Caution for anyone using a refractometer

[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