Decompression – Page 2 – Learn to Dive Today Blog

Suunto D6 dive computer

I’ve been using the Suunto D6 dive computer for about eight months now, having finally got my grubby paws on it just after returning from our last trip to Sodwana. I think it’s about time I write a little review of it, because Suunto have just released the D6i and before you know it my computer will be a museum relic.

Specifications and appearance

The D6 is near the upper end of the range of Suunto dive computers – the model I have will now set you back in the region of R10,000 and there are several cheaper but no less effective offerings. The USB interface cable that will enable your dive computer to talk to your computer will set you back up to a further R1,500 – although this item is frequently advertised on special by Suunto stockists and occasionally as a special bundled with the dive computer, so keep your eyes open. It’s far more usual, however, to have to buy this innocuous-looking cable separately, and gasp at the price.

You can choose an elastomer strap or a metal strap (for about R2,000 more). Although the metal strap looks really cool, it’s not really practical if you dive in varying water temperatures and change the amount of neoprene on your wrist frequently. The computer functions as a dress watch if you want to use it as one – it displays the time constantly when not in dive or memory mode – but it weighs more than a slab of chocolate (130g) and is far too large for the average lady’s wrist, so I don’t use it for this purpose except when travelling (to deny the baggage handlers at OR Tambo Airport the privilege of stealing it).

The computer has a four button interface that I find very intuitive, and I could figure it out to a large degree without reading the manual. That said, if you buy a dive computer, YOU MUST READ THE MANUAL! Don’t be a fool – you want to know EXACTLY why the thing is beeping at you, what it looks like when you go into deco, and be very sure (as one clown – who was buddied with us once because he didn’t know anyone on the boat – wasn’t) whether the “3” you see on the screen indicates a time in minutes, your current depth, or the number of brain cells you have. Read the manual!

Air integration and the D6i

The D6 has actually been replaced by the D6i, which is functionally identical but has more internal memory, and is capable of air integration with an optional (heart-stoppingly expensive) dongle that you attach to your cylinder and reads remaining air. The computer will then give you an estimate of remaing dive time based on air consumption to date. I have no interest in this (at the time I bought the D6, air integration was the main distinguishing feature from the D9) – I’d use a pressure gauge regardless, and wouldn’t feel comfortable trusting what I see as an physical, analogue process (displaying the air remaining in my cylinder) to a potentially failure-prone piece of electronics.

I know I may sound like a luddite here, but an experience Tony had on the boat a few months ago confirmed my reservations. Another instructor’s student had an air integrated computer and no pressure gauge (why bother with redundancy?). The air integration with the computer wouldn’t work, but they only discovered this on the boat when they were parked over the dive site, and – after toying with the idea of cancelling the dive, and then swapping kit so the instructor, who should have an excellent feel for his air consumption, had the set up with no pressure gauge – did a very short dive. As a mathematician I can see that having snapshots of your remaining air at 20 second intervals to look at in the dive manager software might be appealing though…

Decompression algorithm

The D6 uses the Suunto Deep Stop RGBM (Reduced Gradient Bubble Model), and allows for continuous decompression as you ascend (instead of forcing you to do stops at particular depths). It also allows you to complete your safety stops at depth – something I haven’t experimented with much, but will be using next time we dive the Lusitania. The computer recommends a safety stop once you exceed 10 metres on a dive, and if you violate the recommended ascent rate it will advise a mandatory safety stop between 3 and 6 metres. I tried to photograph the D6 during a dive where I’d switched on deep stops, but there was a 20 metre layer of green plankton blocking out the light from above, and my flash kept reflecting off the screen protector. Hence the dubious results you see here. I have 28 minutes of no-decompression time remaining, dive time is 11 minutes, depth is 20.9 metres, and my first deep stop will be at 13 metres. Maximum depth (bottom left) so far has been 23.8 metres.

