Actually, a cave diver has access to six lights, the diver's own plus his partner's. Three, though, is the ideal number, one primary and two back-ups.

Why not one light? Because if that light fails then the diver is in the dark, with only his partner's light for illumination. If that fails then both divers are in trouble. If the diver was on his own then the situation is even worse. Many recreational divers die this way in caves. As the NACD brochures say: "the last thing you will ever see is the filament of your bulb dying".

Why not two lights each? If a light fails then you have a back-up to use plus your partner's light, so what is the point of taking any more? For a start, all lights have a very high risk of burning out when switched on, clips can break and lights get lost, they can flood, etc. One back-up is just not enough.

Three lights means that if your primary goes out then you have a back-up. If the back-up fails then you still have one more. The chance of a back-up having a problem is around 1 in 100, so there is a good chance it will happen. The chance of two back-ups not working is 1 in 10000, a huge jump, which is the reason two back-ups are carried. The odds move in your favour with three lights, but not with just two.

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Why not four lights (three back-ups)?

For the primary and two back-ups to fail is 1 in 1000000. The chance you will need it is so remote that it probably will never happen, so you need to find somewhere to stow a piece of equipment that is totally useless. Three back-ups is overkill. The odds of a dive team having all six lights fail is a staggering 1,000,000,000,000 to 1. The back-ups alone have a combined burn time of at least 12 hours.
 

Sharing air in an overhead environment can only be done safely with a long hose regulator.

If your partner is out of gas, he needs air now and the chances are he will grab for the first regulator he sees, the one in your mouth. If he does, and he will, then he gets the long hose straight away.

On a mixed gas dive, the divers may be carrying gases which would be toxic to breathe on the bottom. It is essential the diver with the problem gets the right gas for the right depth and the only way to be sure is for him to get the regulator which you are breathing from. 
In zero visibility when you are in touch contact only, the only way to find the right regulator is to take the one from the mouth.

The regulator which you are breathing from is the only one which you know is functioning. All the others could be full of sand and grit, the valve may have rolled shut, anything could have happened. The only one you are sure of is the one in your mouth. Do you want to give a panicked diver a regulator which might not work?

If the hose is stuffed in bungee against the side of the tank then what do you do if it works free? You cannot replace it yourself. If it is grabbed by your partner then the kinks on the hose can tangle and get trapped in the bungee. Wrapping it is the only sensible option.

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...I'LL GET STRANGLED IF SOMEONE GRABS THE LONG HOSE!
Not if you wrap it the right way. It will uncoil instantly.

...THE LEFT VALVE CAN ROLL SHUT AND MY BACK-UP WOULD BE SWITCHED OFF AND I WOULD NOT KNOW UNTIL I SWITCHED AND CHOKED AND DROWNED!
Better you deal with it than your partner. When a valve knob rolls closed it often breaks off. If the long hose is on the left and it rolls closed (and breaks) then you have nothing to share with if you need to. You can breathe off the wing which is why the wing feeds from the right post.

...THE LONG HOSE IS HARD TO BREATHE FROM, WHY SHOULD I BREATHE FROM IT?
The long hose is not hard to breathe from. Very long (10m+) hoses can develop resistance in flow, a 2m hose will not. Anyway, if this was true then would you be happy handing the worst performing regulator to an out of air partner? It does not make sense. Use more helium if you are worried about gas flow.

...IT GETS IN THE WAY WHEN I AM BREATHING FROM A STAGE BOTTLE!
Try it, it does not. When not in use it is clipped out the way to the shoulder d-ring. There are thousands, maybe hundreds of thousands of stage dives where the divers use the long hose.

...MY STAGE BOTTLES GET IN THE WAY!
If you wear a stage on the right then it will get in the way. Unless you use steel stages, then there is no need to have anything on the right, all stages are worn on the left. Accept the whole system, it does not work with just bits and pieces.

...MY BACK-UP FREEFLOWS WHEN I SWIM!
Use a low performance second stage, like a TX20 or R190. Assuming that you never dive deep on air, then breathing from it will never be a problem.
 

With a stage on each side, when you swim they will flap outwards as you go. You are increasing your profile and causing drag. The right stage will stop the long hose from being fully deployed and the light cannister will get in the way of it too. It is difficult to route the regulator hose from the right stage too.

By putting stages on the left side only (all of them) reduces drag and makes you more streamlined. The long hose is clear to uncoil and the cannister is reachable. As the battery forms part of your weight system, having a stage on the right means it cannot be ditched. All the hoses can be run from the left, behind the neck and round to the right, keeping them streamlined. If you use a scooter then they are out of the way because you are steering it with your right.

