SG and PPT in refractometer
 

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?!

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

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.

A digital probe compensates for temperature.

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

So
1oz fresh out of tank water measured 1.025
1oz tank water left out covered for 1 hour measured 1.025
1oz tank water left in fridge covered for 1 hour measured 1.025

After this I don't think tempature is a factor.

Is that why mine says automatic temperature compensated?

Lol. I'd say the experiment is a bust if his is also an ATC unit

Lol doh, mine is ATC 10-30C

It won't matter what temperature your water is at.
1 drop of water will become the temperature of the instrument in a few seconds anyways.
Put the ATC refractometer in the fridge, or out in the sun, and that 's what it will compensate temperature for.

Sorry for getting back to this thread after a long time.

Anyway, my question was that in various forums, I always see people telling to measure PPT instead of SG and that's where I am scratching my head. Like does SG scale move up or down and ppt remains fixed? Because as far as I can see, the scale is fixed while for ATC, the metal strip expands or contracts for temperature compensation to show 1.026 - 35ppt ratio. Is measuring SG and PPT the same in a ATC refrac then?

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.

Didn't know that
Thanks

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.