Salinity Temp Compensation with Refractometers

[Bryan]

All:

Question, most if not all the refractometers are ATC, but I have a question. If I take a sample from my tank that is 80 degrees and the refractometer is at 70 degrees and the sample tests at at 1.025, then is not the salinity of the tank actually 1.023 ?

[disc1]

Yes and No. The specific gravity changes with temp. The refractive index changes with temp. But the actual salinity (grams of salt per kilogram of water) does not. That's the ppt number. Now on the scale in the refrac, they have the SG scale lined right up with the ppt scale. In reality, it is measuring neither. The refrac is measuring refractive index. It's not terribly realistic, but what it's actually doing is giving SG and ppt values at some standard temp that would match that RI. I think it's supposed to be 20C, but someone may correct me on that. Then, what the ATC does is adjust that scale again to hold the calibration by correcting for the temperature effect on RI.

So the refrac is telling you the specific gravity would be 1.026 if the water was at 20C, but not what the actual specific gravity is at 80F. What I mean is you would measure something different by putting exactly 1mL on a balance. But, the number it is giving you is what you want. The fish don't care one little bit about the specific gravity of their water. We are only using specific gravity as a surrogate measurement for salinity.

Now, when measuring with a hydrometer, you are directly measuring specific gravity. In that case, a correction may be necessary.

Also, when measuring with a non ATC refrac, a correction for temperature will need to be calculated, but I don't think it's going to be based on the difference in SG at temperature. The proper math would be based on refractive index change with temperature and the temperature and salinity used when calibrating the refractometer.

[disc1]

then is not the salinity of the tank actually 1.023 ?

Another (shorter) way to answer this is just to say salinity is never 1.026. That number is a specific gravity number, not a salinity number. We equate that to 35ppt salinity (because it's easier to measure SG) but in reality the two numbers are only the same at a certain temperature.

So if you are measuring salinity by refractive index, then any corrections need to be made by refractive index to get the salinity number, specific gravity never figures into it. And that's what the ATC does for you.

[Bryan]

Thanks for the reply, very enlightening. Did manage to find the manual for the refractometer and it states the refractometer must be calibrated at 20 Celsius and it is accurate from 10-30 Celsius once calibrated. Like most refractometers it says to calibrate with RO/DI water but I assume if one calibrates with a 35 PPT salt-water standard at 20 Celsius I would be OK.

Cheers