h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hi there - just happened to find myself here and saw your SCUBA diving articles; impressive stuff! I've got a few points to add, if I might be so bold:

Not sure whether this applies worldwide, but here in the UK it's important to make sure your cylinder is within its test dates; they have to be hydraulically tested after 3 years of use, and every 2 years after that. They should have a visual inspection inside and out every year after the first 3 years, too. (I think those intervals are right; I have a feeling they might have changed recently...) Dive centres should refuse to fill your cylinder if it's out of its test date, but the odd tank can slip through.

If you own a compressor, it's important to make sure that it's in good condition - compressors contain filters to remove oil and water from air before it's pumped into the tank, and these have to be changed regularly. If any oil vapour gets into your cylinder, it won't do your lungs any good at all; oils attack the delicate membranes within. If you're compressing on or near a boat whilst on a diving trip, make sure the intake is nowhere near the exhaust of the engines. Otherwise, you'll end up breathing exhaust fumes, and breathing carbon monoxide at pressure is NOT a good idea. (It binds to your blood's haemoglobin in place of oxygen, so essentially suffocating you from the inside)

Henry's law governs the solubility of gases, which put simply states that increasing the pressure will increase the solubility of a gas; the classic example is opening a bottle of Coke (or any other carbonated beverage). It's quite happy when under pressure in the bottle, but as soon as the pressure is released, the carbon dioxide rushes out of solution and escapes as a burst of small bubbles. It's the same with your blood - ascending too fast will cause huge numbers of bubbles to appear. If you're lucky, they'll just form in your joints and skin - you'll get joint pains and skin rashes, which is a mild form of decompression sickness. If you're not so lucky they can disrupt your nervous system - symptoms include tingling and numbness, paralysis, blindness... This is not good. Whilst on the subject, in the event of a diving accident, it's important that the casualty is NOT given a gas called Entonox (nitrous oxide); this can exacerbate decompression sickness. There's evidence to suggest that the administration of pure oxygen can help speed recovery, but the best treatment is definitely within a recompression chamber.

As well as clearing your ears as you descend, it's important to equalise the pressure in your mask by breathing out through your nose. If you don't, the air surrounding your nose and eyes will be at a pressure lower than the water around you, and so you'll get mask squeeze. If you don't do anything about it, the reduced pressure can cause tiny blood vessels in your eyes to burst, producing spectacular red-eye effects. This looks grim. The guy who signed me up to the University club here had bright red eyes for this very reason; it was a little unsettling...

Another danger involved in SCUBA diving is that of getting trapped in a wreck. This one's easy to avoid, as long as you can stay away from temptation, but it's important to be happy and familiar with your kit before diving - the standard SCUBA kit will have many hoses, loops and other dangling attachments that can get caught round protruberances or between rocks. It's not the sort of stuff you'd want to tug on to get yourself free, either. Once in a wreck, it's possible to kick up a lot of silt and the visibility that was perfect when you entered can drop to nothing very quickly. In that case, finding the way out becomes a major challenge.

I've dived here in the UK for the last few years, and had some fantastic experiences. Not quite the same as diving in the Maldives, perhaps, but definitely rewarding. The key is to make sure you're aware of the dangers - if you know how to avoid most problems, and how to deal with any rogue ones that arise, you can reduce the risks tremendously.

Glad you enjoyed it! My diving experience revolves mostly around the Hawaiian Islands, where I lived for over four years, and I dove whenever I could. Now I'm in California, and aching to try out a few spots around here, but they're harder to get to, and more expensive, plus I don't know any divers here. Sigh.

I covered VIP testing of air tanks in the one on equipment, and I was trying to avoid redundancy in the two articles. Here in the States, VIP tests are required annually.

The part about oil and CO getting into the tank through the compressor is an omission... thanks for the tip. I'll put that bit in.

The Henry's Law bit is good, too... will help explain why the blood absorbs so much oxygen and nitrogen at depth. I was so focused on the effects that I may not have fully explained the cause.

I'd never heard of that bit about mask squeeze... I've seen people surface with serious mask squeeze before, and of course we rag on them for being rookies, but never heard of anyone bursting a vessel over it.

The part about wrecks is worth a mention, and I think I'll also mention caves, while I'm at it.

Thanks for your help! So often these things don't get looked at until an editor gets hold of them, and the omissions remain until someone points out what a dumbass I am when it debuts on the homepage as an Approved Entry. Feedback is always appreciated.

That's quite all right; it's nice to read an article I feel I can actually contribute to

Might add a couple of other bits and pieces when I get a couple of minutes - the BSAC training breaks down 'burst lung' into three conditions - air embolism, spontaneous pneumothorax and interstitial emphysema (if that's how you spell it!)

If I recall correctly, spontaneous pneumothorax is where there's a major tear in the lungs and air escapes and gets trapped between the lungs and the chest wall. As the diver ascends, this air expands, compressing the lungs and making breathing more difficult. Interstitial emphysema is, I think, the condition of air escaping inwards from the lungs and getting trapped in the internal body cavity. The symptom for this condition is that the skin round the base of the neck feels and sounds 'crackly' because of the air trapped underneath.

I'll check on those and get back to you...

Cave diving's a good one - I've tried several different 'adventurous' sports so far, but I draw the line at cave diving. I'm not at my best in confined spaces, and my sense of direction is appalling. I can say with reasonable confidence that if I tried cave diving, I'd end up dead. You've got to be mad to try that

All the best,
Azimuth

Call me mad then...

I've never done the real subsurface spelunker thing, though, where you have to bring extra rigs and mark the way out with line and all that. I've just dove into some modest sized caves, the kind where you can see the way out from the deepest accessible point. I recommend you check one out, if for no other reason than to see what happens to your air bubbles at the roof of the cave. With any amount of surge in the cave, they pool together and dance like mercury. Very cool effect.

Ah, no - it's twisty-turny-passage cave-diving that I won't try... you're right; the mercury effect is very cool. Actually, I've dived a site off St. Abbs in Scotland (called Tye's Tunnel, I believe) which was pretty impressive. There were a couple of small airholes to the surface halfway along the passage, and it gets progressively narrower as you go along. It's pretty dark inside, so torches are a necessity. If you get the tide right, you can swim straight through into the next bay, but the waterflow through the tunnel stops you if your timing's out. If you turn round and retrace your steps, the tunnel has another spur that rises gently then stops in a dead end; the buddy pair in front of us went up that and got stuck. They then came back down and met us going the other way, leading to a faintly amusing 'four SCUBA divers in a confined space' moment.

Quite an eerie experience, and open enough for me not to get the creeping horrors

Azimuth