refractometer way off ... grrr

[sowellj]

So I am in the process of setting up a 180. The refractometer was calibrated against RODI. It seemed as if I was adding a good bit of salt, but I was measuring periodically but was no where near 35 percent. I then calibrated the refractometer using 53 uS/cm conductivity solution and it read 25. Recalibrated the refractometer, now tank water reads 45. I am inclined to believe that the test solution is correct but I need to run to the LFS and get a cheap hydrometer just to make sure I am in the ballpark. Never thought that the difference between calibrating off of RODI vs. Conductivity could be so great.

[username in use]

Anytime you calibrate an instrument, no matter what it is, you want to calibrate it to the middle of the range you plan to be testing. The reason being that everything out there is going to deviate slightly as you get farther away from the calibration point. Look at it this way, say there is a vertical steel bar that is semi bendable. Someone asks you to bend it 1/4" to the left and hold it there so they can read the measurment, not so hard. Then they ask you to bend it 37 1/4" to the left and hold it right on the dot so they can measure. A little harder because the bar will start to provide more resistance as you pull it farther from center. Not a great example, but kind of the idea.

[sowellj]

Yeah, what worries me a bit is that it is a new refractometer and a new calibration solution .... so it is difficult to say which one is off. To further muddy things ... the hydrometer gives a reading the doesn't really clear things up ... basically is halfway between the difference between the 'starndard rodi solution' and the 'calibration solution'. What do you guys think?

[jedheuer]

I think if your calibration is relativly new then trust that. Dont trust your hydrometer. Those are notoriously inaccurate. If you still doubt get another vial of calibration fluid.

[detroit_fan]

calibrating with ro water is never a good idea, the 35ppt solution is much better for what we do. i wouldn't put much stock into what the hydrometer says, they aren't accurate. If possible can you take a water sample to a close LFS and have them test salinty so you can verify your new solution and refractometer are reading correctly? Maybe even have another reefer buddy check it for you.

odds are the solution calibrated it correctly, but it would be nice to verify it.

[sowellj]

For now I will split the difference. I'll make a trip to the LFS tomorrow and get it worked out.

[hyperfocal]

This might be useful:

http://reefkeeping.com/issues/2006-12/rhf/index.php#17

[sowellj]

Thanks hyper, looks like I may be heading back into the lab to make my own calibration solution.

[sowellj]

So ... I made up a NaCl solution, 365 mg NaCl in 9.635 mL water. This solution came out at 35. Thanks again for the link hyper. So, as the article says, salinity standards may have the same conductivity but different refractive indexes.

[hyperfocal]

No problem, glad it worked out for you.