Conflicting Refractometer Calibrations

aquaman67 · Nov 7, 2016

So I have a Captive Purity refractometer. The instructions say to calibrate with RO/DI water.

I decided to make some calibration fluid to test with. I have access to lab grade deionized water, a graduated cylinder and a triple beam scale. I put 6.2 g of salt into 182 mL of deionized water and let it dissolve for 15 minutes.

I cleaned my refractometer with deionized water and tested. My refractometer was 0.003 low. So I adjusted it. When I retest with deionized water it doesn't read zero. It reads 0.003.

So.

Should I follow the manufacturer's instructions or the calibration fluid?

I'm pretty confident I made the fluid correctly.

Thank you

cmcoker · Nov 7, 2016

I would calibrate it to 1.026, Randy has a diy recipe in his article. He's helped me calculate one using KCl..

Brew12 · Nov 7, 2016

aquaman67 said:
So I have a Captive Purity refractometer. The instructions say to calibrate with RO/DI water.

I decided to make some calibration fluid to test with. I have access to lab grade deionized water, a graduated cylinder and a triple beam scale. I put 6.2 g of salt into 182 mL of deionized water and let it dissolve for 15 minutes.

I cleaned my refractometer with deionized water and tested. My refractometer was 0.003 low. So I adjusted it. When I retest with deionized water it doesn't read zero. It reads 0.003.

So.

Should I follow the manufacturer's instructions or the calibration fluid?

I'm pretty confident I made the fluid correctly.

Thank you

The reason most manufacturers have you calibrate it with RO/DI is because it is easy.

That said, you created a saline solution, not a seawater solution. They will have different refractions so may not be comparable. In this case, I would recommend calibrating at zero.

aquaman67 · Nov 7, 2016

Never mind.

I accidentally made the conductivity standard. Not the salinity standard.

Oops.

This is what I get for reading on an iPhone 5

It should have been 161 mL not 182...

Brew12 · Nov 7, 2016

aquaman67 said:
I used Randy's formula from his Reefkeeping article.

http://reefkeeping.com/issues/2006-12/rhf/

What did you calibrate it to? 35ppt?

I can't help but think the .003 error you are seeing is tied to this.

"A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with."

From this article
http://reefkeeping.com/issues/2006-12/rhf/

Randy Holmes-Farley · Nov 7, 2016

aquaman67 said:
So I have a Captive Purity refractometer. The instructions say to calibrate with RO/DI water.

I decided to make some calibration fluid to test with. I have access to lab grade deionized water, a graduated cylinder and a triple beam scale. I put 6.2 g of salt into 182 mL of deionized water and let it dissolve for 15 minutes.

as you noted,

3.65 weight percent sodium chloride (not salt mix) will match 35 ppt seawater (sg = 1.0264) when tested by refractometer.

This discusses it:

http://www.reefedition.com/refractometers-salinity-measurement/

aquaman67 · Nov 7, 2016

Thanks Randy.

Randy Holmes-Farley · Nov 7, 2016

You're welcome.

Happy Reefing.

aquaman67 · Nov 7, 2016

Just an update. I had a chance to read the article Randy listed.

I can report the 3.65 g of salt in 96.35 mL of water read 35 ppt and deionized water read zero.

My tank was a little high at 1.027 but at least it's not 1.030 like I thought it was when I used the wrong calibration solution.

Thanks again.