Suunto D6 during a dive - first deep stop is due at 13 metres — Suunto D6 during a dive – first deep stop is due at 13 metres

You’re most likely aware of this, but a dive computer does not measure anything that is going on in your body with respect to dissolved gases. Dive computers use mathematical models – based on the original dive tables, only more sophisticated – that approximate, for the average person, how much nitrogen has gone into solution in the body’s tissues, and how fast it is being released, based on your dive profile. They measure depth temperature, and time, that’s all. For this reason many dive computers, including the D6, have an option for you to set a more conservative calculation algorithm if you’re at higher risk of DCS – for reasons of increased age, high body fat percentage, or any of the other DCS-predisposing risk factors. You can also adjust the partial pressure settings up and down if you so desire, but anything higher than 1.4 bar (ata) strikes me as reckless.

Nitrox and no-fly time

It goes without saying that the D6 is Nitrox capable, and it’s very straightforward to set the Nitrox mix. After one dive on Nitrox, the option to do a repetitive air dive disappears, and you have to manually set the oxygen percentage of your mix back down to 21%. I think this is to force you to think about what gas is in your cylinder. The D6 also handles switching to a richer mix for decompression, and this optional second mix may be set through the same menu system as the primary nitrox mix.

The D6, again like most dive computers, gives a no-fly time after you’re done with diving for the day. This time is usually well under 18 hours, but you’d do well to follow DAN guidelines for flying after diving (usually 18 hours after your last dive) and not bank on the reading given by your computer. Do not be like Gerard, who shall remain nameless, and mistake the time display on your computer for the no-fly time. After a dive on the Aster that ended at about 3.30pm, he announced that his no-fly time was “fifteen hours and twenty nine minutes.” A few minutes later, to his puzzlement, it was “fifteen hours and thirty four minutes!”

Dive Manager software

The Suunto dive manager software, that allows you to examine the details of your dives on your computer screen at home, is not compatible with Apple Macs, so I had to find another solution. I’ll review the software I do use, MacDive, in a separate post. Apparently from “fall 2011”, whenever that rolls (rolled) around, the Suunto software – DM4 – will also be compatible with Apple computers. I’ll test it when I get a chance, and let you know what it’s like… As is apparently wildly popular these days, one can also share one’s sporting activities on the Suunto Movescount site via an automatic link-up from within the software interface. And, no doubt, publish them to facebook.

Electronic compass

Suunto D6 with elastomer strap (right) and titanium strap (left)

One of the major appeals for me of the D6 – and I think the feature that bumps its price up so much higher than the D4 – was the integrated electronic compass, which can be accessed at (almost) any time by holding down the top left (Select) button. In the picture at right, the D6 with the elastomer strap is on the compass display (the one on the right is in the memory log display mode which can’t be accessed during a dive). If you’re not on a dive when you use the compass, obviously the depth and dive time won’t show.

The D6i has updated the compass to allow accurate readings when your wrist is tilted; the old D6 (the one I have) is not as tolerant and you’ll need to keep your arm level as with a standard dive compass. Unfortunately the D6 doesn’t record the compass heading along with the temperature, depth and other dive statistics during the dive – or, if it does, the download software I use doesn’t access it. I suspect the former is true, since I installed Suunto’s own dive manager software on Tony’s PC to check, and there was no sign of compass headings. Boo.

What’s in the box

In the box was the computer, a strap extender, the instruction manual, a disc with the Windows-compatible dive manager software on it, and two or three scratch guards which are trimmed to fit the D6’s screen. I’ve done close to 70 dives with my D6 so far, and the scratch guard is scratched and still doing its job well. Tony’s computer, the Mares Nemo Wide, is protected with some cheap cellphone screen protectors we got from Look ‘n Listen. You can buy a generic size, and then trim it down to fit your phone (or dive computer, as the case may be). We didn’t expect this makeshift scratch guard – which is NOT designed for regular immersion in salt water – to last beyond five or ten dives, but over 100 dives and it’s going strong. I think I paid R60 for the pack of screen protector stickers, and we’ll get nine Nemo Wide-sized ones out of the package.

