Defensive Diving – Part 1

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Defensive Diving – Part 1

Vehicle-drivers survive on the roads by adopting techniques that take all contingencies into account – scuba-divers should do the same, says SIMON PRIDMORE

1218 technique screen is dark

What happens when you look at your computer on a dive and the screen is dark?

WHEN YOU LEARN O SCUBA-DIVE, you are given a little knowledge and taught some basic skills. You take a theory test and demonstrate that you can perform the skills and that’s it – you get a licence to dive, to rent equipment, to buy air-fills and to join dive-trips.

Subsequently, as you do more diving, you improve your skills and you experience various problems. By achieving mastery of the skills and dealing with the issues you encounter, you acquire the ability to anticipate problems and avoid or manage dangerous situations.

This, as you might recognise, is similar to the process of learning to drive a car.

In diving as in driving, however, what guarantee is there that experience will teach you everything you need to know?

You regularly encounter drivers on the road and fellow-divers on the dive-boat who, despite being quite experienced, seem to have poor skills and inadequate knowledge, to the extent that they present a potential danger to themselves and others.

You too might secretly be aware that your own skills are not as finely tuned as they could be. Also, experience is often counter-balanced by overconfidence and complacency.

In the motoring world, to develop safer and more skilled drivers, there are training courses in something they call defensive driving. You can choose to take a defensive driving course, or you can be required to take one as part of a traffic-court sentence.

The concept was first introduced in the USA in the 1960s. The Safe Practices for Motor Vehicle Operations manual defines defensive driving as “driving to save lives, time, and money, in spite of the conditions around you and the actions of others”.

Among other things, drivers learn how to anticipate and assess dangerous situations and make well-informed decisions. As well as being shown how to drive sensibly and safely, they are taught useful things such as how to use less fuel and save on vehicle wear and tear.

They are also given guidelines on being courteous to other road-users.

In scuba-diving, the closest we come to a defensive driving course is Rescue Diver or the equivalent, but this is usually more about emergency responses than personal skills and awareness development.

Some aspects of Divemaster training match the concept of defensive diving, but this is a professional course. Not many divers will actually experience this.

In this short series of articles, I describe strategies that I see as intrinsic to the defensive-diving concept.

I should make it clear that when I use the word defensive in a diving context,

I am copying the example of the motoring world, where defensive means safe, careful, conservative and thoughtful.

In other contexts the word “defensive” can mean negative, fearful or resistant to change. These are certainly not traits that help a scuba-diver in any way.

Appeared in DIVER December 2018


Your dive computer is a useful tool but it is a battery-operated electronic device with only one or two O-rings protecting its complex electronics from high-pressure water that is always trying to get in and fry them. Your computer can fail completely at any time.

At some point, every dive-computer dies while it is being used, and Murphy’s Law of Scuba Diving dictates that yours will fail when you need it the most.

This should give you ample incentive to keep looking at your computer regularly during every dive. If you do this, and one day experience the sinking feeling of glancing at it and seeing that the screen has gone completely dark, you will still be able to remember your depth and time from when you last looked at it a couple of minutes earlier.

This will give you confidence, and help you manage a safe ascent.

You do not want to suddenly notice that your computer has gone blank when you have not paid any attention to it since the beginning of the dive.

In that situation you don’t know how deep you are, how deep you have been, nor how long you have been down. All you can do is guess at your decompression status as you make your ascent.

Or your computer might only partially fail and give you incorrect data, which can actually be worse. Here are two short stories that show how dive computers can go wrong in ways you might not have imagined.


Sandra’s computer seemed to be working fine during a long dive on a deep reef wall, until she ascended at the end of the dive.

She noticed that the depth it was showing seemed to be shallower than she thought she was. Sure enough, as she went up, the depth counted down to zero, and the computer switched to surface mode.

As far as the instrument was concerned, the dive was over. However, Sandra was still several metres under water.

Luckily, she had a team-mate close by who had done more or less the same dive. She swam over, asked to see his computer and saw that it was reading 6m. Sandra thought fast and concluded that it was likely that the computer had been reading 6m shallow throughout the dive. Therefore she had no idea if she had gone into deco or not.

Her team-mate had not accumulated any required decompression, which gave her some reassurance.

Just to be certain, she did an extended safety stop and then went up.

She assumed that the computer problem might be related to a low battery, so she changed the battery out and dived with it again, this time taking along a back-up computer for insurance.

The same thing happened again. She checked that the pinhole leading to the depth transducer was not blocked, and continued to dive with both computers over the next few days.

After about a dozen dives, the malfunctioning computer went back to normal. She still has no idea what happened.


Burt was a divemaster in Guam and had been diving with his group on a shipwreck. As he ascended, he passed

a diver from another group, who was hanging on the shotline.

The diver signalled that he needed help and showed Burt his computer, which was telling him that he had an hour of decompression to complete. The diver’s gauge showed that he didn’t have enough air for anything like another hour.

Burt noted the dive-time elapsed on the diver’s computer and took out his back-up decompression tables. He assumed the diver was on a repetitive dive, and looked at the required stops for a dive at the maximum depth of the site for the total time the diver had been in the water so far.

Even within these parameters, the table required far less decompression than the diver’s computer was showing.

Burt wrote: “Do This, Don’t Worry” on a slate and then listed the decompression stops and times given by the table. He watched the diver until he had safely ascended and was back on the boat.

When the diver later sought him out to thank him, Burt refrained from giving him a hard time about going into decompression when he evidently didn’t really understand what he was doing.

Instead, he diplomatically explained that many dive-computers are not designed for dives with decompression stops and often penalise divers unnecessarily heavily if they go into deco, particularly on a second or third dive of the day.


1218 technique eye on your computer
Keep an eye on your computer.

How do you know when your computer is giving you the wrong information? Studying different dive-tables will help you identify the sort of no-decompression limits you should be seeing at various depths, and give you an idea of usual decompression-stop patterns, should you stray into deco.

If you upgrade from one computer to another, consider keeping the old one and continuing diving with it in a zippered pocket as a back-up, in case your nice shiny new one starts misbehaving and you need a second opinion.

Finally, if you’re going to do planned decompression diving, buy a computer that is specifically built for the purpose.

In next month’s DIVER, I will outline a few more defensive diving strategies.

This article is adapted from Simon Pridmore’s new book:

Scuba Exceptional – Become the Best Diver You Can Be

It’s now available in both paperback and ebook versions via Amazon and other online bookstores worldwide.


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