Subject: Keeping Soda Carbonated

There are several devices on the market that pump air into the 
headspace of soda bottles and purport to save the soda from going 
flat.  What is the physical basis behind this?  

Henry's Law states that the concentration of a solute gas in a 
solution is directly proportion to the partial pressure of *that* gas 
above the solution.  Since there is only a tiny bit of carbon dioxide 
in the air, pumping a little bit of air over soda would not increase 
the partial pressure of carbon dioxide over the soda to any great 
extent and therefore would not offer an advantage in keeping soda 
fizzed when compared to just capping the soda immediately after use.  

What do you think?  Any insights?  Thanks.