Calibrate Refractometer with RODI?

[MReef20]

So, I've got a BRS refractometer, along with the BRS calibration solution, but I had a minor drop a while back, so I wanted to get another one (this time the Marine Depot version) to make sure all was good.  I went ahead and got the Brightwell calibration solution along with it (I believe the BRS solution is made by Brightwell).  When I calibrated the new refractometer with the new calibration fluid, it showed low as compared to the other solution.  I checked with my calibrated BRS device and got the same result.  In other words, it would seem that the two solutions were showing different readings.

So, I put RO water on both and set to 0 (the BRS one was already showing 0).  I then checked against the two solutions again, and the BRS solution showed 35ppt on both, and the Brightwell one lower (like maybe 33-34ppt).  Is that an acceptable/alternate way to calibrate to validate which calibration fluid is correct?

Edited February 28, 2021 by MReef20
typos

[cjmdh]

I've seen swings due to temperature of both the refractometer and the calibration solution.

[AquaKey]

@MReef20 are you waiting a little bit for the sample to equalize with the temp of the refractometer?

I've found distilled water to be a good way to calibrate.

[EMeyer]

RODI water is in principle the wrong way to do this. On some refractometers, you may get lucky and get away with it, if the refractometer has a perfectly linear response. (I wouldnt bet on this, for instruments that cost on average $30)

 

When we calibrate a refractometer we are performing a 1-point calibration. Anytime you're doing  a 1-point calibration, that point needs to be as close to the target level. The farther you get from your calibration point, the larger the errors will be. So calibrating with RODI is great if you want to read RODI. But if you calibrate with RODI and read seawater, you'll get errors. 

 

I like the homemade calibration solution, pretty easily prepared with household materials

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

[MReef20]

Thanks for the input...the readings got a little closer when I replicated conditions as best I could for both - still not exactly the same though.  I've heard of the bottled solutions not always being 100% accurate, so I agree that the homemade solution is probably the most reliable.

 

Thanks @EMeyer for the good explanation re: why not to rely on RO water to calibrate.