Refractometer Calibration

Refractometer

A bit of looking on line, here is the information on the one that I am using (As stated earlier, probably a NaCl, as it was not all that expensive)

RHS-10ATC: 0 - 10% Salinity Refractometer with ATC:

This model is a salt concentration refractometer with a special scale. It has a built-in automatic temperature compensation system, for accurate measurements without recalibration.

Measuring Range: 0 - 32% Salinity
Resolution Factor: 0.1% Brix
Compensation Temperature Range: 10 degree C - 30 degree C
Accuracy: +/- 1.0 PPT +/- 0.001 SG
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With this base line accuracy and then the .0015 error you are referring to it is not hardly any better than a plastic jar with a floating pointer. But it is easy to read!

 

I'm not sure I understand. If it measures NaCl from .0014 to .0016 SG or 1.0263 or 1.0265 SG, it's pretty darn accurate. The problem only occurs because whe are trying to measure all of the ions in NSW as opposed to NaCl which is pretty-darn pure. It's only when when we extrapolate measurements from NaCl to NSW that we have issues.

Trust me, I'm not against refractometers at all. In fact, I use one and I recommend them. However, the thought that because it LOOKS EXPENSIVE, AND IS EXPENSIVE, makes it accurate is a fallacy as few refractometers are made for the reefkeeping hobby.

VERY FEW of us have a refractometer calibrated to NSW as it's most likely calibrated to NaCl. Most of us have purchased one after using an ATC hydrometer for a while. When we find a difference in SG, we assume that the hydrometer is wrong. The reality is that more than likely, the hydrometer was more correct (provided it was rinsed with RO/DI water after each use and not dropped).

Here's why I like refractometers. They may be off but they will be consistently off. Hydrometers will vary in the degrees that they are off unless you are meticulous in their care. As grubbsj said, refracts are also easy to read.

 

Ok i have a question. I use a refractometer, its a SYBON Opticon series FG100sa. Probably a NaCl refract so does this mean if i test with RO/DI ans it reads 0 that its correctly calibrated?

 

The Sybon's are pretty nice units. They are a Specific Gravity meter (I.e. for NaCl and not NSW). If you calibrate to 0 with RO/DI water it is going to read .0015 SG too high. In other words, if you were trying to match 35 PPT or 1.0265 SG, you would really need to add enough salt until you arrived at 1.0280. (1.0280 - .0015 = 1.0265)

Or you can buy 53 microSiemens calibration fluid everywhere and adjust to 35 PPT as opposed to 0. salinity calibration fluid - Google Search

There is an upcoming article that has not been finished yet on this subject. Regardless, Randy Holmes-Farley released a couple of graphs that are going to be in it.

NOTE: TO UNDERSTAND THE GRAPH IT'S IMPORTANT TO UNDERSTAND THAT THE REFRACTIVE INDEX OF NSW IS 1.339. THIS IS EQUIVALENT TO 53 MICROSIEMENS OR 35 PPT SALINITY.



Using the method, I'm discussing. I.e. using a real standard and calibrating to 35 PPT isn't without it's faults either. It will be highly accurate at reef tank temperatures but if you were trying to use hyposalinity in a hospital tank, the refract becomes more inaccurate, the lower in SG you go.

 

Very interesting... So most of our tanks are likely to be running at 1.024 rather than 1.026. that is not a huge difference. Possibly causing some issues in maybe inverts but would consistency be far more important than a deviation of 0.0015. One would think a salinity of 1.0245 that was constant wouldn't cause too many problems with the life we keep in our tanks

Jason

 

Thanks inwall. I haver never calibrated my refract yet, i bought it in april and the manufacturers instruction said it was pre-calibrated. I check it with RO/DI once in a while and it always read 0. So i always thought that it was ok and well calibrated. Was i wrong? I keep my salt at 1.026 does that mean that its at 1.0275 in reallity? since my refract is 1.0015 off?

Thanks

 

That's an excellent point Jason. I would rather have my tank consistently at 1.024 than 1.026 one day 1.028 the next and 1.025 the day after. I like to keep my reefs at 35 PPT because corals are osmoconformers and that is their average salinity. (I say average because they experience sudden upwellings of cooler, less saline water at times and major storms drop the salinity often as well for patch reefs and sometimes even major reef crests). Shrimp, snails, and many crabs do much better at the higher SG levels when coincidentally, many fish do better at the lower SG levels.

 

This is the second best way to calibrate your refract. It is just that it is calibrated for NaCl and not NSW. As a result, make the mental adjustment.

It's the other way....it's really 1.0245 even though it reads as 1.026.

 

Thanks for great info inwall!

 

This has been very helpful. Thank you for sharing your work.