New Refractometer

[FireEater]

I just got a refractometer, it is a Sper Scientific 300006.

It reads in percent from 0 to 28. How do I convert what number I read into the actual salinity level?

My tank now reads 3 1/2% with it.

[Ninong]

My tank now reads 3 1/2% with it.

That is the actual salinity level and it's perfect. That's 35 PSU (practical salinity units) or 35 ppt (parts per thousand).

[Ninong]

I've never heard of that model. Is it an ATC (automatic temperature compensating) model? Is it a seawater refractometer (which I seriously doubt) or a saltwater (NaCl) (brine) refractometer?

Most of the cheapy hobby refractometers are saltwater refractometers (not seawater) but they work just fine. You just have to adjust the reading a little. Most of the ones I have used are only about $45 retail and they're ATC. They usually show the scale on the left side of the vertical line in specific gravity and on the right side in PSU.

[Ninong]

Dr. Randy Holmes-Farley covers refractometers in excruciating detail in this article.

[FireEater]

Thanks for the info.

Sper Scientific
Salt Refractometer 0 ~ 28%, 300006

Measures salt content in sea water, aquariums, breeding ponds, or pickling-brines. While the scale is calibrated for sodium chloride, it can also be used for calcium chloride, barium cobalt, magnesium, cesium, potassium, sodium chloride, cupric sulfate, potassium iodide, citric acid, acetic acid and vinegar. Comes with a hard carrying case and calibration tool.

Dimensions:
6¾" x 1½" (171 x 38 mm)
Weight: 3.1 oz. (89 gr.)
Range: 0-28% Sodium Chloride
Resolution: 0.2%
Accuracy: ±0.2%
Warranty: 5 Years

[Ninong]

Sper Scientific
Salt Refractometer 0 ~ 28%, 300006

I found it online here. That model (300006) does NOT have automatic temperature compensation (ATC). The model with ATC is 300011.

ATC simply means that when the temperature of the product sample varies from 68F (20C), readings are automatically adjusted to compensate for temperature variance between 50F-86F (10C-30C).

I don't know enough about refractometers to be able to tell you how much different it makes whether it's an ATC model or not. (Yours is not.) However, even if the tankwater is at 82F, a tiny sample (one or two drops) should reach equilibrium with ambient room temperature very quickly once it is deposited on the glass plate. In other words, it seems to me that ATC would be much more important for work outdoors where the ambient temperature might be in the 90's or maybe in the 50's (or lower?) depending on the time of year. Of course, you could always collect a substantial sample and then take it indoors somewhere where the temperature is closer to 68F before using the refractometer to measure it.

Here's the bottom line on ATC: It's nice to have but I don't think it's going to be a deal breaker in your case. I think you'll be OK with the one you have but the ATC model would have been better.

Getting back to the difference between "saltwater" and "seawater" refractometers: Virtually all of the cheapy refractometers that we hobbyists purchase online are calibrated for sodium chloride, not seawater. This is not a problem provided you realize that the reading you see (since it is calibrated for NaCl and not seawater) on the scale is a little higher than the actual density. In other words, if it looks like it's 35ppt on the scale, that means that your actual salinity is about 33.6ppt (I think. I'm not going to look it up. Maybe Randy covers that in his article.) In my own experience, I automatically adjusted down by one full PSU, even though the adjustment should have been slightly more than that to be completely accurate. In other words, if I wanted my salinity to be 35PSU (35ppt), I made sure it measured 36PSU (36ppt) on the scale. That's was close enough for me.

So, if you read 3.5% on the scale of your NaCl refractometer, it means that your actual salinity is slightly less than 34PSU (34ppt) and not 35PSU (35ppt). (P.S. -- Based on Randy's article, if your reading is 3.5% (35PSU,35ppt) on your NaCl refractometer, it means your true salinity is 33.3PSU (33.3ppt).)

Dept. of Corrections:

It turns out the correction is a little greater than I thought to convert the NaCl reading to true seawater.

Here is Randy's explanation:

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place.

Randy also discusses ATC (automatic temperature compensation) in his article. You will have to read it for yourself to decide if you think it's important or not.

[FireEater]

Thanks for the reply. I'll read into the article to see about making the adjustment. I think once I make the adjustment I will be good to go.

I got this unit by trading a baseball size Cactus Pavano that was stinging neighboring corals to death.

[Ninong]

Thanks for the reply. I'll read into the article to see about making the adjustment. I think once I make the adjustment I will be good to go.

If you're talking about recalibrating it using a standard solution (35PSU), I wouldn't bother. Just remember in your head that whatever it reads, the actual salinity is about 1.7PSU lower. That's what I did with my refractometer. In fact, most hobbyists who use refractometers aren't even aware that their readings are off. They think their refractometers are seawater calibrated when in truth they're not.

I doubt that the lack of ATC will make much difference as long as you're testing indoors with reasonable ambient room temperature. Just don't hold the refractometer too close to your tank's lights or anything like that that might heat up the glass plate. The refractometer is calibrated for 68F (20C), so that's not too far off from what most people keep their homes, assuming they have air-conditioning in the summer. I don't think your reading will be much different if the ambient room temperature is say 74 degrees. If the ambient temperature is 94 degrees, then you're on your own. I haven't bothered to look up the appropriate correction factors for temperature variation.

I always tried to get a reading of 36PSU on my refractometer, knowing that that meant my actual salinity was between 34-35PSU -- my goal.

[FireEater]

Great, I'll go with that then.

Thanks!