SG and PPT in refractometer

[mrhasan]

Ok I have always wondered about this so I am putting it forth to get some answers. All refractometer comes with two scales, sg on the left and ppt on the right. While Sg is temp dependent, ppt is not. It is always said to watch for ppt and not sg to get the right salinity. Now, I fail to notice how can two fixed scale on a refractometer give a temperature dependent and independent reading using the same white straight line. I never noticed the sg scale to move up and down or anything, its always 1.026 to 35ppt no matter what temperature the solution is in. Digital meter can distinguish but how can a refractometer do it?!