At what point (length) would a snorkel type tube become ineffective?
There are a few considerations here. First, is resistance and strength of your breathing muscles (of which the diaphragm is only one). Clearly 1' of 1" tube is not too much resistance because that's what a snorkel is. You could calculate how much resistance that is, and create a longer, but larger diameter tube. I imagine your breathing muscles would have no problem sucking air through a 6' tube if it were, say, 6" diameter. But that leads us to the bigger problem...
A standard snorkel doesn't hold much air in it's tube
. When you breath in your second breath through a snorkel, the first part of what you breathe in is your own used air in the tube. Used air has too much CO2, which you're trying to get rid of. Keep re-breathing the same air, and pretty soon you've used up all the O2 and your getting other problems from accumulating CO2.
Lucky for you, when you inhale your own used air in your snorkel, it's only a small percentage of what you breathe in that breath. In the 6"diameter 6'long tube mentioned above, you'd be breathing in and re-breathing your own air because when you exhale your entire lungful of used air would remain in the tube. That's part of why scuba regulators have a flapper valve that sends the exhaled air in another direction. Snorkels don't have this feature. But they do have a purge valve....
WARNING: This is where Medsailor should have stopped his post.
What if you put a standard one way valve on the top of your snorkel. Something like this:
Then attach your large diameter (read low resistance) tube to the top of this. What you would do is draw in air through the low resistance tube, then the one way valve, then the snorkel. When you exhale, the one way valve would stop the air, forcing it out the purge valve at the bottom of the snorkel.
Crazy? Probably. Dangerous? No, not if you test it in your living room first and don't go deep with it. Is it the most hair-brained-hacked-together-improvised-idea-ever? Not by a long stretch!
The famous improviser Bernard Motessier created, and successfully used
a system of multiple tubes and upturned and submerged buckets that makes my idea look like a commercial pearl diver's hookah.
His system can be found on page 167 of Sailing to the Reefs
. Sailing to the Reefs - Google Books
P.S. From a medical prospective, I can assure you you can't collapse your lung from sucking too hard. If you could there would be a whole lot of.... um... well... oh never mind.