Reef Chemistry Question of the Day #264 Temperature and Refractometers

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #264

Most reef aquarists that use refractometers are aware that the refractive index changes with temperature. Some handheld refractometers account for temperature by employing a bimetal strip inside them. This bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. This movement is designed to exactly cancel temperature’s effects on refractive index.

Suppose that you have a refractometer that DOES NOT have such a temperature correction, and is properly made to read seawater salinity at 20 degrees C (68 degrees F).

If you measure salinity at 30 degrees C (86 degrees F), the reading you get from the refractometer will incorrectly give approximately what value when the water is really 35 ppt salinity?

A. 30 ppt
B. 34.1 ppt
C. 35.9 ppt
D. 40 ppt

Pick the closest choice to what you think the answer is.

Good luck!

















































.

 

[Crabs McJones]

I'm going to guess A. I know the value is lower because of a quote I read from you. It I dont know the exact math to know the actual salinity. So shot in the dark. A final answer

Edit: changed my answer to A

 

[Randy Holmes-Farley]

I'm going to guess B. I know the value is lower because of a quote I read from you. It I dont know the exact math to know the actual salinity. So shot in the dark. B final answer

I didn't know the magnitude until I just worked it out for this question.

 

[largepolyp]

im gonna just guess d . but i have a question for you Randy what do you use to measure your salinity? im tryin to decide if its worth it to upgrade to a digital or the tropic marin hydrometer. TIA

 

[Randy Holmes-Farley]

im gonna just guess d . but i have a question for you Randy what do you use to measure your salinity? im tryin to decide if its worth it to upgrade to a digital or the tropic marin hydrometer. TIA

I like using conductivity. It's quick and continuous in situations like acclimation of a new creature.

Optimal is something like a used Orion from Ebay (too expensive new). I have the Model 128.

The Pinpoint is also OK, but takes longer to equilibrate to the water temperature, so is slower to get a reading.

 

[Alfrareef]

I’ll go with B.

 

[Chuk]

I get 1.028 sg using my home brew correction chart which is like 37 ppt. I’ll go with d since that’s the closest price is right style

 

[Sleepydoc]

I had to read the question a couple of times - thought it said that the actual reading was 35 ppt but it’s actually asking for what would the refractometer reading be when the water is 35 ppt (at 30º C)

It’s going to read less dense at a higher temperature, so it’s either A or B. I don’t have the formula, but 10º C is a big enough difference that I’ll say A.

 

[Chuk]

I had to read the question a couple of times - thought it said that the actual reading was 35 ppt but it’s actually asking for what would the refractometer reading be when the water is 35 ppt (at 30º C)

It’s going to read less dense at a higher temperature, so it’s either A or B. I don’t have the formula, but 10º C is a big enough difference that I’ll say A.

You're right I read that wrong. 1.028 is what the reading really be would be if you read the 35 ppt at 30C. Again with the hombrew chart I get 1.024 which is just under 32 ppt

 

[dragon99]

Going to read low, but not sure by how much. Going with B.

 

[MnFish1]

Reef Chemistry Question of the Day #264

Most reef aquarists that use refractometers are aware that the refractive index changes with temperature. Some handheld refractometers account for temperature by employing a bimetal strip inside them. This bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. This movement is designed to exactly cancel temperature’s effects on refractive index.

Suppose that you have a refractometer that DOES NOT have such a temperature correction, and is properly made to read seawater salinity at 20 degrees C (68 degrees F).

If you measure salinity at 30 degrees C (86 degrees F), the reading you get from the refractometer will incorrectly give approximately what value when the water is really 35 ppt salinity?

A. 30 ppt
B. 34.1 ppt
C. 35.9 ppt
D. 40 ppt

Pick the closest choice to what you think the answer is.

Good luck!




B












































.

 

[MnFish1]

Reef Chemistry Question of the Day #264

Most reef aquarists that use refractometers are aware that the refractive index changes with temperature. Some handheld refractometers account for temperature by employing a bimetal strip inside them. This bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. This movement is designed to exactly cancel temperature’s effects on refractive index.

Suppose that you have a refractometer that DOES NOT have such a temperature correction, and is properly made to read seawater salinity at 20 degrees C (68 degrees F).

If you measure salinity at 30 degrees C (86 degrees F), the reading you get from the refractometer will incorrectly give approximately what value when the water is really 35 ppt salinity?

A. 30 ppt
B. 34.1 ppt
C. 35.9 ppt
D. 40 ppt

Pick the closest choice to what you think the answer is.

Good luck!

















































.

B

 

[JimWelsh]

According to this calculator, which agrees with the data found in this paper, the answer is about 28.6, so I'm going with A.

 

[Randy Holmes-Farley]

According to this calculator, which agrees with the data found in this paper, the answer is about 28.6, so I'm going with A.

FWIW, I did find a surprising amount of variability as I checked some different sources. My first source matched yours and I went with A being below 30. A second was just about 30.

 

[Randy Holmes-Farley]

And the answer is...A

If you measure salinity at 30 degrees C (86 degrees F), the reading you get from the refractometer will incorrectly give approximately what value when the water is really 35 ppt salinity?

A. 30 ppt
B. 34.1 ppt
C. 35.9 ppt
D. 40 ppt

It surprised me when I looked into it in detail, but the temperature effect is very large, making the accuracy (or not) of the ATC of significant importance with a refractometer. As Jim noted, some papers suggest the value is a bit below 30 ppt.

 

[chipmunkofdoom2]

I like using conductivity. It's quick and continuous in situations like acclimation of a new creature.

Optimal is something like a used Orion from Ebay (too expensive new). I have the Model 128.

The Pinpoint is also OK, but takes longer to equilibrate to the water temperature, so is slower to get a reading.

Not to clutter up this thread too much, but I have the Pinpoint and can confirm this. I usually have to set a timer for 4 - 5 minutes to allow the reading to stabilize. Other than that, no complaints. I definitely like measuring conductivity a lot better than measuring specific gravity or refractive index.

 

[Randy Holmes-Farley]

According to this calculator, which agrees with the data found in this paper, the answer is about 28.6, so I'm going with A.

How did you use that calculator, Jim? Did I miss an easy way to enter temperature?

I looked up the refractive index drop that was reported in a paper, and used your linked calculator to get about 30 ppt, which was higher than the "below 30 ppt" that I got from my own article data table on refractive index vs salinity, so I changed the description in A.

 

[JimWelsh]

How did you use that calculator, Jim? Did I miss an easy way to enter temperature?

I looked up the refractive index drop that was reported in a paper, and used your linked calculator to get about 30 ppt, which was higher than the "below 30 ppt" that I got from my own article data table on refractive index vs salinity, so I changed the description in A.

You have to scroll over to the right on the "Refractive Index Calculator" worksheet, to cell V4.

 

[Victoria M]

Please say that ATC refractometers are good to use. They are the auto temp corrected ones, right?

 

[Randy Holmes-Farley]

Please say that ATC refractometers are good to use. They are the auto temp corrected ones, right?

They are supposed to be. [emoji23]