There is only 1 type of helmet:

Air exhausts at the sides. On some helmet designs there are conventional exhaust valves fitted which help to flush the helmet of the expired carbon dioxide.

The shallow water helmet has been around since the early 1800's. After all: the Deane helmet and the first Siebe helmet were essentially shallow water helmets even though they had a short dress attached.

The shallow water helmet is of a very simple design that is as follows: a cast bronze hat that needs no additional weight or a metal hat with additional weights is put over the divers head and rests on the shoulders. Air is supplied through a valve or nipple. Due to a small over pressure it exhausts by leaking out around the bottom, at the diver's shoulders. Also typical for shallow water helmets is the sturdy handle cast to the top of the dome, which helps the tender lifting the helmet on and off the divers head. The mask types are put over the face. Air exhausts at the sides. There is an airhose attached and the divers descends. That's all there's to it ! When using a shallow water helmet of the helmet type you have to be extra careful: when you bend over too much the air will escape and the helmet will fill up with water. Keep that in mind please ! You can encounter factory made as well as home made shallow water helmets.


Shallow Water Helmet Diving

Of the various types of air supplied diving equipment, including the SCUBA equipment, surface supplied closed dress system and mask diving equipment with air supplied either way, the simplest type of equipment is the shallow water helmet. The rigid helmet may be made from steel , copper, bronze , Plexiglas or even wood. This system use the least amount of air and in the 1940s and 1950s many amateur made their own diving helmet for shallow water exploration. The history of the shallow water helmet goes back much further though and has been used since the middle ages and earlier.
The Shallow water helmet was popularized by William Beebe who used this type of equipment whilst exploring the sea bed in The Caribbean and in Bermuda.
The Shallow water helmet is now only used in the tourist industry where its simple and safe use is ideal for short underwater excursions.
Although simple in design many of the physical principals used in its operation apply to Standard dress diving, surface supply diving and SCUBA diving.
The main advantage of shallow water helmets over other types of equipment is they use the least air of all.
As we may know the deeper we dive the more the air we have shrinks in volume so as the depth increases so the pump delivering the air must be capable of delivering a greater volume of air. The pump must not only supply enough fresh air for the diver to breath but also to flush out the increased volume of carbon dioxide from the diverís exhaled breath.
Depending on the concentration of carbon dioxide in the helmet if the pump were to fail the diver may only have minute or so to carry on breathing before he suffers blackout, therefore the pump is of great importance hen using shallow water helmets.

The greatest drawback from using a shallow water helmet is that of stability. A shallow water helmet usually displaces about 1 cubic foot of water so by the law of displacement it must weigh 60 pounds or so or else it would just float away. Weighting the helmet properly therefore is most important. It the centre of gravity were too high it would tend to throw the helmet off when the wearer leaned forward whilst if the centre of gravity were too low it would make bending forward very difficult.

Older helmets tend to have flat windows protected by grills or mesh but modern helmets have curved windows which afford great vision for the wearer.
Apart from the helmet and pump the basic equipment comprises an air line a pressure tank and a lifeline. Generally the minimum size of air hose over short lengths is 5/16 inch
And always must terminate with a non return valve at the helmet end.

The lifeline and air hoe are seized together and come under the divers arm from behind and up to the non return valve and then fastened to the handle or some point on the helmet. The air line should never be tighter than the lifeline. The diver should only descend slowly to allow the pump to equalize the pressure or the helmet will slowly fill up with water.
Suppose the helmet has a volume of one cubic foot and the diver descends to 33 feet to the sea bed the pressure will have doubled and the air inside the helmet will have shrunk to half the volume thus half filling the helmet with water. If the rate of replenishment of air is just one cubic foot per minute then it will take half a minute to clear the helmet of water!
25 feet per minute is a good rte to descend but be aware of the carbon dioxide build up if there is not sufficient over pressurization of air entering the helmet.

As previously stated the diver must be careful that he does not lean over too far and loose air from the helmet, replacing it with water. Or worse still tip the helmet right off.
It has been the practice to attach the helmet to the wearerís body however this is not recommended as should the diver fall a heavy helmet may drown the diver and prevent his escape to the surface.
Should the helmet fill with water as can sometimes happen the diver should not panic as providing the pump is still working it should take no more than 30 seconds to clear the water , and here we are reminded how important the non return valve is to safety as this prevents water filling the air supply tube. .As it would take at least this time to discard the helmet and swim to the surface it remains the best option to stay with the helmet and wait for the water level to recede.

As the Diver descends he notices that a deep breath causes the water level in the helmet to rise since some of the air volume is not taken up by the Diverís lungs. A deep breath is about 1/9 of a cubic foot in volume so the water will rise a maximum of 1 Ĺ inches in a 15 inch high helmet. This rise whilst may cover the chin will be generally insufficient to cover the mouth. A greater volume of air delivered from the pump will overcome this rise in water level.

At the end of the dive when using a descent line the diver may climb the line or be hauled up the line whist remaining erect at all times. Divers ascending too quickly have reported giddiness on ascent due to the rapidly expanding air and the greater rate of ventilation of the helmet. This may be due to the change in concentration of carbon dioxide which he has become used to breathing at depth.
This experience rapidly subsides as the diver breaks the surface. The rate of ascent must never exceed 25 feet per minute and the diver should never hold his breath whist ascending as the expanding air in his lungs will burst the tissue. Pumping should not stop until the helmet is safely removed from the wearer.

As with all diving activities Shallow water helmet diving can be hazardous and should only be attempted after professional training and practicing in confined water such as a swimming pool. Rescue divers should always be present.