can someone please educate me on how the Steripen pre-filter is supposed to work? I tried to use it on a recent elk hunt, and it didn't seem to let any water into my Nalgene bottle. I screwed the filter onto the bottle with the filter in it and dunked the bottle in a pond. After about a minute, virtually no water had gone into the bottle. I then opened the little water bottle top and dunked it back in the pond and made sure the water bottle top did not get submerged. Still virtually no water got into the bottle. I ended up just taking the filter part out and using the top just to protect the bottle threads from contamination and to insert my Steripen. Am I using this thing wrong, or is maybe this one just defective. The mesh seems incredibly fine, but I would imagine not enough to keep water out.