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...I'LL GET PULLED OFF SIDE!
With steel stages you will. Use aluminium and you will never even notice them.

...I CANNOT STAND UP WITH THEM ON!
Do not stand up! Moving on a boat can be a problem, wear a stage on each side until you enter the water, then swap it over a metre or two below the surface before you start the descent.

...I'LL MIX THEM UP! I KEEP MY OXYGEN ON THE LEFT AND MY 50/50 ON THE RIGHT!
What happens if one day you put them on wrong? It is an easy slip to make, this is just not a workable way of identifying gases.

Each bottle is marked. Check your depth, identify the bottle, take its regulator and put it in your mouth, check the bottle and depth again and switch the valve on. If you can breathe then you have the right regulator.
 

A traditional diving light combines the battery andlamp in one unit. The trouble with that is total unit must be very big if the unit is to be bright and burn for a long enough time for a dive. Holding this in one hand is not the easiest.

In the 1960's, American cave divers started to make lights where the battery was worn on the belt and a cable led to a remote light head. This meant that batteries could power the light for a very long time, but the lamp which the diver had to hold could be small and light. "Goodman" handles enabled the light head to be worn on the back of the hand, keeping the fingers free for work. They are now standard equipment for cave divers and for open-water technical divers they are becoming more common.

A cave diving light needs to be long lasting. Next to air, light is the most valuable resource underground. Extreme Exposure and AUL make very good quality cannister lights which will power a 50watt light for four hours. WKPP divers use high intensity discharge (HID) lights which can double the battery life.

Lead acid batteries are reliable and long lasting, and they recharge fairly quickly without having a memory i.e. they do not need to be fully discharged before recharging. Their output drops slowly during use, and on long dives the light may go yellow, indicating the battery is low. 

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I DON'T NEED A LIGHT THAT BURNS FOR FOUR HOURS FOR A WRECK DIVE!
There are small cannister lights available which burn for shorter periods of time, and cost little more than a large traditional dive light.

WHY NOT USE NiCad BATTERIES WHICH DO NOT GO DIM?
They have a memory, i.e. they need to be fully discharged before they can be charged. They also take forever to charge for even the smallest batteries.

Going dim is a warning that light failure is close. NiCads do not fade, they just go out dead with no warning. This could be a problem on cave, wreck or night dives.
 

Having just one wing may seem like a big risk. But if the equipment is balanced and weighted properly then the drysuit should provide adequate flotation and buoyancy control. In open water diving the diver will probably have a lift bag or at the very least a "sausage" type marker. Even the small sausage markers still have two or three kilogrammes of lift, which combind with a drysuit should be plenty. At the end of the dive, stage bottles can be dropped or clipped to the ascent line, or the battery pack can be ditched too.

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... WHAT IF I AM WEARING A WETSUIT?
Do not use steel cylinders. Only use steel back cylinders with a drysuit. Use aluminium. You will need a small weightbelt and even if you dump this you will still be able to maintain adequate buoyancy to do a safe ascent and to float on the surface. Steel twinsets will sink you like a stone if you have no buoyancy control. There have been many deaths attributed to this.

... WHY NOT JUST WEAR TWO WINGS?
They cause drag and are not needed, even more so with a drysuit. It simply patches and underlying problem, that the diver is not using balanced equipment and correctly weighted.
 

Quick releases made of plastic are very fragile, even if they are metal then they can release by accident. If the harness is just one long piece of webbing with no breaks then it is going to take an awful lot of force to break it. If the harness is adjusted properly then it should not be difficult to get out of.

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... WHAT ABOUT RESCUES? A QUICK RELEASE WILL GET A DIVER OUT OF THE HARNESS QUICKER!
Not necessarily. The quickest way is to grab the shoulder strap, take your shears and slice through it. Even without doing this, it is not that hard as long as the harness is sized properly. Dumping some air out of the victim's wing can help.

I tried an experiment a few years ago on a rescue course. With traditional jacket style BCD's, the rescuers had no problems getting the victims free. I did the same drills using a Dive Rite Deluxe harness. This has quick releases on one shoulder only. Every single rescuer made the same mistake and severely delayed the rescue. They assumed the harness was like the jacket BCD, that it would have quick releases on each shoulder and every one wasted vital time searching for the second release (which was not there). Even divers who had dived with me before and knew the harness forgot as soon as any stress was put on them.

You could add two quick releases but then you are doubling the number of failure points.