Dive watch

Over the last ten years the popularity of the dive computer has increased substantially, but has this replaced the good old dive watch?

Time is extremely important to the diver; overstaying your planned bottom time can have dire consequences in the form of D.C.I. (De-Compression Illness) or ‘The Bends’ as it used to be known. The dive watch is used in conjunction with a set of dive tables; the tables telling the diver the maximum amount of ‘bottom time’ that can be spent at a certain depth and the dive watch giving the diver the means to time the dive accurately.

The Dive Computer

These come in all shapes, sizes and types depending on what your intended use will be. The main function of the dive computer is to replace the dive tables and because it has an in-built timer many are now diving with no dive watch. The dive computer does so much more than dive tables and is infinitely flexible, whereas dive tables are quite inflexible and should only be used underwater as an emergency measure, hence the old saying ‘Plan the dive and dive the plan’. The dive computer monitors the depth and time extremely accurately, its algorithm converts this into the ‘on gassing’ and ‘off gassing’ of nitrogen during the dive. A rough dive plan is still made but if it is changed during the dive the dive computer will update the information on the N.D.L. (No Decompression Limit) constantly, often giving an extended safe N.D.L. on the vast majority of dives. It also monitors safe ascent speeds, safety stops and in the event of exceeding the N.D.L. it will plan the safest way to surface; advising of any emergency decompression stops that may be required. They are often linked electronically to your gas supply, under normal circumstances this would be ‘breathing air’ but modern dive computers can easily handle dives using enriched air (Nitrox) or the more technical mixed gas dives.

The Dive Watch

Not so many years ago it was often said that there were ‘old divers and bold divers, but no old, bold divers’ as any mistakes in the planning and subsequent execution of a dive could and often did result in tragedy. The dive watch was therefore an extremely important piece of equipment and its reliability paramount. The conditions in which a dive watch must perform are severe; constantly under pressure, being knocked about and often in extremely cold water. Through all this the diver’s life is reliant on its ability to keep accurate time. Rolex sum it up with their iconic ‘Submariner’, advertising it as a ‘self-winding, diver proof, time recorder’.

What Does the Diver Require of a Watch?

• Itobviously needs to be able to function under water, withstanding not just static but dynamic pressure (when moving).
• For the no-decompression diving limit of 40m the timepiece should be rated at a minimum static pressure of 20 bar (200m).
• It has to have a second hand; this is not for precision timing but to give a visual reference that the timepiece is still running.
• An elapsed time bezel that can be set to the minute hand at the start of the descent; it is extremely important that it rotates counter-clockwise only so that if it is knocked it can only reduce your bottom time (the time from the start of your descent to the start of your continuous ascent to a safety stop or the surface) thus increasing the safety of the dive.
• It should have a robust strap that can be extended to go over a ‘wetsuit’ or ‘drysuit’.
• It must be easily legible in all conditions including complete darkness with no torch.

Quartz or Automatic?

Generally, the deep dive watches are automatics. This is because the water temperature drops with depth and an automatic movement is unaffected by these lower temperatures, indeed, they will function quite normally in extreme polar temperatures. Quartz movements are powered by a battery and batteries don’t like cold temperatures. Often quartz dive watches incorporate various visual safety features to reduce the possibility of a complete failure on a dive.

Helium Escape Valves

You will see many deep dive watches sporting a ‘helium escape valve’. This is to eliminate the risk of the watch exploding due to the build-up of helium gas inside the watch unable to escape quickly enough during decompression; the Rolex ‘Seadweller 4000’ is equipped with such a valve. So, you will be wanting to know where the helium comes from and how it finds its way into the watch. This is nothing whatsoever to do with the diving itself or the water; all this comes into play when saturation diving only. Roughly speaking, breathing air  consists of 79% Nitrogen and 21% Oxygen, the oxygen is used by the body and the nitrogen builds-up in the tissues over time to saturation level. As the depth increases (normally anything over 60 metres) the oxygen becomes toxic due to the increase in pressure (Dalton’s law of partial pressures explains this), therefore, the deeper the dive the percentage of oxygen in the mix needs to be reduced to keep the partial pressure of oxygen (PPO2) within the safe limit; this is called a hypoxic mix. At depth the nitrogen also causes problems in the form of nitrogen narcosis, producing a narcotic effect that affects the ability of the diver to think rationally. To eliminate this we replace the nitrogen with helium, this mix is called ‘Heliox’. Another bonus of using helium is that it is less dense, making it easier to breathe at greater depth; deeper dives use mixtures like ‘Hydreliox’, a mixture of hydrogen, helium and oxygen and ‘hydrox’, a mixture of hydrogen and oxygen for even deeper. As it takes many hours for a diver to decompress, the divers are kept in a state of tissue saturation for the duration of the job, this can be many days or even weeks; hence the term ‘saturation diving’.

This involves the diver breathing this mixture and the pressure maintained in the sealed diving bell and then transferred via a pressure lock into the pressure chamber on the dive vessel. This is maintained for long periods until the work is finished and the diver can decompress over an extended period; this description being vastly oversimplified for the purpose of this explanation. It is the time in the diving bell and the pressure chamber where the helium gets into the watch via the seals. Why? Helium is the smallest atom and finds its way past the seals, although some watch manufacturers say that their seals do not allow this. Now, someone is going to say that a hydrogen atom is the smallest as it’s atomic mass is 1 and a helium atom has an atomic mass of 4; this is correct, but the physical size, or to be more specific, the atomic radius of the helium atom is smaller. On decompression the helium inside the watch cannot escape as quickly, this results in a pressure imbalance between the inside of the watch and the inside of the decompression chamber; the valve allows the helium to escape quickly so that the watch does not suffer damage, without the valve it is possible for the watch’s crystal to blow out as this is the weakest part. Diving watches are designed to withstand vast external pressures not internal pressures.

So, Is a Helium Escape Valve Really Necessary?

The answer to that question is ‘probably not’, if only for the fact that the watch isn’t really necessary. The watch is far more important in scuba diving than in saturation diving. In saturation diving the divers will have a team looking after absolutely everything to do with their well-being; the divers have virtually no control over their own safety with anything regarding time. Scuba divers on the other hand are very much self-sufficient and look after their own relatively simple schedule.

The Sensible Solution

For normal no-decompression diving down to 40m I always plan the dive on tables, wear a dive watch and set the time elapsed bezel at the start of the descent. I then use the computer to monitor the dive taking advantage of any extra bottom time available and the additional dive time at a shallower depth. As a diver you are always looking at your computer and gauge console every minute or so and should the computer suddenly stop working you are in a strong position to control the situation safely using your watch and depth gauge; there is a set safety procedure for this. It’s always good to have a back-up plan and although I would go diving without my dive computer I certainly wouldn’t go without my dive watch.

Happy bubbling!