help on my own diving helmet!

[helmetdiver4849]

hey, i've always loved the concept of the metal deep sea diving helmets, but the only problem is, they are incredibly expensive and being a student i can't afford too much.
So i had the idea of making my own diving helmet from varnished marine plywood or possibly fiberglass. Now i've worked most of the issues out, but i need to know what kind of valve i would need to let air out of the helmet, but not in the helmet. if someone could weigh in and give me some help i would really appreciate it!

if it's an open-bottom helmet, then you can just have a pipe jutting out the side and then facing down, if you're overly concerned about exhaust. -Alden