Salinity Measurement Help Please

jml1149 · Dec 9, 2016

I currently use a common 30 dollar refractometer to measure salinity, but I think something's up. I calibrate with the also available 35ppt standard, and then measure the tank. Lately, the tank has been reading around 30ppt, and I'm seeing some STN start.

So poking around online, I find this article with reference to common refractometers:

http://www.theaquariumsolution.com/specific-gravity-salinity-and-its-measurement

If this refractometer is indeed intended for brine, purely sodium chloride applications, and skewed by the presence of other elements as found in seawater, wouldn't calibrating it to the 35ppt standard resolve that?

Now, writing this, I believe the answer is no, unless the 35ppt standard contains the same chemical composition as seawater. Which I believe to be unlikely.

I have read Randy's homemade calibration solution instructions extensively here:

http://www.reefkeeping.com/issues/2004-06/rhf/index.php

But would this not make more sense if we used the same salt we use in our tanks? It would have to be adjusted based on composition and brand, etc., but would that not be more accurate?

Or, is the error, when calibrated, so low, it doesn't matter?

Last question and I'm done. My tank is at 78 degrees, the ambient room temp in my fish room is 65 (in the basement). The calibration solution and meter are at the same temp, but I let the tank water sit on the glass for a minute before reading. Does this affect accuracy, and should I take a sample upstairs and measure there?

Randy Holmes-Farley · Dec 9, 2016

Yes, calibrating it to a standard solution that has the same refractive index as 35 ppt seawater resolves that concern for measurements near 35 ppt, and that is what my standard does.

My standard is not 35 ppt sodium chloride, but has a refractive index that matches 35 ppt seawater. Using salt mix would not be desirable as it has variable amounts of moisture in it,.

Temp does effect the result, but a drop should come tot he same temp as the refractometer fairly quickly. I assume it is an ATC refractometer (automatic temperature compensation)?

jml1149 · Dec 9, 2016

Thank you for the response. Yes, I am using a hand held ATC refractometer. I just purchased the Miluwakee digital refractometer (MA887) though. Have you tested any of the commercial calibration solutions to confirm their refractive index as compared to seawater? Perhaps my calibration solution is bad, or has evaporated some and changed. I will be making your recipe tonight.

With respect to the measurements, I assume an ideal 2L coke bottle will hold 2L, so if I substitute an alternate graduated container, 2L is the goal?

As an aside, I really appreciate the level of detail you provide in your articles and papers. I have been reading them, as I'm sure many have, for many years.

Randy Holmes-Farley · Dec 9, 2016

Thanks. :)

I've not tested them, but they are easy to make so folks should be making them correctly. What brand? They can also evaporate over time.

If you have a scale, use that. Yes, a 2 L grad cylinder is much better than a coke bottle, but follow the recipe volume, not 2 L. A coke bottle holds more than 2 L when full.

jml1149 · Dec 9, 2016

This one:

Aqua Craft Refractometer Calibration Fluid
http://www.bulkreefsupply.com/aqua-craft-refractometer-calibration-fluid.html

And got it, 2104.4 mL of water, not 2L.

Thanks!

jml1149 · Dec 10, 2016

Got it. The calibration solution referenced above states refractive index of 1.3394 at 25 deg. C, which aligns with you had stated in your standard recipe. But because the refractometer is ATC, and the calibration solution is at the right temp, it should be fine correct?

This applies for your standard as well, as long as it is at the same temperature as the refractometer, should be fine at 65 deg F or 75 deg F.

The accuracy of these devices is listed as +/- 2ppm, but if we calibrate the refractometer to 35ppm, then immediately measure a sample of tank water, and it measures 35ppm, the only error in the process would be in the mixing of the standard. Is this correct?

Thank you again for the help, I greatly appreciate it. I'm an engineer, chemistry is a foreign language. But I enjoy attempting to try and completely understand what I'm doing, vs. just following the instructions. Thanks.