Printer jam

I teach both SDI and PADI dive courses, and – as any small business owner does – I am occasionally required to print out some forms, student exam answer sheets, or other supplementary material. Mini cat’s fascination with the printer, and great love of interfering with the pages as they emerge, can make things tricky.

Mini cat jams the printer
Mini cat jams the printer

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)
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
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.

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 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.

  1. 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.
  2. 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

Safety stop on Tafelberg Reef
Safety stop on Tafelberg Reef

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.

Chamber dive revisited

Sealing the inner chamber door
Sealing the inner chamber door

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.

Our hyperbaric chamber
Our hyperbaric chamber

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!


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
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
My Mares Nemo Wide showing the dive stats
Citizen dive computer
Citizen dive computer

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
My trusty Suunto Mosquito showing the dive stats
The Aladdin that Clare usually wears
The Aladdin that Clare usually wears

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
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.

Goot checks his computer
Goot checks his computer

Bookshelf: Diving science and physiology books

Sound boring? Doesn’t have to be. While your Divemaster instruction manual might make this stuff sound very dry, learning about what happens to the human body while breathing gas under pressure doesn’t have to be. Find some inspiration from this reading list when your theory gets too boring, or remind yourself of what you’re supposed to know already (if you’re a bit rusty).

For beginning divers:

Recommended reading for Divemaster candidates:

Scuba accidents and how to avoid them: