MacDive – a digital dive logbook for Mac OS X

It was frustrating to discover, when I bought my Suunto D6 in 2011, that a Mac-compatible version of the native Suunto software did not exist. This has changed in the intervening years, but I don’t care – my search for software that I could download my dives into led me to MacDive, and I’ve been using it ever since. Rather than tying me to a single brand of dive computer, MacDive handles almost any dive computer you can think of. The software currently costs $25, which is approximately one million South African Rands at current exchange rates, but it is excellent value and updates frequently as support for new devices is added.

Screen shot of MacDive in action
Screen shot of MacDive in action

Here’s a screen shot of MacDive, with the ten dives we did in Malta in 2011 selected. The dive profile on the screen is from one of the two incredible dives we had on the wreck of the Um El FaroudThe summary information at the bottom of the screen hides a lot more detail that can be stored about each dive, including gear configuration, dive operator, the name of the boat (if used) and Divemaster, and so on.

MacDive also calculates summary statistics for the dive sites that you load, and allows you to interrogate your dives by country, by date, by the computer used, and even by diver. The software would allow you to share the logbook with another user (or users), and if you look carefully you’ll see Tony’s name on the screen. That’s because I downloaded the profile of his dive at Doodles on that fateful day when my Suunto D6 fell through the bottom of the ocean. When I did that, I specified that he was the diver, and not me.

Dive site summaries in MacDive
Dive site summaries in MacDive

MacDive is full-featured software that not only allows you to record a lot of detail about each dive, but to add photos if you wish. The software matches the timestamp on the photo to the dive profile, enabling one to have fairly detailed information about depth and temperature when an image was taken (assuming your dive computer and camera have SYNCHRONISED WATCHES). I have exported the data out as a .csv file, which is excellent if you want to perform some analysis of trends in your air consumption, for example, or draw pretty graphs.

MacDive is compatible with a wide range of dive computers – I’ve used it with a Suunto D6, a Suunto ZOOP, and two different Mares Nemo Wide computers – which makes it ideal for a multi-computer family where the backup dive computer may be of a different make to the primary one. A full list of the supported devices can be found here and if your device is not supported, the developers of the software are open to adding it if you submit a request. In most cases the installation of a USB driver is required to make the dive computer talk to the software, but this is a once-off requirement and hardly onerous.

The forums and FAQ have been very helpful when I have had difficulty setting up different makes of dive computer, and the entire user manual is online in wiki format. With reference to the manual I have renumbered my dives to maintain the correct sequence, and merged multiple dives that were interrupted by an interval in water shallower than 1.2 metres that was long enough that my computer decided the dive was over. The database format is very flexible and allows one to keep the sequence of dives looking as neat as they would in a paper logbook.

I have been using MacDive for four and a half years, and have been very happy with it, particularly the fact that it isn’t tied to any single type of dive computer. If you’re looking for an electronic logbook and running the Apple OS, check it out.

Suunto D6 in full panic mode (part II)

Yesterday I told you about a dive on Doodles, a reef in southern Mozambique, during our trip to Ponta do Ouro last month. Doodles has a maximum depth of about 14 metres. After forty five minutes’ dive time, my Suunto D6 began to register extreme depths (89 metres maximum), and to give various instructions about decompression ceilings and times, accompanied by strident warnings about exceeding my PPO2.

Suunto D6 after missed decompression
Suunto D6 after missed decompression

While annoying and potentially dangerous to lose the services of my dive computer mid-dive, it was also an excellent learning opportunity. Because I usually try quite hard to be safe and not to upset my computer, and dive within the conservative, recreational limits that I am trained for, I never get to see any of this behaviour from the instrument. (Fortunately the dive was shallow and I still had plenty of no-decompression time left so it was far from an emergency situation.)

After the dive, I soaked the computer in warm fresh water, and it gradually came out of dive mode over a period of about ninety minutes. During the course of this simulated ascent, the required decompression times and depths calculated by the algorithm were not adhered to, so the computer entered an error mode, which, according to the manual, indicates that “the risk of DCI has greatly increased.” (In fact, from all the beeping and flashing, I suspect the computer thought I was dead or close to it.) This error mode does two things: it disables the dive planning capabilities of the computer, and it locks you out of dive mode for 48 hours.

