Salinity discrepancy ICP vs Refractometer

[bjsreefer]

Hey all! Just got my ICP test results back from Oceamo and am a bit confused… I have a calibrated refractometer and I have exactly 35 ppt salinity in both my 90g and my nano. When I got my results yesterday though my ICP said 31.5 and 31.8 ppt. Everything is thriving, which I didn’t think they would do if I was that low. Do I trust my refractometer or try to increase salinity by 3 ppt?

 

[Miami Reef]

A very accurate 35 ppt refractometer standard can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams or mL of water.

https://reefkeeping.com/issues/2004-06/rhf/index.php

 

[bjsreefer]

I mean is premixed refracto juice not sufficient for calibration?

 

[Randy Holmes-Farley Verified Community Expert]

I mean is premixed refracto juice not sufficient for calibration?

If it is made incorrectly or was adulterated somehow, yes.

I have high confidence in Oceamo salinity measurements.

 

[bjsreefer]

I mean is premixed refracto juice not sufficient for calibration?

If it is made incorrectly or was adulterated somehow, yes.

I have high confidence in Oceamo salinity measurements.

Wouldn’t my livestock be in trouble if it was that low though?!

 

[rtparty]

I mean is premixed refracto juice not sufficient for calibration?

After years of trying them, I don’t trust any of them. Too many variables and no way to know if it’s correct with just a refractometer.

The diy solution Miami referenced is the way to go

 

[bjsreefer]

A very accurate 35 ppt refractometer standard can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams or mL of water.

https://reefkeeping.com/issues/2004-06/rhf/index.php

So just table salt? I just got a digital scale so I think I could do this easily

 

[Randy Holmes-Farley Verified Community Expert]

So just table salt? I just got a digital scale so I think I could do this easily

Yes, I detail recipes here:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/

 

[Randy Holmes-Farley Verified Community Expert]

Wouldn’t my livestock be in trouble if it was that low though?!

At 31.5 ppt? Not necessarily.

 

[Miami Reef]

So just table salt? I just got a digital scale so I think I could do this easily

Yep. Morton’s iodized salt is the way to go.

 

[bjsreefer]

Yes, I detail recipes here:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/

so I did the mix but my scale only goes to .1g... I went til it was 3.7 then normalized back to 3.6 assuming I must be right about smack dab in the middle. But my refractometer read that as almost 40, which is more than the 3ppt that I was different on... now i'm worried I'll kill everything if I try to bring it to that...

 

[Randy Holmes-Farley Verified Community Expert]

so I did the mix but my scale only goes to .1g... I went til it was 3.7 then normalized back to 3.6 assuming I must be right about smack dab in the middle. But my refractometer read that as almost 40, which is more than the 3ppt that I was different on... now i'm worried I'll kill everything if I try to bring it to that...

Scale up the mix to make a liter or more to reduce the scale rounding error.

 

[bjsreefer]

This does make sense since my precision hydrometer was reading lower than 1.026 and I thought it was just an error with the hydrometer basing my beliefs on the refractometer... teaches me to not trust the tried and true!