Buying it

Continuing with the subject of good value, one more tip for the bargain hunters. I actually bought my D6 from Cape Union Mart. They stock Suunto sports watches, and were able to order me a D6 from Suunto in Finland. I had to wait six weeks for it to arrive, and it cost R8,700. What made the deal very sweet was that by buying it on my Discovery Card which gives me a 20% discount at Cape Union Mart (thanks to my years in the Vitality program and points status), the computer ended up costing just under R7,000. I paid a further R1,200 for the download cable (I got that at a dive centre). If you have a few weeks before you need the computer, or are prepared to wait in exchange for some savings, it’s worth getting a quote from Cape Union Mart as to what they’ll charge you. If you’ve got a Discovery Card it’s a no-brainer. Email them via the website for a quotation, and they’ll tell you to print that and take it to your nearest Cape Union Mart to place the order. I had to pay a 50% deposit.

Update (late 2012): Based on feedback from other divers who have shopped for Suunto computers lately, it seems that Cape Union Mart isn’t doing this any more, unfortunately – but it’s worth asking anyway!

Maintaining it

Finally – if you have a dive computer and live in Cape Town, take it to Orca in Claremont to get the battery changed when necessary, and ask for Chris the “worship manager” (that’s autocorrect gone wild on “workshop manager”) to do it for you. Tony’s students have had baaaaad experiences (a hair across the seal, anyone?!) at other locations. There’s usually not much you can do if the service centre doesn’t seal the computer properly and it floods – your only recourse will possibly be to your insurance company.

(No) decompression diving

Tony has posted about decompression diving before, but it’s been on my mind a lot lately after the winter diving this year.

If you’ve done a recreational scuba diving course, like a PADI or Naui Open Water course, you’ve learned how to do “no decompression diving”. Your entire dive is structured so that, should the need arise, you could ascend directly to the surface (in other words, without having to do a safety stop or decompression stop on the way up) without undue risk of getting bent. This is why you learn how to use the dive tables or the dreadful eRDP, and why you have time limits depending on what depth you go to. This kind of diving is considered (and indeed, is) far less risky than “decompression diving”, which involves overstaying the limits prescribed by the dive tables combined with staged ascents. This means that solving a problem by surfacing is just not an option (unless you want to create a whole other problem for yourself).

Tony safety stopping at Tivoli Pinnacles

As a beginning diver, it’s often a moot point that you only have x minutes of bottom time at a given depth: your air consumption is usually so poor that the air is gone long before the bottom time is up! As you dive more and more, however, the time limits really come into play and you may find yourself ascending with lots of air to spare, but because your computer tells you it’s time to go.

Particularly on repetitive dives (I’m thinking of a day when I did three long dives to at least 20 metres, all before lunchtime) you’ll run the risk of going into deco (in other words, incurring a mandatory stop on the way up in order to get the nitrogen out of your body tissues). Diving on Nitrox can help a lot with this, and it’s healthier, but there may well be occasions when you do find yourself in deco even after just one long (or deep) dive.

The first time it happened to me I got a bit of a fright. I’d read my dive computer manual back to front and made sure I understood all the warnings and alarms, so I knew exactly what it was telling me, but I wasn’t happy. We ascended a bit, my computer went out of deco, and after my three minute safety stop it gave me the all-clear to ascend. I had an idea in my head that going into deco was REALLY BAD, and I confessed it shamefacedly to Tony on the boat afterwards. He wasn’t too fussed; and as Grant pointed out, “There are worse things than going into deco – like not doing your deco stops!”

A little reflection, however, convinced me that I hadn’t done anything wrong (except perhaps not forseeing that this was going to happen and mentioning it to my buddy, who happened to be Tony!). I’ve done over 150 dives in the two years I’ve been diving, my air consumption is excellent, and I’m very calm underwater. Tony and I know each other well enough that our underwater discussions can be quite detailed. I’ve since allowed myself to go into deco a few times, never more than a few minutes because I dive on a single 10 or 12 litre cylinder, and I’ve done the stops that my computer requests of me (usually only an extra 1-3 minutes plus a 3 minute safety stop). I can tell you that this can be a little boring, unless you have jellyfish and seals for company, or an entertaining dive buddy!