The D6 in gauge mode
The D6 in gauge mode

I had never gotten the computer into this state before, so I was keen to see how it behaved when I took it on a dive in error mode. You can see in the photo above that I am wearing Tony’s Mares Nemo Wide (aka the flatscreen TV) to give me actual information about my no-decompression time, depth and dive time, but I took my D6 along for the ride. It is in gauge mode; this means it gives you only measurements, and is the setting a free diver might use.

The measurements available in gauge mode are: depth (18.4 metres in the photo above), the maximum depth you’ve been to on this dive (19.9 metres), an elapsed dive time (17 minutes), and water temperature (not shown) but it refuses to calculate a no-decompression limit for you. This would usually appear where the Er appears in the picture above.

Depth profile (with warnings)
Depth profile (with warnings)

For your enjoyment, here’s another screen shot of the dive profile from MacDive, with the warnings expanded. Click on the image to see it full size. It is clear that the first warning beeps I heard during the dive were because of elevated PPO2 levels. At 89 metres the device immediately put me in deco, and then as it “ascended” fairly rapidly, it gave a warning about oxygen toxicity (OLF or oxygen limit fraction as used in the Suunto algorithm) and an ascent rate warning. On the right, at about 10 metres, a warning is given that the depth is still below the required level to complete the decompression.

All the green circular icons appearing around the middle of my dive, where the computer thought I was at 35 metres, indicate that the computer registered that I surfaced, but not for long enough to show on the dive profile. Weird!

My D6 remained angry for 48 hours after the dive at Doodles; by this time, we had finished our diving for the week. I’m not sure whether the problem with the pressure sensor is a permanent one (requiring repairs, a service or a new dive computer), or whether it was just dirty or stuck and will have resolved itself next time I dive with the instrument. I’ll be wearing a spare dive computer when I do, just in case.

Suunto D6 in full panic mode (part I)

On the second dive we did at Doodles during our trip to Ponta do Ouro, something happened that caused my dive profile to look like this (click on the image to embiggen):

Profile (from MacDive) of a dive at Doodles
Profile (from MacDive) of a dive at Doodles

Fortunately that something was not me falling down an 89 metre deep hole in the ocean floor whilst breathing off a 12 litre cylinder of air, 45 minutes into a dive. It was something going wrong with (we think) the pressure sensor of my Suunto D6 dive computer. I have had the computer for a few years, and apart from the compass appearing to have packed up, it has been a fantastic device.

The first inkling of trouble that I had was when the computer started beeping at me, and when I looked down to see what was up, it said we were at 89 metres. Sunlight was falling on me, so I thought this unlikely. I checked with Christo, and he didn’t think we were that deep either. Nor did his computer. Fortunately I had been diving with a group for a few days, and we were well within our decompression limits at a depth of 14-15 metres. Losing the computer three quarters of the way through a dive wasn’t the disaster it could have been.

I took this picture shortly after the beeping started. The display at the top of the screen shows that the computer is reading 76.7 metres. The maximum depth of 89.0 metres shows at the bottom left. The bottom right shows the oxygen partial pressure, at 1.9. This is greater than the 1.2 limit that I have set for myself inside the computer. The computer thinking that I had rapidly exceeded the maximum PPO2 is probably the initial cause of the beeping I heard.

Suunto D6 showing 76.7 metres' depth
Suunto D6 showing 76.7 metres’ depth

In the middle of the screen you can see a large number 8.0, which is the depth to which the computer wants me to ascend in order to do a decompression stop. Next to that number is an arrow below two horizontal lines, and the number 35. This is how long I should stop at 8 metres in order to fulfil the calculated decompression obligation.

The D6 thinks we have ascended to 47 metres now
The D6 thinks we have ascended to 47 metres now

After a few minutes (I was actually busy ending my dive with a 5 metre safety stop) the computer believed I had ascended to 47.3 metres. By that stage it wanted me to go to 11 metres for a 45 minute stop. When I surfaced after about 60 minutes of dive time, the computer stayed in dive mode, thinking that I was still at a depth of about 35 metres. It continued registering a slow, quite smooth ascent for another hour and a half, as seen in the dive profile above.

