calibrating refractometer

[kolei]

So I got a refractometer and have a question about calibrating it.

 

I bought the Calibration/Reference Fluid from DFS which is suppose to be at 35 PPT.

 

The instruction that comes with the refractometer says to use distilled water and calibrate it to 0 PPT. If I calibrate using distilled water and set the blue/white line to 0, the calibration liquid is around 45PPT.

 

If I recalibrate using the Calibration Fluid and set the line at 35 PPT, when I test it with RO/DI water the blue/white line is below 0.

 

Which one should I calibrate against?

[Mr. Fosi]

Calibrate to the one that is closer to what you will actually measure.

[Warehouse41Ant]

Well, it's not possible for RO/DI to have a negative reading. Should be zero. A negative would somehow mean that you have less than no salt in the water. I would calibrate it so that your RO/DI registers zero.

[Mr. Fosi]

It is possible for RO to read lower than zero after calibration at a higher point if the optics aren't just right.

 

Just means that his calibration curve isn't a perfectly straight line. In this case, it is better to calibrate close to the values you expect to be measuring to minimize inaccuracy.

 

I'd double check this refrac at a range of salinities against another refrac or a hydrometer you know to be correct.

[kolei]

Thanks for the input. Calibrating it using the calibration fluid seems to get me closest to what my hydrometer is reading. I'm going to take it to the LFS and do some salinity tests there.

[Warehouse41Ant]

Hmm...didn't know this. Thanks for the clarification, Fosi.

[Mr. Fosi]

No probs.

 

This serves as a reminder that an instrument can be only as good as the measurements it generates.

[kolei]

No probs.

 

This serves as a reminder that an instrument can be only as good as the measurements it generates.

 

That is why I want to make sure it is calibrated correctly before I can trust it.

[Scott Riemer]

That is why I want to make sure it is calibrated correctly before I can trust it.

Double checking it is a good idea. But, FWIW, it also depends on what your LFS uses to measure their salinity. Mine uses a crappy swing arm hydrometer. So I'm going to trust my refractometer calibrated with 53.0mS salinity calibration fluid.

[Phixion]

Well, it's not possible for RO/DI to have a negative reading. Should be zero. A negative would somehow mean that you have less than no salt in the water. I would calibrate it so that your RO/DI registers zero.

 

Yes and no. What this means is that the lense/prism in the refracto is inferior, and will not calibrate perfectly to one end or the other. In theory, a perfect unit should calibrate RO/DI at 0 and the solution at 35. But many cheaper refractos have less than perfect optics, and will almost never calibrate RO/DI to 0 and measure the solution at the proper 35ppt. Always best like to calibrate closest to what you'll be measuring as Fosi said, which in this case would be the tank water which should measure out at around 35ppt or 1.025 S.G.