I am very, very glad that I have my own dive computer now, and make sure that I understand exactly what it’s telling me at each point in the dive. I’ve been on a deep dive, and two young divers who didn’t have computers spent the entire dive below me. I went into deco and got a one minute stop on the way up; they must have incurred a much longer obligation, but didn’t know about it because they didn’t have dive computers. If you’re at special risk of decompression sickness due to age, weight, fatigue or other considerations, a dive computer is even more important. Small changes in depth can make large changes in the amount of bottom time you have remaining, and unless you are glued to your buddy’s side all the time, relying on their computer to keep you safe is risky.

Dive sites: SS Lusitania

According to Peter Southwood, on the wikivoyage page for the SS Lusitania,

The wreck of the Lusitania is considered by many Cape Town divers to be one of the top wreck dives of the region. It is fairly deep, the wreck is quite broken up, but still interesting, with a number of identifiable components, and the visibility is often quite good. However, it is a physically challenging dive, quite a distance from the launch sites, and conditions are not often suitable, so it is not dived very often. No doubt these factors add to the mystique.

Tony and I had heard about the mystique surrounding this wreck, but because we hadn’t had an opportunity to dive it since he’s been in Cape Town (about two years now) we didn’t know much about it. Beautiful weather in the middle of winter provided an opportunity to take the long boat ride out of False Bay, past Cape Point, and further south to Bellows Rock. Bellows Rock is named for the way in which the water smashes over the top of it, and the break is apparently visible from Macassar – at the northern end of False Bay – when the swell is large.

The ship is now very broken up, after 100 years lying below Bellows Rock

Wreckage lies tumbled among the smooth rocks

Here and there are brass fittings and window frames

Railings (perhaps) from the SS Lusitania

Rocky reef on which the SS Lusitania is lying

The Lusitania was a large Portuguese liner that ran aground on Bellows Rock in late April 1911, carrying about 800 passengers. All but eight survived (a lifeboat capsized), and after a few days the ship slid down the side of Bellows Rock to where it now lies in 37 metres of water. A light Nitrox mix will help increase dive times here. She’s an old wreck – as old as the Clan Stuart – and very broken up. The wreckage is readily discernible, however, because it is not very encrusted with marine life at all. The surrounding reef, made up of granite boulders with a some nice holes, overhangs and places to look underneath, is very colourful and well encrusted with sponges. There are also massive numbers of West coast rock lobster!

Strawberry sea anemones on the SS Lusitania

Sponges and soft corals on the SS Lusitania wreck site

Grant gave us a bone-chilling briefing that had me convinced (if one of the other divers on the boat hadn’t done it already by telling me with dewy eyes that this was “the Mount Everest of diving”) that this is quite a higher grade dive that requires wits, fitness and no small measure of courage for a scaredy-cat like me. The wreck lies right next to Bellows Rock, which is surrounded by a large area of breaking waves and white water. Descents at this site must be rapid – no messing around on the surface doing buddy checks (“PADI stuff”, as Grant called it once) – and descents are equally demanding. There’s usually a current pushing towards the north west, and this will force one onto the rock unless you take care. Grant advised us to swim gently in a north easterly direction (more or less across the current) as we ascended, and to do our safety stops at eight metres instead of the usual five. He warned us not to surface if we saw white water above us. He told us that he has “survived” being washed over Bellows Rock itself, but that it’s not an experience one wants to have unless it’s absolutely necessary.

