The Submersible Decompression Chamber is essentially similar in design and construction to the recompression chamber. They differ however in their uses. The recompression chamber is used for medical purposes, however the decompression chamber is used to solve the problem of long stage or decompression stops required by divers to avoid the effects of decompression illness. It was discovered that the use of oxygen speeded up the removal of nitrogen from the diver's body. In order to safely administer oxygen a submersible decompression chamber was developed by Siebe Gorman & Co Ltd.
Extensive trials at depths up to 340 feet were carried out and a safe depth of 300 feet was adopted and used until 1945. Further experimental work was carried out between 1946 and 1948 in Loch Fyne in Scotland and as a result of this work Royal Navy divers successfully reached a depth of 540 feet.

The Davis S.D.C. (Submerged Decompression Chamber) on board ship

The chamber is constructed of steel and is cylindrical in shape similar to an observation bell. The chamber can be entered under water through an inward opening hatch which is large enough for a fully dressed diver. When the chamber is landed on deck of the diving vessel or dock it can be exited through a smaller inward opening hatch at the top. The whole chamber is generally tested to a pressure of 130 lbs square inch. The chamber is lowered into the water to the required depth from above and is suspended pending the return of the diver. There are lights fitted along with an emergency supply of air. Normal air is supplied from the diving vessel and an amplified communication system provides contact with the surface. In order that a diver may breath oxygen during the decompression period a Novus breathing set is provided for the diver.

Prior to the chamber entering the water the cylinders are charged and the systems checked to ensure they are operating correctly.. The large hatch is opened in the bottom and the diver and attendant climb in from the top. The lower hatch remains open during submergence whilst the top hatch is closed. The chamber is lowered to the required depth , usually 60 feet and is effectively a diving bell at this stage. The pressure inside is equal to the pressure outside thus preventing the chamber from flooding. In the event of a small drop in pressure inside a non return valve similar to that fitted to a standard helmet prevents air from being forced back up the air supply hose. The diver now dresses in his suit and descends a short ladder out of the chamber. The divers descends to the place of work and begins his tasks. He is tethered to the chamber by a bowsing in line. On finishing the task the diver now can return to the chamber by ascending the shot line, observing stage stops if required until he reaches the ascent ladder at the bottom of the chamber hatch door. He ascends the ladder until his upper body is clear of the water and his helmet and weights may be removed. As on the surface at any point when the diver may fall, a safety lanyard is attached to the diver.. The air and telephone cable or breast rope at this time comes from the surface and must now be discarded through the open hatch to be hauled to the surface. The telephone cable must first be blanked off to prevent water entering the fitting. When the air hose and breast rope are clear of the chamber the lower hatch door may be closed. The diver is still pressurised at a depth of 60 feet or 20 metres ( 3 atmospheres) and now the chamber can be hauled on board the salvage vessel whilst undergoing the decompression process.

On the left hand picture the diver is entering the chamber after the dive.

The attendant has removed the helmet. Air can be seen escaping from the bottom of the chamber. We can tell the diver has finished his dive as the air hose and the breast line has been discarded. The diver can be seen wearing the deep water breathing apparatus which is a form of rebreather Visit the going deep section to find out more about rebreathers and standard diving.

The right hand picture shows the salvage vessel on the surface and a sunken ship on the sea bed. The chamber is suspended mid water and the diver is on the seabed. The breast rope and air hoses can be seen coming from the salvage vessel. The only contact the diver has with the chamber is the shot rope at this stage.

Whilst there is just the attendant in the chamber there is enough air supplied for one person but as soon as the diver has his helmet removed the air supply must be doubled from the surface. At this stage it is critical there is no build up of carbon dioxide from exhaled air. The air pressure is reduced at intervals to simulate the divers ascent in water and when he has notionally reached the surface the upper hatch can be opened and the diver and attendant can climb out.

Once inside the chamber the diver can safely breath oxygen from an apparatus called the Novus set. There is a carbon dioxide absorbent in a sealed canister and a breathing bag. This speeds up the decompression process. At first when the diver uses the Novus set his expired breath contains a high percentage of nitrogen so this must be bypassed from the breathing bag or it will dilute the oxygen too much. After eight minutes the diver recirculates the expired air into the bag so as not to waste too much oxygen, occasionally purging the bag with oxygen to avoid the possibility of any build up of nitrogen. The decompression time for the diver is calculated based on him breathing pure oxygen. In the event that this is not possible through equipment failure the time for decompression is multiplied by 2 1/2 times.

The oxygen for the Submerged Decompression Chamber is stored outside the chamber in a 100 cu foot cylinder. This is charged to 120 atmospheres. The oxygen then passes in a hose to the Novus equipment inside the SDC. There is a depth gauge inside the chamber and internal and external lighting. There is also at least two emergency self contained Novus sets as well as a Davis submerged escape apparatus(DSEA)for use by the attendant in an emergency.
There are also other items such as tools ,a torch etc.... The air supply for the SDC is provided from the salvage vessel from large electrically driven air compressors producing compressed air at a pressure of 4000 pounds per square inch. The air is then stored in cylinders for use as required.

The diver is seen approaching the ladder underwater.
The diver emerges from the Davis Submersible decompression Chamber after completing his decompression.

It was found there were other uses for the Submersible Decompression Chamber. On one occasion in very strong tidal currents 2 attendants were lowered to just above the sea bed in 280 feet of water and with the use of grappling anchors were able to retrieve the lost item. By taking the bight of the anchor line and the lamp cable they were able to manipulate the anchor accurately by the illumination of the lamp. After retrieval they cast the lamp cable and anchor cable out and returned to the surface as normal , closing the hatch door when they reached their first decompression stop. There were clear advantages in this operation over having a diver in the water at that depth. The Davis Submersible Decompression Chamber was made even more effective with the addition of side windows similar to those in an observation bell.


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Copyright by Diving Heritage 2008