The computer was still very grumpy when it took itself out of dive mode in a mug of warm water in the bathroom of Planet Scuba, while we were eating a post-dive lunch. The ascent profile it registered did not fulfil the calculated decompression obligations, and it showed that I had violated my decompression ceiling (either by taking too long to ascend to start the stops, or by not spending enough time at the stop depth).

I have put myself briefly into deco a few times (usually during days of repetitive diving during dive trips, and once or twice in Cape Town with two relatively deep and long dives in a day), so the readouts from the D6 on this dive are not unfamiliar to me. I have never before, however, seen such large numbers or heard so much beeping! I have always corrected the situation by ascending a little way, which removed the decompression obligation and returned the display to what I am familiar with as a religiously recreational, no-decompression diver. In this case, the D6 was doing its own thing and nothing I did seemed to influence the depth reading it gave.

I found this to be an excellent opportunity to re-familiarise myself with the behaviour of the Suunto D6, and also to see what all the error messages look like on the dive profile when I downloaded the computer into the MacDive software I use for electronic dive logging. The story does not end here – if the computer did not think I was dead from DCI, it wasn’t going to let me dive any time soon – check back tomorrow (or the day after).

Long term test of the Suunto D6

I’ve had my Suunto D6 dive computer for three years, and while wearing it I’ve done 150 dives for a total of 90 hours underwater.  I’ve also been a remarkably good girl, diving within my qualifications (PADI Deep Specialty) for a maximum depth of 38.6 metres.

Slightly aged Suunto D6
Slightly aged Suunto D6

I discovered that my Suunto D6 needed a new battery while waiting at the airport to board a flight to Durban, on the way to Sodwana for three days of warm water diving. Tony had suggested more than once that I check it prior to our departure, but NO, I was too busy and important to do THAT! Fortunately a spare dive computer was on hand, so I dived with one of Tony’s Suunto Zoops on the trip.

(As an aside, the D6 won’t download dives from the computer into the MacDive software when it has low battery. It goes into data transfer mode when you plug it in, but refuses to download – I got error code -7. Once the battery is changed, downloading is again possible.)

Changing the battery on a dive computer can be a fearsome experience – at least in South Africa. Unless you can change it yourself (like the Mares Nemo Wide), it often entails sending the computer to Johannesburg to the agent who imports that particular brand. Then, in a kind of awful lottery, you wait for one of three possible outcomes, all seemingly equally likely:

  • after the agent acknowledges the safe arrival of your dive computer, a few weeks pass and the agent swears blind he hasn’t seen one of that make for the last five years (this happened to Tony in January)
  • the battery is changed, the computer comes back, the first time you use it it floods, and you have no recourse to anyone
  • the battery is changed and the computer works just fine (phew!)

The odds are never in your favour.

Fortunately Duncan at Orca Industries in Claremont is able to change Suunto D6 (and other) batteries. I think he’s the only person in Cape Town who can do the Suunto D series. (It’s a ten minute process, but mine took five days because the computer ended up under a shelf somewhere for four and a half days before Duncan was told about it!)

Overall I am extremely pleased with my D6. I have minor quibbles. These are chiefly related to the intricacies of doing an air dive after a Nitrox dive, and the fact that it weighs as much as a fully grown labrador retriever, so isn’t suitable for most ladies to wear as a watch. I also don’t trust the compass very much, for no particular reason other than it’s digital. It’s great to dive with, though, and I wouldn’t hesitate to recommend it to others.

Underwater selfies

An underwater selfie from Tony
An underwater selfie from Tony

Tony got a new 5 millimetre Mares Pioneer wetsuit to replace his Flexa 8-6-5, which was getting a bit long in the tooth. When I was clearing photos off my old underwater camera, I found this little photo shoot, in which he appears to have been testing his gear in our swimming pool… Under the pool cover.

The view from underneath our pool cover
The view from underneath our pool cover


The water temperature was a sweet 25 degrees, according to the Mares Nemo Wide.