The ride out to Bellows Rock is magnificent, and takes about 20-25 minutes at the speed Grant drives (very fast). The sea was nice and flat inside False Bay, but outside the bay there was a small swell. Even though it was only 2-3 metres, Bellows Rock and surrounds looked like a terrifying whirlpool to me. Grant couldn’t drop a shot line onto the wreck because it would get washed over the rock (and presumably have to stay there forever, which would be an inconvenience). The plan was for us to enter the water in two groups. The first group kitted up, and Grant drove the boat as close as he dared to the white water around the rock, so that we were right over the wreck. He counted down, slowed the boat to a crawl, and the divers were gone. As soon as they rolled off he drove away so that the second group – me, Tony, Cecil and Ivan – could get ready.

Kitting up on the boat (that's not us!) — Kitting up on the boat (that’s not us!)

Our entry probably looked just as scary, but I wasn’t actually looking at the water! Next time I do this dive I’d take more weight – for deep dives my preferred weighting is marginal and I have to swim down the first 3-5 metres; this wasn’t ideal for this dive, and because I was anxious about getting down quickly, I breathed too fast and struggled to sink! Once we were in, however, everything was fine.

The Lusitania was a 5 557 ton vessel, very large. There are big, complete pieces of wreckage scattered about, but very little actual structure remains. Close to Bellows Rock – which drops off precipitously below the water line – are lots of interesting bits and pieces, but getting there and away is difficult because of the current. We spent some time exploring the edge of the wreck furthest from Bellows Rock, and then swam slightly north over some rocky reef that reminded me a lot of Klein Tafelberg Reef in Hout Bay.

Our ascent was uneventful – we followed instructions, did a deep safety stop, and surfaced a safe distance from Bellows Rock. The water was very clean, and very cold, but as we moved further towards the entrance to False Bay the visibility declined somewhat. I’d like to dive this wreck again, but conditions have to be very special – with almost no swell and no wind – for it to be safe. I feel like a Philistine for admitting that the challenging nature of this dive was not compensated for by the specialness of the dive site itself, although it is without doubt a very interesting and varied dive.

A torch reveals the colours of the soft corals all over the site

The boat ride back past Cape Point and along the eastern shore of the Cape Point Nature Reserve was beautiful. We saw several whales, a sunfish (briefly), seabirds, and – for most of the trip – the ocean floor beneath us, as the water was crystal clear. It was, as Cecil said, an absolute treat and a wonderful way to spend a morning (all of it, even the death-defying leaps from a slow-moving boat!).

Dive date: 16 July 2011

Air temperature: 21 degrees

Water temperature: 14 degrees

Maximum depth: 37.0 metres

Visibility: 12 metres

Dive duration: 36 minutes

Tony managing the reel at the safety stop

Enriched Air/Nitrox Specialty

If you’re a Cape Town diver, and serious about enjoying the huge range of wrecks and reefs we have here, there are two Specialties that you should seriously consider.

One is the Deep Specialty, which qualifies you to go to 40 metres. (The depth it qualifies you to go to is actually less important than the skills you will learn on the course.)

The Number One cat helps Tony apply a Nitrox sticker to one of his cylinders

The other is the Enriched Air/Nitrox Specialty. Enriched air is ordinary air that has been enriched with extra oxygen. This reduces the nitrogen concentration, which is a good thing for two reasons.

When we breathe air under pressure, nitrogen is absorbed by our body tissues (particularly quickly by fat). While you’re at depth this isn’t a problem, but it becomes a problem when you ascend too fast and neglect to do the required safety stops or decompression stops. The nitrogen forms bubbles in your blood, brain and joints, and you will get bent. This can be fatal, and it’s a horrible way to go. You can think about what enriched air does for you in two ways: you get extended bottom time within the no-decompression limits, or a margin of safety because if you follow the dive tables for air when breathing nitrox, you will have absorbed less nitrogen into your tissues by the time you ascend. The risk of decompression sickness is thus reduced.
Nitrogen has a narcotic effect when breathed under pressure, and this can impair judgment and lead to all sorts of stupidity on a dive. Less nitrogen in the mix you’re breathing means less narcosis.

