Calibrating refractometer

[Steadysteady]

I have just been looking at calibrating my refractometer. So here is what I did. I ensured that all liquids to be tested were at same temp as tank. I used a calibration fluid from Fauna Marin at 35ppt. I adjusted refractometer to this and then out of curiosity checked some warmed RO which should read 0 but it was approx 2ppt below this. Then I tested some RO at normal temp and it was about 4ppt below zero. So which do I trust. I always just did RO to calibrate at normal temp. Now I'm not sure what way is best to calibrate with the difference. Any input/thoughts welcome

 

[Toads74]

For single point calibrations, always go with the one closest to what you are measuring. In this case the 35ppt cal solution. Subsequently if you run out of solution you would set ro reading to -2ppt instead of zero for that refractometer.

 

[Steadysteady]

Well that intrigues me. So I have been calibrating cold RO to zero on refractometer and getting tank to 35ppt. But now that's a difference of 4ppt so I would be running the tank at 31ppt. Big difference. Begs the question how many people calibrate with cold RO and could have same difference.

 

[bat21]

Can't you just calibrate to zero using the RO water at tank temp? That's what I do.

 

[A.Astore]

Hmm, interesting find. I also followed the instructions for calibration of using RO water. Luckily it seems that as long as you provide a stable salinity and it is within reason the inhabitants seem to still thrive. What made you decide to change how you calibrated it?

 

[thegrun]

Can't you just calibrate to zero using the RO water at tank temp? That's what I do.

No, it will be off, it is much more accurate to calibrate with 35ppt solution

 

[Steadysteady]

I was checking calibration on few test kits against the same solution and got a notion to check the salinity too. That's when I came to it. But even calibrating to zero on RO at tank temp 25degrees has a 2 ppt difference. I hear what you are saying about stability vs actual salinity but I'd like that stability to be at the correct 35ppt if I can. I found it interesting though as I was always just told zero normal temp RO to zero but now I am researching a bit more and it looks like the better option is to go with a calibrated fluid at tank temp. Just thought I'd share and get some input.

 

[Member No. 1]

All you'll ever need to know...

Here:

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

And here:

http://www.reefkeeping.com/issues/2004-06/rhf/

 

[Steadysteady]

Great info. Thanks for the links

 

[DrPheel]

All you'll ever need to know...

Here:

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

And here:

http://www.reefkeeping.com/issues/2004-06/rhf/

Great read ! thanks for sharing

 

[Toads74]

Can't you just calibrate to zero using the RO water at tank temp? That's what I do.

If you want to do that, calibrate it at 35ppt cal solution first. Then rinse/clean the refractometer window and lid in ro water and measure the ro water. Where it shows then, whether 0 or not, that reading is your zero calibration point. That new "zero" point will be different for each device. Should be good enough to use this point for a long time, unless you drop the device or it gets knocked around.

The reason is that there is usually a slope error that inexpensive refractometers don't allow you to correct for. Great detail in the article posted.