Many divers do in fact use EAN80 (80 per cent oxygen and 20 per cent nitrogen) for decompression. This higher oxygen level is very beneficial for decompression, but can only be breathed from about 12 metres up to the surface. To breathe EAN80 deeper than 12 metres or to breathe EAN50 deeper than 20 metres, or EAN100 (100 per cent pure oxygen) deeper than 9 metres, risks a potentially fatal oxygen toxicity hit. Thus, every deco mix, be it EAN50, EAN80, EAN100 or whatever, all have their own depth limits where the amount of oxygen in the mix becomes toxic – and potentially fatal.
In a contrast to open-circuit diving, Paul and I, in common with the majority of technical divers, have for a long time been using closed-circuit rebreathers (CCRs).
Whereas in open-circuit diving the exhaled breathing gas is vented to the surface, a closed- circuit rebreather continuously recirculates the same breathing gas – there is no venting to the surface. One of the benefits of using a rebreather is that a diver can program their onboard computer to never let the PO2 in the breathing gas loop drop below a certain level.
As a diver rebreathes through a CCR, during each breathing cycle (that is, one inhale and one exhale) the diver’s body metabolises some of the oxygen as it passes through the body, producing carbon dioxide (CO2). The expired breathing gas thus contains less oxygen than the gas the diver inhaled. In a CCR, that expired gas is cleaned of the dangerous CO2 in a scrubber canister filled with sofnalime and then analysed in a chamber in the rebreather by onboard oxygen sensors. The results trigger a solenoid switch to open and bleed just the right amount of oxygen into the breathing gas to keep the PO2 at the desired level of say 1.3 bar, no matter what depth the diver is at. So, on the ascent, all the way to the surface, the rebreather is trying to inject oxygen to keep the PO2 at say, 1.3 bar. At 20 metres, a CCR diver can be breathing a PO2 of 1.3 bar – but in contrast to breathing EAN50 on open circuit, at the final deco stop at 6 metres a CCR diver is breathing almost pure oxygen. The diver is getting the optimum amount of oxygen, so beneficial to decompression, at any point.
A modern rebreather, the A.P. Diving Inspiration Vision, popular with technical divers. The corrugated hose leading from the mouthpiece over the diver’s right shoulder is the ‘exhale’ hose of the breathing loop. The corrugated hose leading over the left shoulder to the mouthpiece is the ‘inhale’ hose. The wrist-mounted computer handset is in the foreground. © Bob Anderson
Rear view of a popular closed circuit rebreather (CCR).
The diver’s exhaled breath moves from the mouth through the exhale hose that runs over the right shoulder and into the bottom of the central stack between the two cylinders. From there the exhaled breathing gas passes upwards through a canister holding the ‘scrubber’ sofnalime material, which strips the dangerous carbon dioxide out of the exhaled breathing gas.
After passing through the scrubber, the cleaned, exhaled gas passes into a chamber at the top of the stack, where three or more oxygen sensors analyse the resulting breathing gas to determine how much oxygen the diver has metabolised in the last breathing cycle. Two electronic controllers (essentially minicomputers) then trigger a solenoid (switch) that injects the correct amount of high-pressure oxygen into the breathing mix to raise the depleted oxygen level back up to the desired level (the PO2 ‘setpoint’).
The cleaned, analysed and adjusted breathing gas then passes through the inhale hose that runs over the diver’s left shoulder directly to the mouthpiece, and the breathing cycle repeats. No breathing gas is vented to the surface – it is continuously cleaned, analysed and corrected as it is rebreathed.
The right-hand cylinder holds high-pressure 100% oxygen. The left cylinder holds the ‘diluent’, the desired breathing gas – air or trimix. The small black cylinder on the left holds the diver’s drysuit inflation gas. © Bob Anderson.
Of course, too much oxygen is also a problem. If something goes wrong, say the solenoid switch sticks open and the oxygen level in your breathing gas goes above that set level, audible alarms go off and red lights blink on the heads-up display (HUD) unit that is usually mounted on the corrugated hose breathing tube just below and off to one side of your mask in your peripheral vision. Too much or too little oxygen, and the normally green lights start flashing red warnings.
I generally use a PO2 of 1.3 bar, but with deep repeat dives I back it off to 1.1 bar just to stop racking up too high levels of oxygen over a period of days. I usually manually inject oxygen in my final decompression stops to keep the PO2 at 1.4 bar and shorten decompression.
The wrist-mounted computer handset on my Inspiration Vision CCR. The top left figure reads 0.70 and confirms the pre-set PO2 set point being used. The three figures –in this picture all at0.81 – show the individual readings of the three oxygen monitoring cells that continuously analyse the breathing gas. The readings should be roughly consistent – if one figure differs wildly from the other two then it is an indicator that the cell is possibly malfunctioning.
The horizontal white rectangle at the top is the scrubber monitor: it displays how the scrubber in the back-mounted canister is performing. © Bob Anderson
A fully rigged technical diver with a shallow bailout nitrox cylinder slung beneath the right arm on the ‘oxygen’ side. In this case the cylinder holds EAN50, which has a maximum breathing depth of 20 metres – and this is clearly marked on the cylinder, to avoid the wrong gas being breathed at the wrong depth, a standard tek diving practice. To breatheEAN 50 deeper than 20 metres for a prolonged time risks an oxygen toxicity hit with possible fatal convulsions. The mask strap is under the hood,to avoid it being kicked or knocked off.© Bob Anderson
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27 October 1914 – the first British battleship of World War I to be lost to enemy action
The 598-foot King George V-class battleship HMS Audacious is another important first in British naval history. She had the misfortune of being the first British battleship to be sunk by enemy action during World War I, on 27 October 1914, just two months into the war. She was also the only modern British dreadnought battleship to be sunk by enemy action in the war. The story of the loss of HMS Audacious also involves a famous White Star liner, RMS Olympic, which would carry out a dramatic rescue attempt.
The 23,400-ton King George V-class dreadnought battleship HMS Audacious– the first British capital ship to be sunk by the enemy during WWI. (IWM)
Audacious was one of the four dreadnought battleships of the King George V class provided for under the 1910 building programme. Battleship design had taken a dramatic leap forward in 1906 with the launch of the revolutionary HMS Dreadnought, when the Royal Navy, under the charismatic leadership of the First Sea Lord, Admiral of the Fleet Sir John Fisher, boldly embraced a risky radical alteration of the prevailing balance of naval power with the creation of a revolutionary new type of battleship. HMS Dreadnought was such a quantum leap forward in battleship design that her name would be used to define the whole class of such new battleships – dreadnoughts. Almost overnight, the generation of battleships that had gone before her was rendered virtually obsolete; they became known as pre-dreadnoughts, and although they sailed with the respective fleets in World War I, they were relegated to the end of the battle line or given other rear echelon taskings. Soon, other major naval powers raced to build their own dreadnoughts.
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