It’s not as simple as just putting more oxygen in your cylinder and jumping into the water, however. Oxygen is toxic when breathed under pressure (you just can’t win!) and can cause convulsions. At the bottom of the ocean, a convulsion is bad news. So while you are free to add oxygen to your breathing mix, your maximum depth is restricted by the richness of the mix you choose. Nitrox mixes are referred to according to the percentage of oxygen in the mix. Normal air has 21% oxygen: Nitrox 32 means that the cylinder has 32% oxygen in it.

The Nitrox Specialty is mainly theory – there are some formulas that you need to get to grips with, and you need to understand the two-edged sword that is enriched air. Once you’ve mastered the theory, you’ll learn how to use a Nitrox analyser, and probably do two dives on Nitrox.

As you dive more and more, your air consumption gets better and better. When you get to the point where your dives are limited by the no-decompression limits of air rather than the amount of air in your cylinder, the Nitrox specialty becomes extremely attractive. If you’re in the group of divers (such as older, or overweight) who are most at risk of decompression sickness, diving on Nitrox is a huge investment in your own safety. And finally, if you do repetitive dives (several dives in a day), diving on Nitrox will extend your total bottom time tremendously.

Decompression diving

As a rule recreational diving is no decompression diving. The dive tables are designed to ensure a huge safety margin and you could ascend directly to the surface (no faster than 18 metres per minute) and in theory you should not get bent. Safety stops are just that: a stop, usually at 5 metres for a minimum of three minutes to allow your body to eliminate some of the nitrogen it has absorbed while you were at depth.

So what if you go into deco?

Should you exceed your maximum bottom time there are a few basics and that is where decompression comes in. A regular safety stop is a deco stop and three minutes is considered adequate on most dive planning, but for example a Deep Specialty calls for a simulated deco stop of 8 minutes with some of this time spent breathing from an alternate air source.

By the time a diver gets to this level of training they invariably have their own dive computer and your dive computer will show you that you have exceeded your bottom time and now have a decompression commitment – usually an eight minute safety stop.

When we do these dives we dive on enriched air/Nitrox and leave at least one of our dive computers on air. The result is that the computer set on air will give you a warning that you are now in deco and it will indicate the length of your required safety stop and the recommended depth. Providing you have planned your air consumption and air supply correctly you will be able to fullfill the deco commitment and surface with your computer ready for another dive.

Failure to complete the required safety stop for the entire recommended duration your computer will normally not go into full dive mode until 24 hours have passed. By doing the dive on Nitrox yet leaving your computer on air you give yourself a safety margin, allowing you to experience the actual computer output screen and interpret what it is wanting you to do, without the risk to your health of actually going into deco and being unable to fulfil that commitment for some reason.

Newsletter: Seasons of the sea

Hi divers

We have had some amazing diving days of late. Friday saw conditions at Long Beach that we have been longing for for months. Warm, clean water with an abundance of life. The ocean’s seasons are very interesting aspects of diving for Clare and I, and thanks to Clare’s logbook and amazing photos we have a much better idea now than we had a year ago of what you can find and when.

We have been fortunate enough to see tiny octopus, warty pleurobranchs spawning egg ribbons, klipfish mating, huge rays feeding and shysharks having a feeding frenzy. We visited the cowsharks, watched sadly as injured and hooked sharks struggled to adapt to the harm inflicted upon them by man, and watched a juvenile jutjaw and a doublesash butterflyfish grow from 2 centimetres to close to 6 centimetres before they moved off from their tiny safe house to brave the ocean.

On the weekend Clare found a juvenile sole so small and so well camouflaged it almost avoided her beady eyes. We always find something new and interesting in the ocean. We have watched our small artificial reef go from bits of wood and plastic to a small colony of life. The list is endless.

Diving at this time of year is not to be missed. (That applies all year round!)

