Reef Chemistry Question of the Day #131 Floating Glass Hydrometer

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#131[/HASHTAG]

If you have a floating glass hydrometer in a marine aquarium, the specific gravity is read by matching the height that it floats in the water to a built-in scale on its side. If the water sample is allowed to cool, the hydrometer will:

A. Rise higher in the water, indicating a lower number.
B. Sink lower into the water, indicating a higher number.
C. Rise higher in the water, indicating a higher number.
D. Sink lower in the water, indicating a lower number.

Good luck!





















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[Longnose Hawkfish]

I think C

 

[JimWelsh]

Without cheating, C. Water gets more dense as it cools. That means that it takes less displaced volume to equal the mass of the hydrometer. Less displaced volume happens when the hydrometer is floating higher. This same principle is why the numbers on the scale are increasing from top to bottom, meaning that floating higher will cause a higher reading.

 

[beaslbob]

as the temperature cools the hydrometer becomes more dense sinking more into the water creating a lower reading.

Which is offset by the cooler water being more dense.

Might offset each other if the hydrometer was designed correctly.

I suspect the water being more dense overrides.

rides higher in the water for a higher reading

let's see.

that's : C.

But that's the same as guessing

 

[Randy Holmes-Farley]

And the answer is...C. Rise higher in the water, indicating a higher number.

Glass hydrometers expand and contract much less with temperature than does water.

So as the water cools and gets more dense, the glass hydrometer is pushed up more by the heavier water, and reads a higher number.

 

[beaslbob]

dern

 

[Randy Holmes-Farley]

dern

You got it right!

 

[beaslbob]

are you sure you asked the question correctly.

I was surprised I got it right. LOL