Reef Chemistry Question of the Day #179 Refractometer ATC

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#179[/HASHTAG]

Some refractometers have the attribute that they can correct for temperature changes, and some do not. This attribute is usually called ATC, or Automatic Temperature Compensation.

How do typical refractometers accomplish ATC?

A. The reticule (the part with the scale printed on it) is made from a material with almost no thermal expansion over the temperature range specified
B. The optical lenses are made of high quality glass rather than plastic, so they do not expand and contract as much
C. The crystal used for the prism is designed to have a very low temperature expansion coefficient, like Pyrex glassware does to prevent breakage due to temperature shock
D. They have a metal strip that expands and contracts, moving the optics to adjust for temperature changes.


Good luck!





























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[SROPE]

I'll take a shot with D.

 

[Cory]

I think D also

 

[Downbeach]

D

 

[beaslbob]

d

 

[JimWelsh]

More accurately, it is a bimetal strip, which is an important part of how it works. Answer is D.

 

[aaron23]

d

 

[Eckolancer]

A

 

[Randy Holmes-Farley]

And the answer is...D. They have a metal strip that expands and contracts, moving the optics to adjust for temperature changes.

Good job folks!

I discuss it and other aspects of refractometers here:

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

from it:

Temperature and Refractive Index: ATC

It turns out that refractive index is highly dependent on temperature. When using a refractometer that does not account for this effect, temperature changes can be a large source of errors. Most liquid materials expand slightly when heated and shrink when cooled. For a given material, light can pass through it more easily when it is expanded, so the index of refraction falls when materials are warmed. However, the magnitude of this effect is different for every material, and refractometers must somehow take this into account.

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, and generally does a very good job IF the refractometer is designed to cancel out the temperature effects of the specific material being analyzed.

Because many refractometers are designed to use aqueous (water) solutions, the bimetal strip can be designed to account for the change in refractive index of aqueous solutions with temperature, although it may not be perfect in some situations because salts and other materials in the water can change temperature’s effects on refractive index by a small extent (or possibly to a large extent for very concentrated solutions, such as 750% sugar in water, but seawater is not in that category). Other details of this compensation may cause it to be imperfect (for example, the bimetallic strip provides a linear correction while the true temperature effect may be nonlinear), but those issues are beyond the scope of this article, and in general automatic temperature compensation (ATC) is a very useful attribute for aquarists using refractometers.

 

[beaslbob]

something counteracting the effect (like a bi metal strip) was the only thing that made sense to this old engineer. lucky guess

 

[Randy Holmes-Farley]

Good job!