As a variation on the independent singles v manifolded-doubles theme, suppose I am using independent singles and Murphys Law strikes I have one tank empty or low, and the other has plenty of air but an unusable reg. Is it a viable option to swap the regulators under water Will a regulator still work after the first stage has had a swig of water through it
Dave McLean

What would make your regulator unusable If it fails, a modern downstream regulator will free-flow, alerting you to the problem well before you need it, in which case you would turn off the offending valve and abort the dive.
     I cannot foresee that a regulator would fail after being under tank-pressure for some time, and just at the moment you intended to use it.
     It sounds as if you plan to open that tank valve just before you need to breathe from it. That sounds foolhardy for a back-mounted gas!
     I would not try to swap regulators from tank to tank, under water and at depth. That sounds like a recipe for disaster. If you have a deco-gas, and no other choice, you would be better off breathing from that supply while ascending.
     Contrary to popular belief, an oxygen hit is rarely instantaneous and you would be better off breathing a deco-mix beyond its recommended operating depth than drowning.
     The well-known diver Rob Palmer made five dives over five days to 120m on ordinary compressed air before he was finally killed (probably by an accumulation of oxygen toxicity) on the last day.

A membrane system for supplying nitrox does not require oxygen-clean tanks for any mix below nitrox 40.

Oxygen-clean tanks
I am waiting to get the twin-seven tanks I ordered O2-clean, so that I can dive nitrox on a regular basis. Does that mean I cant fill them with straight air afterwards, or do I need a personal filter (as someone told me) to do this There are certain companies that top up the O2, and then I believe there is a type of compressor that pumps the mix straight into the tank. Does this make any difference when I decide to have a fill with ordinary air
Divernet forum

You must be sure that the ordinary air compressor used has a double filter system, as for use with nitrox. In the UK, nitrox is still often mixed in the divers tank, using the partial-pressure blending system. This means adding neat O2 to the tank, then topping up with the right amount of air.
     You need air of the quality required for this procedure if you want your tanks to qualify as oxygen-clean.
     Elsewhere in the world, nitrox is supplied by a membrane system, so an oxygen-clean tank is not a requirement for mixes of less than 40% O2.
     I daresay the UK will eventually enter the 21st century!
     By the way, if a compressor is properly maintained, no hydrocarbons should pass through it. The double-filtering is there to reduce the chance of moisture getting across and rotting a tank in the elevated O2 atmosphere from the nitrox stored within it.

Making light work of breathing
Why do people talk about work of breathing when referring to the performance of regulator first stages I thought that that was a feature only of the second stage.
Nigel Hewitt

The first stage of a regulator uses a spring to determine the gas pressure delivered to the intermediate hose. It attempts to reduce the pressure in the cylinder to about 8 or 10 bar above ambient, so is depth-related. An unbalanced first stage also depends on pressure in the tank and will tend to deliver less gas as that pressure drops.
     In practice this is noticeable onlyat depth and a diver is not normally at depth when his tank is at this reduced pressure.
     However, it does determine the amount of air available, and a poorly performing first stage might exhibit a drop in inter-stage pressure on each demanded breath when at depth.
     The second stage matches the pressure of the delivered air to that of the surrounding water, so that in theory no effort should be required to breathe. However, as the valve is mechanical, a certain amount of effort is needed to pull it open.
     This is referred to as the cracking pressure. It can often be increased by manually increasing the tension on the valve spring with an external breathing-resistance adjustment knob.
     Effort will also be needed to push open the exhaust valve when exhaling, which adds to the work of breathing.
     So the work of breathing is derived from the total of all these effects, and a two-stage regulator can be judged only when both parts are working in conjunction with each other.
     The graph provided by an ANSTI regulator-testing machine clearly indicates the overall effects.

A yellow safety flag, as recommended by testers!

Yellow flags fly high
I have read that a Health & Safety Executive report found that one of the simplest and most effective diver location aids was a yellow flag. How true is this
Divernet forum

Youre right. Research by Herriott-Watt university in the Orkneys a few years ago proved that the colour most visible at sea was yellow.
     Taking a large yellow flag on an extending pole with you while diving is a simple low-tech answer to getting seen while waiting at the surface, especially as its light and readily portable, even if travelling overseas.
     Personal locator beacons and other devices that rely on battery power are a sophisticated alternative but a flag on a pole is virtually sure to work when you need it and the pole allows you to position the flag higher than the swell.
     A recent trip to the Red Sea indicated that a high percentage of divers now carry these simple low-cost solutions to getting separated from boat cover.

Rebreather scrubber material gets wet because water plays such an important part in the filtration process

Why is scrubber material wet
Why does the used scrubber material of my rebreather always seem to be wet
Divernet forum

The primary constituents of soda lime are calcium hydroxide (70-80%), water (16-20%), sodium hydroxide (1-2%), and potassium hydroxide (0-1%).
     Water plays an important part in the reaction that takes place to bind the carbon dioxide. The gaseous CO2 reacts with water to form carbonic acid. Then the sodium hydroxide reacts with this carbonic acid to produce sodium bicarbonate and water. The sodium bicarbonate reacts with the calcium hydroxide, which has been disassociated into calcium and hydroxide ions to produce calcium carbonate, or limestone.
     The CO2 is now in a relatively stable state. There is a net production of three water molecules for every molecule of CO2 taken in.

Computer settings in your hands
I notice that many new computers allow you to choose an algorithm or a micro-bubble suppression setting. How do you know which setting is right for you
Divernet forum

The short answer is that you do not. Cynical observers might say that the manufacturers add these options to put the decision-making firmly in your hands, and if you do suffer from DCI it will be a case of user-error, in that you chose the wrong setting.
     Decompression theorists deal with exactly that - theory. Haldane did the empirical research a long time ago, but a lot of observational research has been done since.
     Considering the number of man/ dives per year, very few divers get DCI. So choose a computer and algorithm with which you feel comfortable. If it is always out of step with those of other divers, consider changing your option.
     Circumstances also differ. You may think it safer to spend a long time decompressing in the shallows, but this may not be the case if a strong current is running and your surface cover fails to find you afterwards. Better to be bent than drowned after a long time drifting in the ocean!
     Its your call. The computer manufacturers allow you to decide.