Hot air temperatures

[v-600]

I was deleting junk mail when I saw this Floating shark. The part that says "Just fill your air swimmer with helium..." got me thinking about hot air.

Lifting Gas While wikipedia isn't always the most trustworthy I did find these facts:

The average temperature of air in a hot air balloon is about 212 °F (100 °C).

Air (ρair) = 1.292 kg/m3 at zero degrees C

Helium (ρHe) = 0.178 kg/m3 at zero degrees C

The weight of the shark 780g. Napkin maths suggests the shark displaces approx 0.9m3 (and how do I get superscript in posts?).

So, my question/discussion. How feasible would it be to use heated air, rather than helium to lift the shark? Sorry but my scientific knowledge isn't strong enough to work out what temperature you would need to get the air so it has the same density as Helium (or a density low enough to lift the shark).

My initial thoughts are that you would have problems with melting the plastic, so maybe a more durable material would need to be used.

You would also need a pressure valve somewhere so that heated air could escape, lowering the weight rather than increasing the pressure.

I think the weight of the heater and extra batteries could also cause some problems.

So, is it possible, and a good idea?

 

[CycloWizard]

I don't think it's feasible for a few reasons. The temperature of the air would need to be very high and might lead to mechanical problems with the skin material. The heat loss would be very rapid due to the high temperature differential. Buoyancy would be less in the heated air case than the helium case even if the density of gas within the shark were the same in both cases since in the heated air case, the surrounding air would be less dense due to heating. In the end, it's not going to end well.

 

[KillerCharlie]

For a fixed pressure, density is inversely proportional to temperature (rho = p/(RT)).

That means you need 7 times the temperature to get the same density as helium! That's just not possible. My tables here tell me that heating air to 600 degrees barely halves the density.

You could try electrolysis to make hydrogen! If it catches on fire it would probably explode fast enough so nothing catches on fire.

 

[PlasmaBomb]

From your napkin math you are looking for a density difference of ~0.867 kg/m^3.

Assuming room temperature rather than 0 C, then an air temperature of 1000 C would give you a density differential of 0.907 kg/m^3, comfortably allowing your shark to float for milliseconds before spontaneously combusting...

 

[colonelciller]