Refractometer calibration/temperature question

Randy Holmes-Farley · Oct 16, 2014

Sully said:
But, from what I've read and understand is works like this. The refractometer itself needs to be very close to 68 degrees (20 degrees Celsius) when you calibrate it. The solution temp doesn't matter (within reason, I wouldn't use water or solution that is extremely cold or hot. Room temp should work fine). .

That is the claim, but it is incorrect for the reasons stated above.

I agree that calibrating at the same temp as the water to be measured (meaning the temp of the water after it hits the refractometer, which may be a combination of tank temp and refractometer/room temp, but mostly the latter) will give the best result in the chance/expectation that the ATC is not perfect.

Randy Holmes-Farley · Oct 16, 2014

-Logzor said:
I believe salinity varies depending on water temperature, this is why calibration fluid states that it's 1.0265 @ 77 Degree Fahrenheit. My understanding was that the refractometer will automatically correct for this regardless of water temperature.

How does this work exactly and is this function dependent on the original calibration temperature?

This is why I moved to a conductivity meter, had nothing but trouble with my refractometer, especially during the winter months. Could never figure it out, I could place a sample and measure, it would result in a reading, I could leave that same sample sit for a few minutes and the reading would totally change. Maybe mine was broken.

Salinity doesn't vary with temperature. 35 ppt seawater is 35 ppt seawater at any temperature.

The specific gravity will change, but that mostly only impacts hydrometer readings directly.

The index of refraction also changes with temperature, which confounds refractometers.

I describe how ATC works here:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

Note that the reading may also depend on the lights use, so use the same ones for calibration and measurement.

mike007 · Oct 16, 2014

I bought one of the new Redsea refractors and when I calibrate with di water and then check with salinity solution It does not match.

Randy Holmes-Farley · Oct 16, 2014

mike007 said:
I bought one of the new Redsea refractors and when I calibrate with di water and then check with salinity solution It does not match.

Not surprising. Most refractometers sold to the hobby won't work correctly if calibrated with RO/DI and should be calibrated with a 35 ppt seawater standard.

The reason is they were made for measuring brine solutions, not seawater, and brine has a slightly different relationship between refractive index and salinity/specific gravity than does seawater.

Using a 35 ppt standard for measuring near that salinity largely eliminates this issue.

mike007 · Oct 16, 2014

Randy Holmes-Farley said:
Not surprising. Most refractometers sold to the hobby won't work correctly if calibrated with RO/DI and should be calibrated with a 35 ppt seawater standard.

The reason is they were made for measuring brine solutions, not seawater, and brine has a slightly different relationship between refractive index and salinity/specific gravity than does seawater.

Using a 35 ppt standard for measuring near that salinity largely eliminates this issue.

These new Redsea refractors are supposed to made for saltwater not brine. I guess my question is should I trust the di water calibration or the salinity reference solution? They seem to be far apart.

Sully · Oct 17, 2014

I thought mine was working fine. Now I'm questioning it with all that's been posted here.

Eienna · Oct 17, 2014

Well, that's disturbing. When I asked about calibration solution the people I bought my refract from hadn't even heard of it.

saltyphish · Oct 17, 2014

Always use 35ppt calibration solution. Not only is it what is recommended by many manufacturers it is also very close to the target salinity making a more accurate measurement.

Randy Holmes-Farley · Oct 17, 2014

mike007 said:
These new Redsea refractors are supposed to made for saltwater not brine. I guess my question is should I trust the di water calibration or the salinity reference solution? They seem to be far apart.

OK, that's one of the few seawater refractometers. I guess it boils down to whether you have more confidence in the maker of the calibration solution (and its treatment since manufacture, including evaporation), and the maker of the refractometer. It is generally impossible to know which is more accurate with a single test. Maybe just split the difference.