Saturday 25 May 2013

In One Way, Depth is Just a Number...

(I started writing this post back in 2010, came across it, and decided to revive it. As of 2013, I'm a Rescue diver, albeit I do need to go back to Lake Ellerton and practice fishing people out of the water some time soon. Funny how even back then I was picking holes in PADI's teaching methods...). 

And in others, it isn't.

As part of my Advanced Open Water Certificate, I'll be doing a "deep dive" whilst in Bute for the diving weekend I'm going on in March. For those not familiar with recreational diving limits, I currently have an Open Water certificate, which means I'm qualified to dive as deep as 18 metres.

In common with (I suspect) most Open Water divers, I have somewhat bent this rule - without getting myself bent, I hasten to add - whilst diving in the tropics. My deepest dive so far was 23 metres in the Maldives to see an interesting sunken ship. It was at the start of the dive, myself and my buddy had full tanks, I wasn't tired or dehydrated and I kept a close eye on my dive computer and air gauge, so I figured I wasn't running an unacceptably high risk of burning through my breathing gas or running out of bottom time. I was right.

A quick digression on some terms I'm using here:

Bent = getting the bends; decompression sickness causing by ascending too fast or staying down so long you can't come straight up but have to do decompression stops. Recreational or sports divers, like me and like everyone when they first start diving, do the sort of dives which don't require decompression stops, so that at any point in the dive, you can abort it and ascend to the surface, albeit at a slow enough rate that you don't get the bends. This is for safety reasons.

Bottom time = the amount of time you can stay submerged before you have to ascend. Governed by two factors; how much breathing gas you have, and how much nitrogen you have absorbed from breathing gas under pressure. The amount of nitrogen you have absorbed governs how long you can stay under without needing to do decompression stops to allow the nitrogen to leave your system so that you don't get the bends. The two factors are interlinked: If you're at a deeper dive, you go through your breathing gas faster, because it is delivered at a higher pressure. This also means you're breathing in more nitrogen molecules - it's a compressed gas, so there are more gas molecules in each breath you take in - so you absorb more nitrogen and have a shorter period of time you can safely stay under. Also one of many diving terms that causes juvenile humour to occur. 

For my "deep dive", I'll be going deeper than this. It's surprisingly hard to find out exactly how deep online - it's something I'll ask my instructor - but probably around 25-30 metres deep depending on the conditions.

This post starts "In one way" because a novice diver saying "depth is just a number" and leaving it at that implies that person hasn't fully grasped that deeper dives do come with a set of distinct challenges. These include the aforementioned shorter bottom time periods and the fact that they use gas a lot faster than shallower dives. They also include the unique aspect of getting the narcs.

The narcs = nitrogen narcosis. Essentially, the deeper you go, the more the higher concentration of nitrogen you're breathing makes you feel intoxicated. It's most commonly compared to being drunk, although symptoms vary from diver to diver. The narcs are harmless in themselves and go away if you ascend (safely) to a higher depth. The risk comes from feeling a bit drunk 30m below the water. It's even worse for technical divers, who have to fiddle around with their gases to try to come up with a mix that won't send them completely off their heads whilst 60m down.

But in one way, I believe that depth is just a number. Why? To me, it's in the mind. One thing I think PADI's Open Water Diver course does run the risk of is encouraging newbie divers to think "It's okay if things go wrong, I can always go back up again". I think that's the wrong mindset. Sure, there are times when a CESA (controlled emergency swimming ascent - swimming up to the surface if you've run out of air, but keeping your weight belt on so that you don't rocket up there, and exhaling to stop you from getting lung overexpansion injuries) or even the dreaded buoyant ascent are the right, i.e. only, options to handle the situation.

Buoyant ascent = ditching your weights so that you immediately become positively buoyant and fly up to the surface in an uncontrolled ascent. Extremely dangerous as you run the risk of getting lung overexpansion and the bends. Absolute last-ditch measure for getting to the top if all other hope is lost. Comes with a big AVOID DOING THIS IF YOU POSSIBLY CAN in the diving training manuals.

However, I think that's a risky mindset. It encourages you to think that all problems are solvable and there's always a way out, whereas you should be thinking about how to prevent the problems in the first place. You can get the bends coming up from 12m or even shallower. Going deeper increases the risks, but the risk is always there.

Dive safely, folks. And remember... in British waters, there's frequently bugger-all to see at 30m in March in the Farne Islands. 15m is more fun, and there are seals!

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