A natural thing to do is to add redundancy to your equipment so that you can change to some sort of backup in the case something fails. But it is important that in the case of an equipment failure that backup is in fact available and working because otherwise it gives you a false sense of security and is therefore in fact decreasing your true security instead of increasing it.
An important factor in planing your backup is some idea of on one hand an assessment of the threat to your life or health (or whatever) of a failure of the equipment under consideration and on the other hand the likelihood of a failure. In the best of all worlds you would take precautions for even the smallest inconvenience no matter how unlikely you consider it to happen. But thinking about the idea of taking two spare pairs of fins to every dive because you might need them you realize that this is impossible in a real world scenario.
Therefore it is important to determine the most threatening and likely events and to be well prepared for them. Unfortunately there is no objective way to determine them and this involves some personal judgement of your own because in the end it will be your life that is in danger (but do not forget that you also have a responsibility for the people that dive with you and potentially those who try to rescue you in case you screw up). I am writing this page to explain my personal conclusions and maybe you can benefit by considering them when you decide for yourself what kind of equipment is necessary for the dive you are about to make. Of course, all I say is wrong you should not (blindly) rely on what I or somebody else tells you. Use your brain! (This last sentence is the only advice you should trust!)
The most important part of your equipment is your air supply and in case of a failure you should have a good backup plan. Here we will discuss several possibilities for redundant air supply in increasing order of technical and financial demand and find their individual pros and cons.
There is one air supply with an abundance of breathing gas and that is the atmosphere above the surface. But that might be unreachable because it is to far away or you might miss decompression stops (or an all important slow ascend) or hurt your lungs even from very shallow depths like two or three meters which makes an emergency out of air ascent to the surface a bad choice for your emergency plan.
The next choice of emergency breathing gas is the gas in your dive buddy's tanks. It is a question of general dive philosophy whether you should consider your buddy as part of your emergency equipment or not. You could argue that that is what a buddy is for and that you should only dive with a well trained and alert buddy that in the case that something goes wrong will be there and know what to do to save you. There is another philosophy that says that if things go wrong your buddy might also not be within reach or might not react properly or has equipment that in a stressful situation just fails as well. Thus you should better have 'solo competence' and be prepared to solve all problems on your own even when buddy diving.
Assume that you decide that counting on your buddy is what you go for, there are several equipment choices: You could just go for your buddy's only regulator and start buddy breathing. Everybody who tried it knows that even in the well controlled environment of pool training it is one of the more difficult exercises to buddy breath and make an ascend maintaining controlled buoyancy. At least it requires good training and self control to handle this well in a true emergency situation. As a comment I would like to add is that several training organizations stopped teaching buddy breathing in beginner training because they consider it too difficult and not really helpful under stress.
Things become significantly simpler if your buddy has an alternative air source, i.e. some other regulator you can breath from so you don't have to swap mouth pieces. A cheap and very common choice is an octopus, i.e. an additional second stage connected to your buddy's single first stage. I am writing this text to convince you that considering this as a good backup is wrong and in fact dangerous.
On this page, the kind of diving I am talking about is cold water diving that you do in most northern parts of Europe, that is temperature not much above ten degrees Centigrade and usually bad to very bad visibility. Other conclusions might be reached for the diving you do on your dive holiday in warm clear waters.
The most common failure of air supply during such a dive is due to freeze-up of the regulator. Let us discuss some physics to see why this is the case: Because the density of water is highest at four degrees centigrade it sinks down and some meters from the surface the ambient water will be just a few degrees away from freezing point. Thus most of your equipment (especially tank and regulator) will be cooled to this ambient temperature. A second physical fact is that air cools down when you lower the pressure. This is called the Joule-Thompson effect and is roughly the opposite of the effect that causes gases to warm up if you compress them (things are a little bit more complicated because it depends on how you let the gas expand, ideal gases that freely expand do not cool, but air is not an ideal gas). This effect is real and the refrigerator in your kitchen is based on this effect.
