Constant Pressure Suits.

The Duck Beak Valve and Constant Volume Suits.
by Don Wilson. 23 October 2002

In 1966 I received the snorkel lecture on “Protective Clothing” which covered Wet Suits, Dry Suits and the Constant Volume Suit. There wasn’t many dry suits about in those days and it was years before I actually saw one.

The Constant Volume Suit was similar to todays dry suit but the diver wore a face mask with an internal mouth piece and the exhaled air was fed directly into the suit. The theory was that because the diver was taking in air at the ambient pressure -(appropriate to his depth) only a small increase in the pressure inside the suit would cause it to be leaked away and the volume of air would remain constant at all depths. Thus buoyancy control was a doddle!

A 'Spirotechnique' manufactured Cousteau Constant Volume Suit Hood, Squale-made Mask, 'DA' Regulator and Communication System.
Photograph with acknowledgement to the Historical Diving Society Canada

Air was allowed to leak away through rubber “duck beaked” valves fitted to the top of the hood, shoulders and heels so that in any position of the diver the air could always leak out by the highest valve. Because the air breathed into the suit was at body temperature the diver was (In theory) always warm.

It all sounds great, but they didn’t catch on for amateur diving and I only ever saw the one.

Diver wearing a constant volume suit and a hood similar to the one above
on a welding project in the Gulf of Mexico, circa 1968.
Photograph with acknowledgement to the Historical Diving Society Canada.

What’s a duck’s beak valve?

It’s like a flat rubber bag with two corners cut off. It is quite sensitive to pressure, the holes being squashed tight with a positive pressure to the outside and allowing free flow when the pressure is greater on the inside.

This valve was used in the early Mistral valve at the end of the exhaust valve and was often the reason for the fruity raspberry noises heard under water. Mind you - there could be other reasons !

How not to use Duck Beak Valves.
Don shared this educational story with club members at a recent meeting.

Many years ago in a place called Penrallt (clifftop) near Tudweiliog, North Wales, one of the earliest Diving Centres was run by a chap called Terry Pritchard who also ran a seafood restaurant alongside. Each morning Terry, with his buddy Gwillum, would go down the cliffs to dive to catch crabs and lobsters for the restaurants evening fare.

Terry told us that Gwillum had one of these new fangled Constant Volume Suits which, as everyone knows, works by allowing the diver’s exhalations to go direct into the suit and permitting excess pressure within the suit to be exhausted through “Duck beak” valves mounted on the ankles, shoulders and the top of the head.

On this particular morning, having climbed down the cliffs, Gwillum decided to reduce the air already in the suit before fitting his full face mask. So – turning his head sideways, he closed his mouth over one of the shoulder release valves and took a large suck.His face steadily turned a shade of green whilst he choked and spluttered.

It was only then that he remembered that he had farted during the climb down !