While the Mark V helmet is the best known, other types of helmets preceded it.Most common are the commercial and the sponge or lightweight helmets.Abalone helmets had guards with bars over the windows and a Mark V exhaust valve.Agar helmets have rings without bars for the windows and a adjustable single spring head butt exhaust.The Lightweight Fisheries has the Mark V exhaust valve.The Browne Commercial helmet has large screw in Acrylic windows, an integrated air control valve, and a low profile air inlet/communications block.Still a buyer must be wary when bidding or purchasing online.In some cases the seller is not knowledgeable in the item they are trying to sell.
Variants Variants of the commercial helmet include the Fisheries Helmet with a single large window, and the Nuclear with a plastic screw in front window, Helium helmet exhaust valve, and integrated air control valve.
The name Sponge Helmet refers to a particular model of helmet but is also used as a generic term for lightweight helmets which use the same breastplate and top shell.
The Sponge helmet was developed for divers harvesting sponges. Sponge divers would spend most of the day underwater and having the lightest gear possible would reduce fatigue.
The commercial diving helmet traces back to the mid 19th century.
The breastplate is a twelve bolt type (meaning 12 studs pass through the collar and brails) which is very wide across the shoulders, falls fairly deep in front, but falls shorter in the back.
Different fisheries had different requirements so various models of helmets developed.