In the first stage of your regulator, the difference of pressures is largest, dropping from above 200 bar in the tank to about twenty bar intermediate pressure. So the air starting from four degrees ambient temperature is decompressed and thus drops temperature. It might end up well below freezing temperature. If now for some reason there is a tiny residual droplet of water in your air or in you regulator it will freeze to ice and might end up in the valve of the regulator thus blocking it and preventing it from working. The result is a frozen-up regulator.
There are several precautions the manufacturer of your regulator can take to prevent this from happening. Therefore some regulators are better for cold water diving than others but, take my word none is 100 percent save. So you should always take this scenario into your considerations.
From the way the freeze up comes about you can learn several things: First of all it is most likely to happen in the first stage since there the drop of pressure is much larger than in the second stage. Second the more gas you suck the stronger the cooling effect and thus the more likely is the freeze up. Third, because it happens when you inhale it is likely that the valve will freeze in the open position.
So if this happens to you, your own octopus is of no help since it is connected to the same first stage that just stopped working properly. Thus your savety equipment is not your own octopus bud your buddy's. Thus everything I say above about relying on your buddy applies, too. But unfortunately, also your buddy's octopus might not be as useful as you thought: Because you brought your first stage into a situation that caused it to freeze up (including ambient temperature, depth, stress, condition of the compressor at the dive store that filled your tank) your buddy's first stage will be in a similar general situation. But now, two people start to take air from the same first stage. Thi smeans that the cooling effect more than doubles because it is very likely a stress situation with heavily increased rate of air consumption. Thus if you manage to freeze-up your own regulator it is not unlikely that both of you manage also to freeze-up your buddies regulator. The two failures are not statistically independent but given that your regulator failed it is very probable that also your buddy's will give up leaving the both of you in a very life threatening situation.
The problem with the octopus configuration is obviously that you rely only on a single first stage even though the first stage is very likely the place where the problem occurs. Thus the obvious solution is to have redundancy at this point. This is why I think it adds a lot to your safety if you have two first stages that can be closed independently even if they are are connected to a single tank. In addition to the possibility to use your buddy's second regulator as you would use an octopus without doubling the load of his first stage you can, in case of free-flow due to freeze-up of your first stage, use your second, independent regulator.
Even in free-flow, you do not loose all of your gas at once. Rather it takes several minutes to empty a cylinder. This gives you enough time either to savely surface or to change to your backup regulator and then reach to your back and close the valve of the frozen regulator. Usually, this should leave you enough air for a save ascend without relying on the availability and skills of your buddy (Here I am assuming that you don't have too much decompression duties. Otherwise you have have completely different equipment requirements anyway). Hence having two independent regulators means a huge boost to your safety and options in case of a failure of your primary regulator.
One could argue that having two valves on one cylinder gives you more o-rings that might burst and cause problems. However, I would judge the probability of an o-ring bursting while diving (as opposed to on land when opening the valve) is much much smaller than the probability of a freeze-up of a first stage and therefore you gain a lot of safety by adding this part of redundant equipment.
Using two independent regulators on a single tank still leaves you with only one one single reservoir of gas. In case you lose it (in a time that does not allow you to reach the surface safely) once again you have to rely on you buddy. One can argue about the likely-hood of this scenario but in any way it is saver to have the independent second regulator on an independent tank. That can be a pony (note that it has be be large enough, otherwise it does not help, see the discussion in the rec.scuba FAQ) or stages or independent doubles. Manifolded doubles do not count as closing the manifold is as difficult as turning of the valve of one of you regulators. Either you think this is possible, then two regulators on one tank are fine or you doubt that this is possible. Then an open manifold is as dangerous. Note however, that all these options require substantially more equipment and that using independent doubles requires the additional task load of changing regulators every now and then to use the air in both cylinders while maintaining the maximal reserve.
Summing up, I think that using two independent regulators is a good idea to increase your safety and it is significantly better than just using an octopus (which might even be dangerous because it gives you a false sense of security). All I am writing here assumes that you are diving in the recreational or sports diver range and do not have to do more than a couple of minutes of mandatory decompression stops. If you are aiming at more technical diving other considerations regarding your equipment apply anyway.
I would be happy to hear your comments. Please send them to Robert Helling.
This was last changed on 18 Jan 2003.