Subject: Why is my carbon dioxide effusing too fast?

For a project in my chemistry class, my friend and I are testing the 
relative effusion rates of three gases: helium, oxygen, and carbon 
dioxide.  We tested this by filling 30 standard party balloons with 
the gases - 10 balloons for each.  We then measured the difference in 
volume over time by water displacement.  The experiment was relatively 
successful, but our results didn't make sense according to Graham's 
Law.  Carbon dioxide has a particle mass 11 times that of helium so, 
if I understand the law correctly, the helium should have diffused 
about 3.316 times as fast as the carbon dioxide.  However, the average 
percent lost for one day was 59.1% for the helium, and 82.6% for the 
carbon dioxide.  We asked our chemistry teacher why this was 
happening, but between the three of us we couldn't find any rational 
explanation for why the carbon dioxide effused so fast.  Any light you 
could shed on this subject would be greatly appreciated.  Thank you 
for your consideration.

Re: Why is my carbon dioxide effusing too fast?