This is essentially a small diving bell that fits over the user's head and allows him to descend to moderate depths. It does not protect the user from pressure, but could be easily modified to do so. It should be noted that I am not responsible for any injuries sustained while using this device; If, however, you take the proper precautions, the possibility of this is small, even in the event of a pressure failure. This helmet can be made from simple, inexpensive materials. It can be operated by two people, and with some design modifications, could be made self - contained. I have made sustained dives of over two minutes without a problem, at depths of approximately ten feet.
Step 1: Design
This apparatus consists of a watertight helmet which partially encloses the diver's head, and is connected to a topside air supply by a hose. It is chained or otherwise affixed to a ballast weight to counter the positive buoyancy of the air trapped inside the helmet. It is very similar to diving apparatus used in the early days of ocean exploration.
Step 2: Helmet Container
This is the main component of the apparatus. Any watertight container large enough to fit over your head should work; I found Sparklett's water bottles of the variety used in office water coolers to be particularly suitable. For those unfamiliar with these containers, they are large, clear plastic bottles about a foot and a half in diameter, and having a flat base, a mostly cylindrical shape, and a conical top terminating in a neck six inches high and two inches in diameter. I sawed the bottom off of one of these bottles to make my helmet. It is just large enough to fit over the diver's head, is transparent, negating the need to cut a window or porthole, and its neck provides a convenient attachment point for an air hose. A large plastic cement bucket would probably work too, but its lack of transparency would require the cutting of a viewing port, which would reduce the container's tolerance for pressure. You should pad the edges of the helmet to protect the diver's shoulders. I used two pieces of rubber hosing slit lengthwise and taped in place as pads.
Step 3: Air Hose
Many things would conceivably work here. I happened to use rubber aquarium hosing that I happened to have in a shed. This is about half an inch in diameter, and works fine. An old garden hose would probably be the most easily available thing. Just make sure you don't use drip irrigation hosing. It's riddled with little holes, and would leak A LOT. Other than that, just about any watertight hose would work. The way you attach it to the helmet and the pump will depend on the diameter of hosing you use, and the diameter of the helmet hose connection and bike pump nozzle. I'll leave that up to your ingenuity.
Step 4: Air Supply
In the first picture, you see a bicycle pump connected to the air hose to pressurize the helmet. If you use one, make sure it's a large one, capable of supplying enough air; an electric air compressor would probably work better and allow for longer dives. With my design, the diver must come up when the person working the pump gets tired. Probably, you could eliminate the topside air supply entirely and use a chemical re-oxygenation system inside the helmet itself, like on Isaac Peral - class submarines, and on the Columbiad projectile in Jules Verne's JOURNEY FROM THE EARTH TO THE MOON. If anyone knows how this could be done, feel free to comment!
Step 5: Ballast
You'll probably need about 25 pounds of ballast to get the helmet to sink. Old U.S. Army ammunition boxes filled with rocks work great, but failing that, use your ingenuity. Test to ensure that your buoyancy is right, and you can ascend and descend with ease. Ballast could be attached directly to the helmet itself, or hung on a chain or rope, as I did. 19th century divers used cast metal helmets, contributing some weight, and also wore weighted boots and large weights around their necks, one at the front, one at the back, contributing weight (see image). Anything heavy would work; I used rocks, but old metal junk, bricks, cast- iron pots, etc., would probably all work if you had enough of them. Make sure you have a rope tied to the ballast to allow you to haul it out of the water, or pull up the diver in an emergency.
Step 6: Operating
Drop the ballast into the water, holding it just under with the rope. Have the diver drop into the water and put on the helmet, taking care to keep it level. Let the rope out, and pump continuously to keep the helmet pressurized. When the helmet begins to descend, the diver will feel a sensation like a rapid descent in an airplane, and the diver's ears might pop. After that, pressure should remain constant. If the water level in the helmet begins to rise, air is escaping faster than the assistant topside is pumping it in. Some air must escape to stop carbon dioxide buildup, But not too much, or the helmet will flood. A system using pulls on the rope to signal distress is probably a good idea. Make sure to observe proper safety precautions.
Step 7: Closing Notes
Many modifications could be made to this design; the helmet could be sealed around the diver's neck or onto a dry suit to increase pressure protection and allow for greater depth, although it should be noted that this would make it more difficult for the diver to remove the helmet in the event of an emergency. The topside air supply, as stated earlier, could be eliminated and replaced with a self contained air refresher. You could mod it into an awesome steampunk helmet! The possibilities are endless! be careful, and have fun!