The most common type of snorkel is a simple tube that is allowed to flood when underwater. The snorkeller expels water from the snorkel either with a sharp exhalation on return to the surface (Blast clearing) or by tilting the head backwards right before reaching the surface and exhaling till reaching or "breaking" the surface (Displacement method) and facing forward again (Normal position), before inhaling the next breath. The displacement method expels water by displacing its presence in the snorkel thereby clearing the tube of its contents. A more advance technique that takes practice but one that uses less energy and clears the snorkel with much greater efficiency.
Some modern snorkels have a sump in the mouthpiece to allow a small volume of water to remain in the snorkel without being inhaled when the snorkeller breathes. Some also have a one-way output valve in the sump, which automatically drains the sump as it fills with water. A few snorkels have float-operated valves attached to the top end of the tube to keep water out when the snorkeller submerges, whilst more recent designs have a splash deflector on the top end that directs any water that splashes over the open tube to the outside of the tube, thereby keeping the user's mouth free from water.
Snorkels used to be sold with small "ping pong" balls in a cage mounted to the open end of the tube to prevent water ingress, though these are no longer sold nor recommended to be used since they are considered hazardous to the snorkeller. Similarly, diving masks with a snorkel built into them are considered unsafe and obsolete.
The maximum usable length of the snorkel tube is around 40 centimetres (about 16 inches). A longer tube would place the lungs in deeper water where the surrounding water pressure is higher and the lungs would be unable to inflate when the snorkeller inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure. Snorkels also create what is called "dead air space" - when the user takes in a fresh breath, some of the previously exhaled air remains in the snorkel and is recycled into the lungs, reducing breathing efficiency and causing CO2 retention. The greater the volume in the device, the more this problem is exacerbated.
http://en.wikipedia.org/wiki/Snorkeling
