Caution for anyone using a refractometer

[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