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?!

[SteveCGY]

Very good question as I've wondered the same. Following along.

[MitchM]

That scale will be accurate as long as you use the refractometer at the temperature it was calibrated at.
However, don't compare the refractometer specific gravity with the number you get from a hydrometer. They were probably calibrated at different temperatures.
The hydrometer may be calibrated for measuring the specific gravity at 25C water temperature, while the refractometer is probably calibrated for use of the instrument at room temperature, about 20C.

That's why I try to use the ppt (part per thousand ) measurement, but a hydrometer cannot measure that.

[MitchM]

A digital probe compensates for temperature.

[gregzz4]

Raied, why not run an experiment ?
Take out a cup of tank water, cover it and let it sit until room temp
Then compare it to your tank water

[SteveCGY]

Originally Posted by gregzz4

Raied, why not run an experiment ?
Take out a cup of tank water, cover it and let it sit until room temp
Then compare it to your tank water

Good idea I have just done this, also added a cup to the fridge for comparison