Calibrating refractometer

EvilE

EvilE

New member
I recently got a refractometer, and I calibrated it, and used it. My question is how often should I calibrate it?? Once a week, before each use, what??
 
I calibrate mine before each use, but I know it's excessive, and probably unnecessary. Most weeks I don't even have to adjust the nob at all.
 
Randy says in that article,

For many refractometers used by reef aquarists, the manufacturer calls for pure freshwater to be used for calibration. With a perfectly made refractometer (that hasn't changed since its manufacture), that single point calibration at the end of the range (Figure 5) would be adequate, albeit not perfect. A better single point calibration might be performed in the middle of the range being used, and for higher accuracy, more than one calibrating solution would be used.
Click to expand...

If the slope is off then even using a seawater standard will give inaccurate readings for other values. It takes two points to establish a straight line so the more involved calibration is great but only periodically.
 
I have found the pinpoint solution to be unreliable as a refracto calibration standard (checked 8 bottles of the stuff). Others have found the same, such as this post here:

http://reefcentral.com/forums/showthread.php?s=&threadid=1007891

IME, the only way to reliably calibrate is to make your own using the recipe in the link Waterkeeper gave. A friend of mine made some of Randy's DIY in the lab for our club using 99.9% minimum HPLC grade NaCL and 18.2 MegaOhm water. Another way to calibrate would be to find someone with a good floating glass aquarium hydrometer, and calibrate your refracto to the SG measured by the hydrometer. Your LFS may have one.
 
For daily calibration use RO/DI and every couple of weeks use both a DI and 53 microsiemen solution to check if the slope is correct. If you are really in doubt then do the two point and use a third standard that falls between the calibration solutions to act as a true check. That should only be needed if you lose faith in the readings.
 
If it is important to you to have it accurate for Hypo and SW, then you need to calibrate and have it accurate at both ends of the scale.

If you are only using it to test SW, you need only be concerned about the 1.026 end. My refractometer when calibrated with RO/DI is off by .003 when measuring SW. Since I dont care about FW measurement, I want it accurate there.

My refractometer is way off when testing with Pinpoint 53mS calibration fluid
This thread I started prompted Randy to write the article of refractometer calibration.
 
<a href=showthread.php?s=&postid=9569143#post9569143 target=_blank>Originally posted</a> by WaterKeeper
For daily calibration use RO/DI and every couple of weeks use both a DI and 53 microsiemen solution to check if the slope is correct. If you are really in doubt then do the two point and use a third standard that falls between the calibration solutions to act as a true check. That should only be needed if you lose faith in the readings.
Click to expand...

I actually made up some high quality 1.009 in addition to the 1.0264 made up in the lab for checking salinity because I do hypo in my QT. The RO/DI is off by about .002, while the 1.0264 (35 ppt) and the 1.009 are dead on.
 
FWIW, the pinpoint bottle that sjm used in the linked thread is likely on target. Out of the 8 bottles I and a few others tested, only one was on target according to the high quality DIY solution. This was in line with what sjm reported. The other bottles were off by differing amounts so I drew the conclusion that the pinpoint solution was unreliable as a refracto calibration standard.
 
<a href=showthread.php?s=&postid=9569468#post9569468 target=_blank>Originally posted</a> by crumbletop
FWIW, the pinpoint bottle that sjm used in the linked thread is likely on target. Out of the 8 bottles I and a few others tested, only one was on target according to the high quality DIY solution. This was in line with what sjm reported. The other bottles were off by differing amounts so I drew the conclusion that the pinpoint solution was unreliable as a refracto calibration standard.
Click to expand...
So what do you suggest then?
 
Someone needs to sell an accurate calibration fluid. According to the people that make the Pinpoint fluid, theirs is, but apparently it may or may not be.

I tried the DIY route, but I guess I'm "measuring cup challenged". I could not make two batches that came out the same.
 
<a href=showthread.php?s=&postid=9569556#post9569556 target=_blank>Originally posted</a> by sjm817
Someone needs to sell an accurate calibration fluid. According to the people that make the Pinpoint fluid, theirs is, but apparently it may or may not be.

I tried the DIY route, but I guess I'm "measuring cup challenged". I could not make two batches that came out the same.
Click to expand...
After reading all of this-- there really is no true answer. I guess I will just calibrate it with distilled or RO water, and hope it's not far off. I think that RO would give me a close enough reading, I mean if it's off by like .0002 or something, it wouldn't be so bad.
 
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