Refractometer calibration

[Killer bunny]

Ok, maybe I'm going overboard, but... my tank is coming up on 6 months old now (32g Biocube,) and I have been getting consistent SG readings of 1.025-1.026 for at least a couple of months. I test the salinity weekly at water change time and once midweek just to make sure it's ok. I check calibration at every use using calibration fluid. I have heard numerous recommendations to calibrate using calibration fluid as opposed to RODI water, because the fluid is closer to, if not right at the salinity our tanks are kept at. When I had an LFS test the water to verify some other readings, they told me they were getting 1.028. Another store later got the same number. Both stores told me they calibrate using RODI water. So, I tried calibrating mine using RODI water and I started getting the readings the stores were getting. So now I'm wondering which one to trust. I guess if I leave it calibrated with RODI water, I'd be erring on the side of caution, since it results in a higher reading, but I'm just curious if anyone has noticed a difference in readings using water vs. fluid and what they recommend. Also, I'd love any recommendations for refractometers that people really like, as I am thinking of replacing mine anyway, since it is on the cheap end.

 

[Tripod1404]

Use calibration fluid. The other way is wrong, it is like calibrating a scale using a mouse and than using the scale to weigh horses.

Calibration needs to be done at values close to the expected measurement results. This is true for anything that needs calibration.

 

[Killer bunny]

Thank you

Thank you

Thanks tripod, much appreciated - it certainly seems more intuitive to use fluid versus pure water.

 

[billdogg]

Exactly what tripod said.

This topic comes up quite often. The correct answer is to use calibration fluid regardless of what the manufacturer suggests.

 

[Killer bunny]

Thanks

Thanks

Thanks Billdog - I recalibrated using the solution and the tank water tested right where it was before - at 1.026

 

[ReefkeeperZ]

LFS do not tend recalibrate their refractometers very often, and since they get heavy use and handled a lot they tend to be less accurate I would trust yours since you know it is calibrated, or even pick up a second one to verify on your own. I keep 2 refractometers and 3 different hydrometers (collected over the years) to keep each other device honest.

 

[Killer bunny]

Thanks

Thanks

Thanks Reefkeeperz - a second, higher quality refractometer is the next piece of equipment I plan to buy...

 

[billdogg]

I actually rely more on my pair of 30 year old swing arm hydrometers than I do the refractometer. Although both of them are off a bit from what the refractometer indicates is correct, they are marked where they are correct - in effect, calibrated. They are also much easier to use.

 

[jjencek]

I used RODI water thinking I was fine. When I rechecked with salifert fluid after months I was way off.

I thought I was keeping my water at 35 ppt (when calibrated with RODI). When I finally calibrated with salifert, I found out I was at 31 ppt.