SG and PPT in refractometer

[mrhasan]

Originally Posted by hillegom

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.

[hillegom]

Didn't know that
Thanks

[George]

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.