Cupar Sub Aqua Club

BSAC No 1094

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Dive Officer Checklist

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This List of Diving Officer checks comes courtesy of our very own DO, Wayne...

When I last sent around the marshalling list I included some (hopefully) helpful hints and pieces of news. You might recall that I also asked for all incidents – however trivial – to be reported to me, so that information of this sort could be used for us all to learn from.
Well, it seems to me that the best way to circulate this sort of information, and to hopefully stimulate thought and discussion, is to have a regular ‘column’ that doesn’t necessarily need to wait upon the publication of the next marshalling list. Of course a ‘column’ presupposes a regular publication, and since we have decided to let the electronic world (largely) take over from ‘Buddy Lines’ that seems to mean me sending around an email – at least for the non-luddites amongst us!
My apologies for the title of this ‘column’. Trying to find a catchy one resulted in some even more monstrous proposals, and this seemed more positive than “dos and don’ts”.

DO's
Check Weighting
Practise DSMB drills
Monitor Depth & Time
Know your computer

Weighting

1.Do remember to regularly review and check your correct weighting. Diving overweighted, especially in a dry suit, can, at best, result in a tiring dive as you struggle to maintain neutral buoyancy, and, at worst, be extremely dangerous. As one member recently discovered, it can make an immense difference to the pleasure of the dive (even for those that follow) and to air consumption etc. It is also a VERY important safety concern.
2.Do be careful with weights held in pouch type harnesses and BCs. It is recommended that you try to keep this weight to less than about 24 pounds, carrying any extra that may be required in a suitable non-dumpable way. Also, of course, you can mix non-dumpable weights with a standard weight belt for both comfort and security. All you need to be able to do is to drop enough weight to ensure that you will head surfaceward if all else fails. Bob also makes the following excellent point: '“I think there is a tendency for people using a harness system (particularly when changing from a 'conventional' belt) to over-tighten the waist band. This will increase the force pulling on the pouch closure, so increasing the likelihood of weights being lost. For this reason I recommend that the waist band be tightened just sufficiently to prevent the weights shifting.”

Practise DSMB Drills

Do practise DSMB deployment skills carefully and frequently.

Monitor Depth and Times

Do always carry your own depth and timing device(s). Never rely on another diver monitoring depth and timing for you.

Know your Computer

Today’s ‘do’ is prompted by my reflecting on a recent incident, where, I’m glad to say, nothing more serious than a bit of anxiety and the consumption of some precautionary nitrox ensued. However, the report on it got me thinking about the circumstances of a few other incidents that have occurred over the years and the common theme that emerges from them:

“Do know your dive computer”

Those wonderful little beasts sitting on your wrist (my preference) or elsewhere have done a lot to transform the way in which we dive, usually for the better. They give us credit for our actual dive profile, so enable sensible multi-level dives with reasonable bottom times, they warn us when we do (some) silly things, etc. etc. Very few sport divers would choose to be without one (given the finances).
However, they pose two major risks:
First, they sometimes encourage us to ‘push the envelope’ closer to the limit than we otherwise might. If we plan a dive based on tables, we usually assume a square profile, when we will probably be shallower for some proportion of the dive, and we tend to overestimate depth or time to ‘fit’ the tables. The planning is therefore conservative. Your dive computer, on the other hand, unless you either know it to be, or tell it to be, won’t be conservative. It will tell you the limits of what you can do, rather than what might be prudent. No algorithm – in a table or a computer – is foolproof. There will be circumstances under which it has a fair probability of getting you bent. The closer you approach the limits, the higher that probability. Do you know how conservative (or not) your computer is? Do you know how to adjust that conservatism? Have you thought of adding a ‘safety stop’ even after your computer says it is clear to surface?
Second, they sometimes fool us into not thinking; not properly planning our dive. “I’ve got a computer. That’s fine. It will tell me when I’m out of ‘no-stop time’. It will tell me how much deco I need to do. etc.” Sure it will tell you – but what about when it tells you something you don’t expect!?
What about when it tells you that you have 20 minutes of deco to do and you look at your own or you buddy’s gauge and see that there isn’t enough gas to do that? It can happen very easily: you are deep, you have plenty of air and you have only clocked up a couple of minutes of deco. No problem! You immediately start to ascend, but you do it slowly, because that’s the safest thing to do (and there was also lots of interesting stuff along the way). Sometimes just doing that will result in your deco obligation clearing even before you reach the desired ‘decompression depth’, but sometimes it won’t. Under some circumstances it will result in your deco obligation climbing alarmingly. Do you know what those circumstances are?
(If you think that cave diving is scary but deco diving is not, then think again. They are very similar in many ways. In neither case are you completely free to head straight to the surface whenever you need or want to.)
What will your computer do when it thinks you have been ‘naughty’? You haven’t gone into deco, but you have come up a bit too fast (maybe even for just a minute, nowhere near the surface); it didn’t like your see-saw profile; it thought you were breathing too hard and therefore overexerting (happens with some air-integrated computers), etc. Perhaps you have been breathing nitrox, but haven’t bothered to (or can’t) tell it, and you ‘know’ damn well you are unlikely to be in deco, but it thinks you are. What will happen? Do you have to obey your computer’s instructions or get bent? If you ignore what it says, what will be the consequences for both you and your computer?
Well, it seems simple: just take the conservative approach and do what your computer says anyway. But is that necessarily right? What if it says to stay down beyond the limits of your air supply? What if staying down unnecessarily causes you to become severely chilled, thereby increasing, rather than decreasing, your chance of a bend.
You need to understand what your computer does and why and on many occasions you need to anticipate what it will do. It comes down to what you were taught in some of your earliest lessons – you need to plan each dive. Having a computer doesn’t allow you to ignore that, though it might mean you need to think about different things or in a different way.

For reference:
The depth at which you can effectively begin decompression depends on the relative nitrogen load of the various ‘tissue compartments’ in the model your computer uses. There may be an optimum depth for fastest decompression, but every computer model that I know of will give you credit for the ‘off-gassing’ of some of these compartments at considerably deeper depths (albeit more slowly). If you are ‘in deco’ because of a single short deep dive, chances are that it’s the fastest tissues that are supersaturated, and these will start off-gassing pretty much as soon as you start to ascend, and you will often see your deco penalty disappears before you reach ‘decompression depth’. If, on the other hand, you slower tissues are the most heavily loaded, then a slow ascent is likely to load them even more and your decompression penalty will increase.
However, most computers that penalise you for ‘bad behaviour’ with a mandatory safety stop will demand that this stop be completed within a specific depth range, whatever the state of the various theoretical tissue compartments. If it says you must be, for example, between 3 and 6 metres then it will be very unforgiving if you are not, whereas if you are decompressing due to nitrogen loading, then it might give you an estimated time required at, say, 3m, but it will still let you decompress – though more slowly - at 5, 6 or even 8 metres.