Recent dives

We dived the Atlantic early Sunday, Grant taking us to the yacht wreck on Klein Tafelberg reef. We were looking for depth to continue the Deep Specialty and our maximum depth was 37 metres in 10 degree water with amazing visibility, 15-20 metres. We had to perform a simulated emergency deco stop for 8 minutes and during this time we had seals nipping at Cecil and I, and a jellyfish bonanza. We had a dive time of 36 minutes and we dived on Nitrox.

From Hout Bay we dashed to Long Beach to dive with the two Divemaster candidates and continue the Advanced Course doing navigation. Back in the water once more for a Refresher, and home to download the photos. The ocean was warm at Long Beach, 17 degrees, calm and the visibility was 5-6 metres.

Part of the yacht wreck on Tafelberg Reef

Atlantic diving should start to fade soon as the seasons change and the prevailing winds come from the north west. This cleans and cools to False Bay area and the visibility gets better and better.

Trips

We are off to Sodwana on Saturday for a four night/six dive trip, and the group, 13 in total, are all looking forward to this. We will post photos and video when we get back. I think we have five or six cameras for this event so there are bound to be loads of good photos.

Planned dives

We are hoping to book two launches for the Friday after we return from Sodwana, that being Easter Friday and booking is essential. We will plan to go to a wreck for the first dive and possibly a barge wreck or reef for the second launch. I need to give Grant some numbers before I leave for Sodwana so please let me know as soon as possible.

Courses

I am starting a new Deep Specialty course as soon as we get back from travelling. It’s a good idea to do the enriched air/Nitrox specialty at the same time. This combination qualifies you to dive to 40 metres, and gives you longer bottom times and safer diving.

Regards

—
Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog

Diving is addictive!

Chamber dive revisited

We recently did a chamber dive to 50 metres. A hyperbaric chamber is a sealable chamber, or pressure vessel, somewhat like your dive cylinder (just larger), and has hatches large enough for you to climb in. It is connected to an air compressor or a bank of compressed air, and once you’re in and it is sealed the pressure is increased just as the pressure around you increases as you descend. You need to equalise as you do under water, the only real difference being is that you are dry. A fast descent means constant equalizing and ensuring deep breaths are taken. You will experience nitrogen narcosis, the extent will vary from person to person and you voice will change.

The video below shows us counting backwards from five, showing the correct number of fingers and turning our hands round after each number. It’s hard when you’re narced!

[youtube=http://www.youtube.com/watch?v=zWX-X5RngUs&w=540]

As a recreational diver you will find that your PADI eRDP will not allow you to enter a depth greater than 40 metres when you plan a dive, and the recreational dive planner (RDP or dive tables) will not allow planning deeper than 42 metres. The chamber operator or anyone that does deep technical diving will have a program to enable proper dive planning and ensuring the correct dive profile is maintained, by means of decompression stops.

Checking on the first group of chamber divers

On our dive we descended to 9 metres, and paused whilst two way communication was tested and the operator checked everyone was okay. Thereafter we dropped like a stone down to 50 metres in two minutes. Our bottom time was nine minutes. Nine minutes at this depth gives you quite a decompression commitment and we ascended slowly doing several deco stops on the way up with a total dive time of 39 minutes.

My Mares Nemo Wide showing the dive stats

Our dive computers all joined us for this dive and were placed in a bucket of water (some dive computers will not go into dive mode unless the water contacts are activated). I had a Suunto Mosquito, a Mares Nemo Wide, an Uwatec Aladin Prime and a Citizen dive computer as well as a wrist mount depth gauge.

My trusty Suunto Mosquito showing the dive stats

The computers were all similar in readings and were all between 50.1 metres and 50.4 metres whilst the depth gauge showed 59 metres! It’s safer for your instruments to err on the side of conservatism (i.e. tell you you’re deeper than you are, rather than the other way around). This depth gauge probably didn’t know what hit it!

Analogue depth gauge with red needle showing maximum depth

A very important deep skill on a PADI Advanced course is to compare your depth gauge with your buddy and your instructor. There can easily be huge variations in depth gauges.