In general there are 3 types of helmets:

• airhelmets, with airsupply via a hose from the surface. (These include the shallow water helmets).

• regulatorhelmets, where the air, controled by the divers breath, comes from a regulator (a reservoir  filled with air under higher pressure) on the divers back. Compare this with the invention of Rouquayrol and Denayrouze. These hats were last used around 1910. Regulator helmets featured connections for the regulator as well as connections for air supply from the surface. After 1910 most of them were refurbished to ordinary air helmets.

• rebreatherhelmets, where air comes from a closed system. The exhailed carbon dioxide is chemically tied and via an injector oxygen is supplied. The Drager DM-40 is an example of this system. Also, the DM-40 can be used as a mixed gas helmet or with airsupply from the surface.

Types of closed-type helmets

In general the diving-helmets can be divided in open and closed helmets. The majority of the open helmets was not connected to a jacket or suit. The closed helmets can be discerned according the number of bolts of the mounting-system of the corselet onto the suit and the fitting-system of the bonnet onto the corselet . One system, to make a watertight connection is a rubber ring or flenge, connected to the suit, that is clamped between the bonnet and the corselet. At the other system a rubber collar on the suit is clamped between the corselet and the straps and tightened with nuts.

The connection of the bonnet and the corselet can be done by means of flenges, that are bolted together. A rubber O-ring or –gasket between the flenges takes care for the watertight connection. Another solution is a connection of the bonnet and corselet with an interrupted screw (bayonet)-connection. In this case the sealing is done by a leather gasket, that is mounted in the upper ring of the corselet.

Like described hereunder, a variety of systems and combinations of types of bonnets and corselets has been developed since 1837, since Siebe developed his first close dress. Although not complete, examples of makers or countries where these types were made, are mentioned.

• No-bolt type (French Petit/Piel helmets with locking-device)
• Flenge-helmet (direct connected to suit or ring (Siebe Gorman, Carlsson)
• Two-bolt type ( early Dräger helmet, Danish helmets)
• Three-bolt type (mainly French ,German, and Russian helmets)
• Four-bolt type (no specific makers known)
• Six-bolt type / round corselet (Siebe Gorman, Heinke)
• Eight-bolt type / round corselet (Siebe Gorman)
• Twelve-bolt type / round corselet (Siebe Gorman, Heinke, U.S.N. Mark V/ commercial, Galeazzi, Salvas, IAC, Carlsson, Petit/Piel, Cabirol)
• Twelve bolt type / square corselet (Siebe Gorman, Heinke, Russian)
• Two/Three bolt type (in general converted Danish/German helmets)
• Six-or Twelve/Three-bolt type (in general converted six/twelve-bolt corselets with three-bolt flange connection bonnets (different types)
• Twelve/Four-bolt type (Carlsson)
• Twelve/Five-bolt type (Morse and Schrader)