Caution for anyone using a refractometer

[jasond]

I use RO as well....I might have to give this a shot...

[Reefer Rob]

The problem is that a lot of the refractometers that find their way into the aquarium industry are for measuring brine, as in for the pickling industry. Our salt water has a lot of other major elements that also effect the refractive index of the water.

According to Pinpoint, their calibration fluid also works well to calibrate our refractometers.

Here's the original thread that spawned Randy's article.

[Doo]

so - aside from the PINPOINT product - are there other products that can be picked up locally??

[fkshiu]

Originally Posted by Delphinus

Isn't that calibration fluid intended for the electronic salinity monitor? In that case it makes sense to me that it's using a saltwater based reference because it's using a relationship from conductivity to relate back to salinity.

This quote from the article will answer your question, Tony:

"One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. It is sold as a 53 mS/cm calibration fluid for the company's electronic salinity probe (a conductivity probe), but it also is suitable for use in a refractometer. NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The Pinpoint fluid happens to be made to match seawater in other respects, not just conductivity, but other brands, or do-it-yourself 53 mS/cm standards, may not be appropriate to use with a refractometer because, while they have the same conductivity as 35 ppt seawater, they may not have the same refractive index.

For example, standard seawater with S=35 (35 practical salinity units, or PSU) is defined as seawater with the same conductivity as a solution made from 3.24356 weight percent potassium chloride (KCl), and that conductivity is exactly 53 mS/cm (mS/cm, or milliSiemens per centimeter, is one of the units used for conductivity). That solution, however, has a refractive index of about 1.3371, matching seawater just below 26 ppt. So do not assume that all 53 mS/cm conductivity standards are suitable for refractometer calibration.

Salifert has a product called Refracto-Check that they often give away at meetings like MACNA. It is a 35 ppt seawater refractive index standard, but it is not widely available commercially."

I picked up the standard at J&L so it's easily accessible to anyone. The interesting thing is that they had to go digging around the store for awhile to find it so obviously very few people - in vancouver at least - use it for calibrating. I highly doubt very many more people have gone to the trouble of putting Randy's homemade standard recipe together (Randy IS a chemist BTW) so the concerning thing is that nearly all refractometers being used for SW out there COULD be reading incorrectly.

I do realize that the article is very long, but all you really need to read on a practicle level is the beginning and the end ("Commercial Salinity Standards" onward). All that egghead stuff in the middle can be saved for when you have insomnia

[Reefer Rob]

Originally Posted by Doo

so - aside from the PINPOINT product - are there other products that can be picked up locally??

As far as I know Pinpoint is the only one. Thats what everyone who posts on RC is using. Alternatively you could find a buddy that uses a well calibrated salinity monitor on their tank, and use their tank water to check your refractometer.

[Delphinus]

Ah, ok... verrrrry interesting. Thanks for the synopsis, fkshiu.

Reference fluids of 35ppt seawater are a brilliant idea. I really wish most testers we have would have this available. I know Seachem testkits used to have reference solutions but I stopped using Seachem when they changed their reagents so that it the colour changes were so faint it was impossible to use.

The only tester (electronic or otherwise) that I have any degree of confidence in is my Pinpoint NO3 meter, and that's because you need to calibrate at the endpoints of the measuring scale for every test. You can use the calibration fluids as the reference. Reference solutions ought to be an industry standard, that way if you have any doubt about a reading, you can at least still verify your testing. Oh well, [/rant]

[Doo]

Ok - this great - I learned something new...

Now, what do I do to Calibrate my Refractometer - can some one give a play by play???

What solution?
what should it read when I put it on my Refractometer?
If it does not read that nymber, what do I need to move it to so that I know it is correct?

[fkshiu]

Originally Posted by Doo

Ok - this great - I learned something new...

Now, what do I do to Calibrate my Refractometer - can some one give a play by play???

What solution?
what should it read when I put it on my Refractometer?
If it does not read that nymber, what do I need to move it to so that I know it is correct?

Direct from the article:
Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. So my recommendation for calibration is as follows:

1. First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261.

This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I've never understood how that could matter and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article.

Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified to have passed the second calibration step below at least once. This is an offset calibration, as described above.

2. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. This step involves testing it in a solution matching the refractive index of 35 ppt seawater (or some similar solution near the range of measurement). Remember to let it sit for at least 30 seconds so it comes to the same temperature as the refractometer. Suitable commercial and do-it-yourself standards were described earlier in this article. Using one of them, place a drop onto the refractometer and read the value. If it reads approximately 35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940, then the refractometer is properly calibrated and is set to go.

If it does not read correctly, and is off by an amount that is significant relative to your salinity precision requirements, then you need to recalibrate it using this second fluid. I suggest that a salinity error of ± 1 ppt or a specific gravity error of ± 0.0075 is allowable. If the refractometer is off significantly, and you used a do-it-yourself standard made with crude techniques such as Coke bottles, a good next step might be to buy a commercial standard.

To correct errors using these seawater standards, simply adjust the calibration screw on the refractometer until it reads the correct value for the standard (35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940). This type of slope calibration makes the refractometer suitable to read solutions whose salinity is close to seawater's. After such a calibration, refractometers may not read freshwater correctly.

Again, despite the claims in the directions of some refractometers to have the standard at a particular temperature, when calibrating an ATC refractometer with this seawater standard, I'd just use it at room temperature.

If you are using a refractometer for hyposalinity, such as when treating a sick fish, I'd just use one calibrated in freshwater, because that is closer in salinity than seawater to the hyposaline solution usually used (say, specific gravity = 1.009). A new standard for hyposalinity can also be made by mixing one part 35 ppt seawater and two parts freshwater, but that is probably overkill.

Other Tips on Using a Refractometer



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Clean the refractometer between each measurement using a soft, damp cloth. Failure to wipe the prism can lead to inaccurate results and damage to the prism.

Do not immerse the refractometer in water. If the refractometer looks foggy inside, water has entered it. You may or may not be able to dry it out without damaging the unit. Do not measure or clean it with abrasive or corrosive chemicals.

If the scale is completely dark, you may not have added sample to it in the appropriate way. If the scale is completely light, then the liquid's refractive index is above the refractometer's high end.

[Doo]

Very Cool - Thank you!

[Lance]

fkshiu: After reading this thread I compared readings of my refractometer to an old hydrometer I had laying around. Refractometer read 1.026, hydrometer read 1.023. Then I tested the QT tank: refractometer: 1.025, hydrometer: 1.022. Borrowed another hydrometer from a friend: same readings.
So i am going to order some of that solution ASAP!
My corals, inverts and I thank-you my friend. Lance