accuracy.
Conclusion: ATC is a bunch of marketing hype. Within the operating range, the refractometer is still within an acceptable error range. Regardless - a refractometer still works great!
Minh
PS - I used PSU instead of PPT because PSU is easier to calculate. I just want to show how the salinity changes very little over that temperature range.
PSS - for you science buff - I ignored significant figure in the calculation. If I had to to illustrate the insignificant changes.
<a href=showthread.php?s=&postid=9201631#post9201631 target=_blank>Originally posted</a> by jacmyoung
Regardless, should an auto-temp-correct refractometer report accurate sg regardless your water temperature?