Caution for anyone using a refractometer

[fkshiu]

I've been happily using a Sybon refractometer for several years looking down my nose at hydrometer users. I used the "super pure" FRESHWATER reference solution that came with it to calibrate it a few times to zero.

I had read this Holmes-Farley article a while ago: http://reefkeeping.com/issues/2006-12/rhf/index.php#18. Today I finally got around to calibrating the refractometer using a proper SALTWATER calibration solution (http://www.americanmarineusa.com/acc...linity%20fluid).

I thought my SG was 1.026, but after calibration it was actually 1.0235! OK, not the end of the world, but it goes to show that any instrumentation you use is subject to error and must be properly recalibrated on a regular basis!

$5 got me pretty much a lifetime supply of the solution. A good investment methinks.

[EmilyB]

I'm confused. I use a refractometer and calibrate it with RO. Never had a problem in over five years...????

[fkshiu]

Originally Posted by EmilyB

I'm confused. I use a refractometer and calibrate it with RO. Never had a problem in over five years...????

Same story here - never had any problem with livestock.

However, using FRESHwater to calibrate an instrument that you use to measure SALTwater can lead to some error. Anyone who uses a pH controller knows that you have to calibrate it at several different pH points for a proper reading - this is called slope calibration.

Using a SW reference solution as per the article does the same multipoint calibration for your refractometer. Mine was out by about 0.025 SG - not a massive amount, and my livestock has always been fine. I may try to bump it up a little bit to see what happens. It would be interesting to see what an electronic salinity meter would say. Anybody out there have one?

[Delphinus]

Huh, interesting...

But .. I'm still sort of confused. How can a saltwater reference have a SG of 1.000? Doesn't the salt make it, by definition, not 1.000? I thought we wanted to use RO/DI just so that we're getting a "real" 1.000 reading??

[Delphinus]

Uh .. ok, not having read Randy's article for now, but having followed the other link ..

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.

Whereas a hydrometer being a mechanical device where you just look through it, I think you may still want to use RO/DI as your 1.000 reference ...... just my $0.02, I'll try reading the RHF article later when my brain is less tired..

[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

[skylord]

I ran into the same problem. I set up my profilux and after calibrating the probe I was reading 1.019. Checked my cal on the refrac with pure water and it said I was right. Re-calibrated the probe and it was the same. Very frustrating. I get out a floating hydrometer and 4 swing arm hydrometers. I have never trusted swing arm hydrometers but all four read closer to what my profilux was reading than the refrac. The floating was the exactly the same as my profilux. I recalibrated the profilux probe about 20 times and it never changed. Still not being the trusting type I have only brought my salinity up to 1.024. I will get some fresh cal fluid and re-test some more and then bring it up to what I thought I had been running all this time..... 1.025

Scott

[Myka]

Originally Posted by EmilyB

I'm confused. I use a refractometer and calibrate it with RO. Never had a problem in over five years...????

Use the calibration solution and you'll see that you've had a hidden problem for a long time!

You're not supposed to use RO to calibrate.

[Doo]

Now I am lost!

When I bought mine from a LFS they calibrated it for me using RO water - is this wrong?

Where do I get this calibration stuff??

[BMW Rider]

The article describes how to make your own solution. I did so quite a while ago and was pleased to find that my refractometer was dead on at 1.026. I had also initially calibrated it with RO water. I imagine there can be some variation in manufacturing tolerances with these low cost units which would necessitate calibrating it to the desired reading instead of to 1.000. I'm sure if you've sprung for a high end lab grade unit, this would be less of an issue.