Calibrating Refractometer...how do you do it?

[Hawkeye]

All - over the course of several months, a friend of mine has been diligently changing the water in his tank, except only last weekend (when i went over) did he realize his refractometer (from marine depot) was calibrated erroneously. I checked his salinity and it read 10ppm. But his refractometer read it as 35ppm. Basically he had made his FO tank into a brackish/fresh water tank....killing almost everything except the cyano algae and 3 hardy fish (sailfin, clownfish and one other).

How does this happen? Does this mean he never checked his main tank (and only checked the make-up water - and assumed all was well)?

So now that we know, and he's killed 3/4 of his fish tank inhabitants (there were no corals) - what should he do? I have never recalibrated one of these things in all the years i've done this.

 

[steamman]

You can take some RO DI water and adjust the refractometer to zero. It may not be the most accurate way but it will be very close to right on. There is a little screw near the glass under a small rubber cap. You need one of those little eyeglass type screwdrivers. His refractometer should have come with a small driver.

 

[downbeach]

http://www.bulkreefsupply.com/store/aqua-craft-refractometer-calibration-fluid.html

 

[redfishbluefish]

I believe what Jim posted above is similar to what I use HERE…..a calibration fluid that calibrates your refractometer to 35ppt. You are better off calibrating to the number you are interested in then “zeroing” the refractometer with DI water. It’s more accurate that way.

I also find it odd for a refractometer to be “off” that far….reading 35 ppt when it actually is 10 ppt. That’s way off. NOW, here’s the question I have for you Hawk, when was your refractometer last calibrated? How do you know if yours is right?

 

[steamman]

I believe what Jim posted above is similar to what I use HERE…..a calibration fluid that calibrates your refractometer to 35ppt. You are better off calibrating to the number you are interested in then “zeroing” the refractometer with DI water. It’s more accurate that way.

I also find it odd for a refractometer to be “off” that far….reading 35 ppt when it actually is 10 ppt. That’s way off. NOW, here’s the question I have for you Hawk, when was your refractometer last calibrated? How do you know if yours is right?

Zeroing with ro water is better than what he has been doing that is for sure. How far off do you think ro water at zero is?

 

[redfishbluefish]

Zeroing with ro water is better than what he has been doing that is for sure. How far off do you think ro water at zero is?

All I’m saying, regardless of the instrument you are using, is that you are better off using a calibration standard close to what values you are actually attempting to measure.

Your refractometer MIGHT be just fine calibrating with distilled (RO/DI) water. However, if there has been a “shift” in the linearity of the meter, your reading of saltwater at 1.026 sg is going to be off.

Here’s a good read from the good Doctor Holmes-Farley. Check out the section on calibration.

 

[steamman]

I was going to post the same thing but I couldn't edit my post. I waited too long. If nothing else is available pure water works. It is sure better than being 25 ppm off. At least you can get it within acceptable ranges.

 

[dnov99]

Best thing to use is what Paul said, been using that for years. Using RO will be off by a point or 2 over time, which is not where I want to be...

 

[Hawkeye]

I also find it odd for a refractometer to be “off” that far….reading 35 ppt when it actually is 10 ppt. That’s way off. NOW, here’s the question I have for you Hawk, when was your refractometer last calibrated? How do you know if yours is right?

I know! I was shocked. I think he did something wrong with the thing.

As for mine -good question! I've been relying upon the 40 gallon bags per Reef Crystals. What I mean is that I have a 55 gallon bucket that is marked off for 40 gallons. I fill up to the line, and then add a full bag. I then check that with my refractometer.

 

[steamman]

I just checked my refractometer with ro water. The reading was dead on 0. Then I used calibration solution and the reading was dead on 35 ppm. I did not have to adjust a thing. So my ro calibration is just as dead on as using the calibration solution. Take it with a grain of salt. If you have good clean water you can calibrate your refractometer with it.

I am happy for the debate here. It made me recheck myself. I have had the calibration solution and never used it. Now I know that the ro method is accurate. Or maybe I have special RO/DI (magic water).