Calibrating Refractometer

[da1jewfish]

Ok, I am now confused.
I just calibrated it with pinpoints 53.0mS calibration fluid and it was .004 higher than it supposed to be! Then I put some distilled water on it to see what comes up and its .004 below 0. I don't know what to believe now.

Any assistance?

[TXAlbert]

RTFM?

[Mike O'Brien]

Believe the calibration fluid.

[abulgin]

is your refractometer adjustable?? No use in calibrating it if you can't adjust it. If it has a screw, you need to crank it down with the cal fluid until it reads 0.

[da1jewfish]

Whats RTFM?

abulgin, it is calibratable (adjustable). The calibration fluid I have is for 1.026. If I crank it to 0 then when water is 1.026 it will read 0

[cveverly]

I would trust the calibration fluid over distilled water. At least that is what I have read.

I set mine to 1.026 or 35ppt with the calibration fluid in it.

[J. Montgomery]

Go with the calibration fluid. The problem is that most hobbyist refractometers are not accurate over the range from 0.000 to 1.026; therefore you should calibrate it near the value you intend to use. Its not unusual for RO/DI water to read below 0.000 once the refractometer has been calibrated to 1.026.

[abulgin]

ah, I misspoke--dont know what I was thinking. When you use the cal fluid, you need to crank the reading up to 35 ppt. Then, it is calibrated. Sorry for confusion.

[mg426]

Originally posted by J. Montgomery
Go with the calibration fluid. The problem is that most hobbyist refractometers are not accurate over the range from 0.000 to 1.026; therefore you should calibrate it near the value you intend to use. Its not unusual for RO/DI water to read below 0.000 once the refractometer has been calibrated to 1.026.

Listen to this !!!!!!!!!!!!!!!!!!!!!!!

[da1jewfish]

aight, thanks a bunch yall

[Randy Holmes-Farley]

Just so folks understand better, the primary reason that refractometers can be inaccurate for measuring seawater, aside from potential mismanufacturing, is that they are all intended to measure brine solutions. That is, sodium chloride in water. Since seawater has more in it than sodium chloride, that causes problems.

in short, refractometers measure the refractive index of the water, and then try to relate that back to the salinity or specific gravity. But the relationship between refractive index and these other parameters is different for seawater than for sodium chloride solutions.

So the best way around that is to calibrate with a solution that is known to match the refractive index of normal 35 ppt seawater. That can be done with a true seawater like solution (such as the Pinpoint standard) or with a sodium chloride solution that has the same refractive index as 35 ppt seawater.

It cannot necessarily be done with any other solutions, such as other 53 mS/cm conductivity standards, since they do not necessarily have the refractive index of seawater.

I discuss these in more detail here:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

[pelochas]

cool thanks, now i need to recalibrate my refractometer because i was doing the same thing with ro/di water and adjusting to 0 ppt.

where can i get a few drops of Pinpoint Standard or 35 ppt seawater?

[Randy Holmes-Farley]

You can make the DIY in the article, which is only highly accurate if you have a good balance, or you can buy the Pinpoint from many vendors.

[moo0o]

i have a question thought, in the bottle it says something about 77 degrees, does the bottle have to be 77 when you read it, or will it always be at 33ppt no matter what temp

[Randy Holmes-Farley]

35 ppt, and if it is an ATC (automatic temperature correction) refractometer, then any temp reasonably close to tank temp is OK, and if the ATC is perfect, then any temp is OK.