Dive computer at the bottom of our pool
Dive computer at the bottom of our pool

Article: Wired on biometric monitoring of navy divers published an article in their Danger Room section, all about defence, concerning proposed DARPA research on biometric monitoring of navy divers. DARPA, the United States Defense Advanced Research Projects Agency, is the organisation that commissions advanced (and sometimes wildly unrealistic… until it’s realised) research for the US Department of Defense.

One of their latest research proposals is to come up with some kind of device that could monitor a diver’s vital signs while he’s in the water. One of the vital signs that the device should be able to read is the presence of nitric oxide, which works to prevent the bends (decompression sickness: the presence of excessive nitrogen in the blood after breathing compressed air at depth). Nitric oxide would then be added to the diver’s body if the levels of this gas fell too low. There’s a lot more to it…

(As an aside, it would be wonderful if modern dive computers could do this, but they can’t.)

The brief for the required research reads like an excerpt from the script of a James Bond movie. I actually laughed out loud reading the article, but that’s the reason I’m not doing cutting edge research: lack of imagination. It’ll seem ridiculous until someone actually invents it, and you can be sure that it’ll be used in military applications before us civilians get our grubby paws on it.

Read the article here.

Tips on shopping for dive gear

I’ve been diving for a while, owned a lot of dive gear. Here are some tips on shopping for gear, some learned through painful experience!

General rules for buying gear

  • Try it on before you buy it. Wetsuits, booties, hoodie, you name it.
  • Try on your BCD and weight belt OVER your wetsuit – two layers of 5 millimetre neoprene adds a LOT of waistline!
  • Make sure you understand the returns policy of the shop you’re using.
  • Get acquainted with the Consumer Protection Act (if you’re in South Africa).
  • Shop around! Don’t let sales people sweet talk you. They are more interested (generally) in making a sale than in making you a happy diver.
  • Don’t cut the strap of your dive computer shorter unless you’re VERY sure you’re never going to dive in cold water (wearing lots of wetsuit and gloves to make your wrist thicker).

Second hand gear

  • When purchasing second hand cylinders: get them viz’d first (at the expense of the seller) before agreeing to purchase.
  • Try and get the seller to allow you to “test dive” expensive items such as dive computers before agreeing to purchase them.
  • It’s a good idea to check BCDs for leaks before purchasing, unless you plan to use the BCD only for shallow dives, and even then it’s iffy.

Gear to avoid

  • Don’t purchase based purely on colour (ladies, I know it can be very tempting).
  • Be realistic about what you will use the gear for. (Do you really plan to dive to 100 metres, under ice with that regulator?)
  • Don’t fall for wrap around face masks with 3 glass panels (here’s an example) without trying one first – they give rise to very confusing visual phenomena and distort things hugely as they pass across the join in the panes of glass!
  • Avoid BCDs with inflate/deflate handle handles (example here) – I have never yet seen a beginner diver (and even some divers who have done over 100 dives) using one who was in proper control of their buoyancy.
  • Neoprene covers on mask straps (example here) usually only work without a hoodie. They have a tendency to slip off your head during a backward roll off the boat when worn over a hoodie (although some people swear by them!).
  • Smaller volume masks are usually better for beginner divers than huge five litre models! They are much easier to clear.
  • Do you really need a three foot dive cutlass, as opposed to a small knife?


  • Get a second opinion on extensive repairs.

Dive gear maintenance: Wrist mount gauges

I have a range of different wrist mount gauges as I teach a range of courses. Whether it’s a compass, dive computer, or timing device the same rules apply when it comes to maintenance.

They must all be soaked in warm fresh water after a dive.


The bevel on the compass must be rotated several times in both directions whilst in the warm water. This prevents salt from building up underneath it.

Dive computers

Dive computers must have the buttons depressed several times while swirling the instrument in the warm water. This will ensure no salt crystals can build up behind the buttons. I have a Suunto Mosquito that has done over 1000 dives and has never given me a day’s trouble whereas many people have had button trouble with these units. If you have delicate water contacts brushing them with a soft toothbrush keeps them clear of build up.

I also take a soft cloth and wipe silicone over everything, pour it in behind the buttons and then wipe it all off with a clean cloth.

Dive computer in action
Dive computer in action

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.