Physics of Saltwater

[RustyRay]

Hi,

I wonder if anyone knows the answer to this -

When saltwater cools it contracts and becomes more dense. Correct? But why then does my SeaTest Hydrometer read lower when the water cools? And why does it read higher when the water warms? Shouldn't the float arm float higher in cold water? I've tested saltwater in large containers with a free-floating hydrometer and it does the opposite. I suspect it has something to do with the small confined container of the SeaTest hydrometer, like depth to surface area or something.

But anyway, doesn't this make all those temperature compensation charts in marine books useless?

Thanks for any response.

[OrionGirl]

At lower temps, the water can not contain the same amount of dissolved matter. So, salt and other elements precipitate out of cooler water, and stay solvent in warmer water. The hydrometers are set to a specific temperature standard (usually around 78 degrees F), so do not compensate for the temperature shifts. Not sure, but the amount that remains solvent may vary with volume--shouldn't but might. Not sure, I am not a chemist, so this is speculative.

[wastememphis]

I'm a physics major... um, I don't know what to really say about this other than when anything is kept in higher temps its particles move faster, also like when its summer up in buffalo doors expand in their frames, I don't know if that helps but maybe the particles are just less active in the water and are closer togeather, but it shouldn't change the reading that much at all, I figure it's just what the last post said about your suppose to just have your tank at a certian temp for the tester. :confused:

[AnimateAnemone]

This is quite intersting, your hydrometer is somehow not working properly. Colder water is denser and has a higher specific gravity, you can google-search for tables showing the temperature correction factors for hydrometers.

OrionGirl, salt will never precipitate from aquarium water, unless most of it evaporates. Look up temperature-solubility curves for salt.

[RustyRay]

The hydrometer is working properly. I have two of the same hydrometer and they both do it. It's just that there is a physical difference between a free floating hydrometer and box hydrometer like the SeaTest. I'm sure it has something to do with the box shape of the SeaTest. A free flaoting hydrometer will rise higher in cold water as compared to warm water. But the SeaTest box shape doesn't allow water to expand freely because of it's walls, and the surface area is small. I'm pretty sure this is the answer I'm looking for. I just need a scientist to confi

[RustyRay]

The hydrometer is working properly. I have two of the same hydrometer and they both do it. It's just that there is a physical difference between a free floating hydrometer and box hydrometer like the SeaTest. I'm sure it has something to do with the box shape of the SeaTest. A free flaoting hydrometer will rise higher in cold water as compared to warm water. But the SeaTest box shape doesn't allow water to expand freely because of it's walls, and the surface area is small. I'm pretty sure this is the answer I'm looking for. I just need a scientist to confirm that I'm on the right track.