John Bantin

Kit Q and A

An Israeli inventor has developed an apparatus that will allow breathing under water without the use of compressed air tanks. Apparently it uses the small amounts of air that already exist in water to supply oxygen to either scuba-divers or submarines. Are we likely to see this on sale soon?

I am led to believe that, even on the figures supplied by the inventor, Alon Bodner, the device will need to pump 2000 litres per minute. That's around 2 tonnes of water a minute (more than 33 litres a second)!
A pump that powerful would need a lot of batteries. A typical 500W electric pump will shift 233 litres a minute, so we would need about nine such pumps to run off our battery pack, requiring an impossible 4.5kW.
Also, according to Newton's Third Law of Motion, the thrust of 2 tonnes a minute would propel the diver through the water pretty fast! A very modest boat jet-drive pumps half a tonne a minute.
There also seems to be a flaw in the assumption that the air will come out of the water if the pressure is reduced.
That may be true of 'saturated' water in a laboratory, but sea water is not always saturated with oxygen, and even at reduced pressure it might not reach super-saturation and come out of solution. As the bubbles float away from the cavitation area, they will start to re-dissolve.
Amounts of gas dissolved in the sea also vary a lot. The solubility of each gas varies with salinity and even more so with temperature; at a salinity of 35 (average around the UK) and a temperature of 25Â?C (Red Sea in May), the solubility of oxygen is less than 4ml per litre, but if the water cools to 0Â?C, solubility rises to 10ml per litre.
Nitrogen solubility varies in a similar but different way, 10ml per litre at 25Â?C and 15 at 0Â?C. 'Air' extracted at these two temperatures would vary from 29-40% oxygen - quite a challenge for the nitrox decompression tables.
Oxygen-solubility figures describe the maximum amount that could be dissolved, not the true figures. O2 levels vary with biological activity. Next to a shallow reef busy photo-synthesising in bright sunshine, the water is likely to be almost fully saturated, but swim into a dark wreck full of decomposing seaweed, and there may be no oxygen at all.
Don't swim behind your buddy if he is wearing this sort of kit... not only will you be hit by 2 tonnes of water a minute, but the water will have no oxygen left in it!

Air integration puts all your data, including tank contents, in one display.
Tanks talking to computers
What is the point of a gas-integrated computer? (from Divernet forum)

A gas-integrated computer uses a pressure sensor to read the remaining pressure in the tank. The most popular method now is to transmit this data via a radio-transmitter attached to the first stage of the regulator to the computer wrist unit, though earlier console versions used a high-pressure hose.
The computer then displays remaining tank-pressure (in psi or bar), along with remaining airtime that it calculates from the ambient depth and the breathing-rate of the diver up to that moment. It assumes that you will go no deeper and that your breathing rate will not dramatically increase.
Naturally a reserve pressure is built in, and the diver usually chooses this when first setting up the computer.
It is wise never to let the time-to-surface, which may include mandatory decompression, be longer than the remaining-gas time.
Some gas-integrated computers can track only one tank. Others can also track gas-switches for the purpose of accelerated decompression. Others still can track gas in up to three cylinders.
Many divers use gas-integrated computers alongside conventional pressure gauges, which offer the small advantage of an immediate read-out when turning on a tank.

Prepared for the Red Sea
What's the right suit for use in the Red Sea? (from Divernet forum)

What's right for you may not be right for me. The Red Sea varies greatly in temperature between Djibouti in the south in summer and the Gulf of Aqaba in the north in winter.
In January in Egypt, I have been cold in a 7mm semi-dry and wished I had a drysuit. It's less the temperature of the water, though this can be as low as 18Â?C, more the prevailing north-west wind - very chilly when you get out.
In summer a 3mm suit may well be adequate. I always recommend a suit that covers your arms and legs to protect you from man-eating plankton and accidental cuts and abrasions to water-softened skin.
Take a suit that can be used in layers. A thermal undervest can make the difference between a good time and misery - and a hood can come in handy. Get a suit that fits properly and that you can get in and out of easily.
Always err on the side of too much insulation.

Weighting depends on the amount of material in a diver's suit, rather than on their own bulk
Fat or tall, it's all the same
Do fatter people need more lead weights than thin people to make them neutrally buoyant under water?

Although fat is less dense than muscle, most humans are more or less neutrally buoyant depending on their lung volume (breathe in, you float; breathe out, you sink).
What makes you positively buoyant is your suit. The more material needed to cover you, the more buoyancy and the more weight you need to compensate.
I'm a tall, skinny man weighing in at 90kg. Though not fat, I need a lot of suit to cover me. This equates to a lot more lead than someone who is only, say, 1.6m tall. If I dived with no suit (brrrrrr!) with a neutrally buoyant aluminium cylinder, I would need no lead at all.
So it's generally not how fat you are but how big that counts.

The Buddy Auto Air - AP Valves' integrated alternative air system. These need regular servicing, like any other regulator.
Ditching the octopus
Many BC manufacturers show an integrated air source inflator as standard equipment. I would like the definitive answer on the use and safety of these for sport and technical diving.
Mathew Kelly.

The idea is a good one on the face of it because it gets rid of a hose. However, if you had to breathe for any length of time from a second stage at the end of the corrugated hose of your BC, you would realise how awkward it is, and it certainly is not suitable for use by another out-of-air diver.
These second stages also need just as much care and freshwater rinsing as the main second stage on your regulator, yet sadly they tend to get the same rough treatment as BC direct-feed inflators, so are constantly in need of retuning.

I'm sucking water
I have always been more than happy with my regulator but recently it seems to be giving me a wet breathe. Any ideas why?
(from Divernet forum)

There are two common possibilities, the most obvious being that the mouth-piece has developed a small hole that lets water in. Have a good look at it, stretching it a little as you do, as it's notoriously difficult to spot small holes in black rubber.
It's also possible that the silicone rubber mushroom exhaust valve is no longer sitting properly. If it has gone curly, you can usually see this by looking at it through the exhaust-T.
If you have any doubt in either case, get the parts changed. With the dust-cap firmly sealed over the gas intake of your first stage, inhale gently through the second stage.
If you can suck any air at all there is a leak that, under water, will let water back in the other way.

Glueing up my BC
I bought a BC that was old stock but unused. However, air appears to leak from it through a small hole or tear that's almost impossible to see. The importer no longer has any replacement stock but suggests I repair the BC with Aquasure adhesive. Will that be safe?
Adrian Richy

The air is at ambient pressure, except when the BC is fully inflated, but if you have a hole high in the buoyancy cell, it will rise and escape through that. This is very common with BCs after heavy use, especially in tropical conditions.
With a very small hole a sealant such as Aquasure seems ideal. After all, it is very effective at sealing perforations in drysuits, so why not a BC?
Apply the sealant when the BC is well inflated. A tiny trail of bubbles escaping will do little to affect your buoyancy control during a dive, and you can usually top the air up if you need to.

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