Decompression and Charging

I know, I know. I hear you. But mine is a future with room for books of both the ‘real’ and the ‘e’ kind.  Although I do think my pocket Sony Reader is just too darn small. I have managed to find the bloody thing again and am currently charging it; my plan being to read the mighty Stephen King’s Under the Dome which I downloaded last Boxing Day. I am pretty sure if that had been a book, I would have finished it by the end of the next day. But perseverance and noble spirit and yaddah, yaddah…

While we are waiting for the collected and constructed words of Stephen to charge themselves into a user friendly fomat, here is a partial transcript from  an e-mail I received from my underwater loving friend:

Dive plan was to the bottom of the pinnacles of an earlier dive, though for longer and with different start and end points. That first dive had been memorable: a cold, clear winter dive of stunning visibility offset by a soft green light, filtered through the algae, increasing in intensity with decreasing depth.

From the surface, near the middle of Dorothea Quarry, drop down on to a ridge at around 40 metres. Pause, check gear, check gas and check bearings. Continue down past the entrance to an old tunnel bored into the ridge. The tunnel dog-legs and comes out the other side. (Note to self: need to tackle that one day, although I hear the roof is cracked and sagging in some spots so best to do it soon.)

Down past the tunnel entrance and down, levelling out at around 62.2 metres or 205 feet. Continue west-north-west with the main ridge below at 67 metres, the sides dropping down another 40m to the bottom, although my torch shows only the first 10 metres.

I am with two divers who are on CCR’s – Closed Circuit Rebreathers. This system re-circulates the gas by passing it through a scrubber to remove the expelled CO2: so no bubbles.

The temperature is 5 degrees.

The only noise comes from my breathing. I am on OC – Open Circuit, expelling air into the water with every breath. As the expelled gas heads to the surface, it expands as it rises. Each bubble will be seven times the size at the surface as when it left my body owing to the pressure at this depth. This also means I breath my supply at seven times the normal surface air consumption (SAC) rate.

I am carrying four cylinders: two times 12 litres as my bottom, or back gas. For this dive, the mix is 18% oxygen, 30% helium and 52% nitrogen. The added helium displaces the nitrogen and oxygen, reducing the effect of nitrogen narcosis – fuzzy head, slowed thinking and reactions, reduced peripheral vision. The reduced oxygen, reduces the risk of oxygen toxicity caused by increased partial pressure of oxygen which could lead to convulsions and loss of consciousness.

For decompression, I am also carrying two times seven litre sidemounts, each holding a nitrox mix that contains 52% oxygen. This will reduce decompression times, but I cannot breathe this below 20 metres or 65 feet owing to the risk of oxygen toxicity.

After the 67 metre ridge, we come to the bottom of the pinnacles, and now we need to start rising as my decompression obligation is nearly 40 minutes, after nine minutes into the dive.

We fin slowly around and up the pinnacles, rising north-north-west to a road at about 24 metres or 80 feet. We follow this road south, continuing to rise through 20 metres.

Change gases and regulators to my left sidemount.

Change computer settings to make sure decompression obligation is accurate.

Rise to 6 metre depth and begin decompression for the next 54 minutes. For the next 54 minutes, I can’t go any shallower than this. I have at this moment, significant nitrogen circulating in my bloodstream. This nitrogen will not be diffused by my own cells and needs to be replaced in my blood, with oxygen. Because it is not diffused, the nitrogen maintains a bubble like status in my body. If I surface now, the reduced pressure could cause these bubbles to expand and fizz causing DCI – Decompression Illness, or ‘the bends’.  If I surface now, death would be a real possibility; crippling brain damage a certainty.

I gaze up at the surface and think about the juxtaposition of the normally life supporting atmosphere above me, which if I were to enter now, would kill me; and the normally hazardous underwater environment, which for the next 50 plus minutes, is my protector.

This is usually the time when I relax, reflect on the dive, and think about the next one.

Apparently quite the spot for UK diving

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