Salinity Calibration Conundrum

[Hooloovoo]

I need some help figuring out how to accurately determine my tank's salinity. This is for a new tank with no livestock added yet. I have an Apex Controller with conductive salinity probe, and a refractometer. I would tend to trust the refractometer more than the conductivity probe, but I am having some concerns about the calibration of the refractometer.

The refractometer is a JBJ CScope that I purchased over at World Wide Coral's brick & mortar location in Orlando. It's the same model they use. The instructions that came with it said to calibrate with distilled water, which I did initially. When I got my Apex installed and calibrated the salinity probe, the results were way off from what my refractometer showed. I tried recalibrating the conductivity probe several times, but its results stayed consistent.

After some investigation, I came across a post suggesting that it was more accurate to calibrate refractometers with a 35 ppt refractive calibration solution, so I purchased some from BRS and recalibrated my refractometer accordingly.

Here is the calibration solution I used:

This brought the refractometer and conductivity probe measurements much closer to agreement, and I was satisfied with the result until...

I took a water sample over to World Wide Corals to have them test it (mainly because I was getting conflicting results from my Red Sea and Hanna Alk tests, but that's another story). They told me that my salinity was way too high. I explained my previous calibrations, and they told me that they alway calibrate to 0 using RO/DI water. They went ahead and checked their calibration using RO/DI while I was there, and then confirmed that their tanks were showing the proper values with this calibration.

Now my question is, should I calibrate to 0 using RO/DI, or should I continue to calibrate to 35 ppt using my refractive calibration solution? My instinct would be that the calibration solution would be more accurate, but World Wide Corals is obviously very successful keeping their corals at a salinity based on measurements obtained with RO/DI calibrated refractomers.

I decided to take a series of measurements using both calibration methods to confirm that it does make a difference.

Here is the reading I get from my display tank after calibrating my refractometer to 0 ppt using RO/DI:

Here is the reading I get from my display tank after calibrating my refractometer to 35 ppt using the calibration solution:

Meanwhile, my Apex conductivity probe reads 35.1 ppt.

 

[bertoni]

If the refractometer calibration fluid was made properly, calibrating with it will be more than accurate enough for our purposes, and possibly much more accurate than calibration with RO/DI:

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

If the Apex has been calibrated properly, and the refractometer agrees with it when calibrated with the Aqua Craft product, I would believe the 35.1 ppt and ignore the store. Their demonstration proved nothing. We have no idea how accurate their SG device is.

 

[mcgyvr]

Calibrate with the calibration fluid..
Calibrating with RO is not correct and will lead to this problem..

Their refractometer is not calibrated properly. Yours is.. trust yours

 

[ReefsandGeeks]

I'd also trust yours after calibration with 35ppt fluid. I'd also caution to make sure you double check your refractometer's calibration before making any adjustments to your tank based on salinity. My refractometer drifts quite a lot if I let it sit for a week, even in a controlled environment. ;My procedure is to always test a drop of calibration fluid and make sure the meter is reading correctly before ever testing my tank water.

 

[outssider]

I'd also trust yours after calibration with 35ppt fluid. I'd also caution to make sure you double check your refractometer's calibration before making any adjustments to your tank based on salinity. My refractometer drifts quite a lot if I let it sit for a week, even in a controlled environment. ;My procedure is to always test a drop of calibration fluid and make sure the meter is reading correctly before ever testing my tank water.

+1 always use calibration fluid and always calibrate or check the calibration before every use.....refractometers drift especially if the ambient room temperature has changed. The store is wrong....very wrong....

 

[Timfish]

I use tapwater to calibrate my all refractometers. As pointed out by Holmes-Farley some refractometers are designed to test saltwater made with just sodium chloride and will be roughly 2 PPT low testing natural or synthetic seawater when calibrated with RO. If a refractometer designed to test sea water it will be accurate if it's calibrated with tapwater, RO, RO/DI, distilled or with a 35 PPT seawater standard. For my refractometers that are for just saltwater and not seawater I just add 2 PPT.

 

[bertoni]

That calibration might work, but it also can fail. Even if a refractometer is intended for measuring ocean water, it can have defects in its manufacturing or problems induced by handling. I would calibrate with a saltwater standard, in all cases, but I am cautious.