Immersion cooler design (which ones in?!?!)

[BenB]

Just making my immersion I/C but (as per title) which end of the coil should be the "in".

Now I'm working on the principle that the top of the wort will be hotter than the bottom due to convection. Therefore, to obtain a constant temperature gradient (thereby removing most heat in a similar way to a CFC) you want the coolest water going into the bottom of the spiral and the warmer going to the top. Right?!?! After all you can't really ask much more of the water leaving the cooler than to be at the temperature of the hottest bit of the wort...

Then again if the top of the wort was cooled very quickly (ie cool water into the top of the coil) it would sink (relative to the hotter stuff at the bottom) which would introduce convection currents that might stir up the fluid and thereby ensure a more homogenous temperature.... (ie no "hot spots" within the wort).

Has anyone done a test to see which is better? Otherwise I see an experiement coming on!! Of course for the moment it doesn't make any difference as it will need two tails whichever way and the only difference is which end I put the in hose!!