Question for the scientists

[Sula]

"ATC Refractometers cannot measure Specific Gravity. "

What are you saying they are measuring?

"I could be wrong but I believe traditional Refractometers measured Degrees of Brix for winemaking. Sugar content or something like that."

Sugar content in liquid...probably by measuring the SG.

The traditional use of refractometers that I am referring to is in a clinical laboratory setting, where they were used to test and report specific gravity of body fluids.

 

[Anonymous]

"What are you saying they are measuring?"

Refractive index which is closely coupled to salinity.

Let me put it this way.

If I have a liquid that I know has a salinity of 35ppt and I heat it to 85F (my nano tank) and test it with my ATC refratometer it will show a Salinity of 35ppt (ppt scale on the right) and a S.G. of 1.026 (S.G. scale on the left).

If I cool the water to 75F (my pigmy seahorse tank) it will show a Salinity of 35ppt and a S.G. of 1.026 when I use the refractometer per instructions.

I know that the water has a greater density at 75F but the refractometer didn't indicate that. Saltwater at 35ppt cannot have the same S.G. at 75F and 85F. But it will have the same Salinity (for hobbiest purposes).

Specific Gravity (density) is related to temperature when measuring salt water, Salinity does not change significantly with temperature change. ie. the S.G scale on the ATC refractometer is inaccurate on my reef tanks but the salinity scale is right on.

 

[Sula]

Thank you, now I understand what you are saying.

What I was having trouble with was the statement that SG could change and Salinity would not. Since as you say the only measurement made is of refractive index, I was trying to figure out how the SG and Salinity could be independent measures to you. Because all I could think was how easy it would be to alter the SG without changing salinity and come up with a whole different reading for both.

But if I understand you correctly, you are saying that with respect to sample temperature fluctuations, the refractive index reading on an ATC refractometer does not change, and since you know the SG does change and the salinity does not, the salinity reading is more accurate.Right?

So just to play devils advocate, is it possible that sample temperature isn't that big an issue? After all, you just use 2-3 drops of sample. If your refractometer is at 78 degrees and your sample is 75 degrees, I'm thinking that by the time you lift the refractometer up to your eye, the sample has warmed up to 78 degrees. Same should hold true for cooling down. Which would mean that the actual reading of both samples are being taken at the same temperature, which is the temperature of the refractometer. Any thoughts?

Sorry to take this thread on a big tangent, it's just interesting to me....

 

[Anonymous]

The refractometers on the market for the hobby are ATC (Automatic temperature compensating). The Salinity reading will be correct for reasonable water temperatures. It's actually advised to let the water cool for 30 seconds on the lens before reading. I have tested this because I have tanks at different temperatures. I have also tested using a fairly accurate glass hydrometer.

As far as "is it possible that sample temperature isn't that big an issue"? That's up to you. Typically 1 or 2 ppt isn't all that significant. For me it often is. For anyone doing a hyposalinity treatment 1 or 2 ppt is significant, 14ppt is the desired salinity to kill parasites. Lower salinity will stress the fish more and higher salinity might not kill the parasites. For acclimating Starfish or large snails, 2ppt is very significant.

For me, the 2 ppt is critical. Rotifers will die of osmotic shock if placed in a salinity more than 5ppt. But, they grow best at lower salinities. So, I need to keep them at a salinity exactly 5ppt lower than the larval rearing tanks I have for Clownfish and PJ Cardinals. If I'm off by 1 or 2 ppt then either my rotifers grow too slow or they die when I feed them to fish larvae.

The bottom line is you have a precision instrument capable of giving accurate salinity readings. Why not use it in the most accurate way possible.

But if I understand you correctly, you are saying that with respect to sample temperature fluctuations, the refractive index reading on an ATC refractometer does not change, and since you know the SG does change and the salinity does not, the salinity reading is more accurate.Right?

Exactly. So, if you're using a hydrometer, use the S.G. scale + temp to derive the salinity and ignore the salinity scale on the hydrometer. If you're using a refractometer, use the Salinity scale, it will be accurate, ignore the S.G. scale.

 

[btodd43]

Guy- sounds reasonable to me -my refractometer is ATC -the scale on the right is in ppt ,the scale on the left is salinity I believe . But the 2 scales don't move and at 35ppt you are at 1.025.
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Hero Honda CBZ

 

[Anonymous]

Guy- sounds reasonable to me -my refractometer is ATC -the scale on the right is in ppt ,the scale on the left is salinity I believe . But the 2 scales don't move and at 35ppt you are at 1.025.

The ppt scale on the right is salinity (35ppt, 36ppt, etc.), the S.G. scale on the left is Specific Gravity (1.025, 1.026, etc.). If it makes more sense to you to just maintain levels at 1.026 on the S.G. scale there's nothing wrong with that. I just believe it's important to know what the numbers mean. Use it however works best and most convenient for you but try to understand the different scales in case it's ever more important than just mixing saltwater.

 

[btodd43]

It's too early.You must have had your coffee already....Thanks for all your help Guy.
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Indian Creek Designs BKO

 

[Anonymous]

:lol: :lol: :lol:

It can get even deeper but I choose not to go there.

Randy-Holmes Farley does a much better job of explaining than I if you read his articles. He also describes a home-made calibration solution using table salt to make sure your refractometer is accurate at 35ppt (parts per thousand). It's a worthwhile excercise if you ever need to be precise.

 

[btodd43]

uh huh.
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recessions

 

[Sula]

uh huh.

:lol:

 

[Sula]

Tell you what Guy: I work for a clinical laboratory. And the next time we're getting rid of old instruments, I'm gong to keep my eye out for you. No telling what kind of fun you could have with a chloridometer (measures actual Cloride ions) or an Ion-Selective Electrode method........

 

[Anonymous]

Cool!