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Who looks out for you when you have diving equipment problems? Diver does. In our quarterly series, John Bantin fields your queries about gear, and calls on the suppliers for help where appropriate. If you have a problem with kit, let him know.
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Neither the A-clamp nor DIN cylinder valves currently in use will be suitable for use with nitrox after new regulations come into force |
Can you explain the new rules regarding valves and fittings on nitrox cylinders?
J Summers by e-mail
European law due to be introduced next year (2005), and enshrined in Euro standards BS EN 144-3 and BS EN 13949, demands that any cylinder with any amount of oxygen in it over and above that found in normal breathing air should have a new and unique type of first-stage valve connection.
This includes all mixes from 100% to 22% O2. This new valve connection will be a DIN-type but with a different thread diameter (M26) from DIN valves already in use.
Nitrox regulators will therefore have to be dedicated to nitrox tanks, although there will be nothing to stop you filling a tank so-equipped with double-filtered air.
Many manufacturers are already working on producing stocks of regulators with new fittings, because they believe demand will be high. This new standard has been introduced to protect those divers who fill cylinders with oxygen by the commonly used partial-pressure method from what may not be oxygen-clean equipment.
What's the story regarding breathing knobs and venturi switches on regulators? Does increasing the cracking pressure reduce air consumption?
Nikolai from www.divernet.com/forum
Manufacturers design regulators to give as clean a flow of air as possible, with a good venturi effect, which is what happens when a wind increases velocity because of a constricted flow.
However, "venturi effect" is also a basic aerodynamic law which describes a pressure-drop across a surface. The air rushing past the back of the diaphragm tends to produce a reduction in pressure which causes the ambient (water) pressure to push it in more than it should.
This causes the lever of the demand valve to open the valve more, causing a greater flow of air. The effect is exponential and the final result is an uncontrolled free-flow.
This normally happens where pressure differences are great enough to allow things to get out of control: ie, at the cusp between water and air.
Most designers build in a foil that can be angled to interrupt the clean flow of air through the body of the second stage. This is used pre-dive; you pull it out of the way once under water (dive). Hence the term "venturi plus/minus" lever. The venturi effect is employed only when submerged.
Some manufacturers avoid using the venturi effect in the design of their regulators altogether. Mares, for example, uses a patented bypass tube instead, so no venturi plus/minus switch is needed.
The breathing resistance control alters the amount of cracking effort needed to pull open the valve in the second stage. It does this by adding to the spring-tension of the valve and it does affect the work of breathing.
The only real reason to increase this spring-tension is if the regulator has been set up wrongly in the first place.
I admit that many service technicians set up regulators so that they breathe too lightly and tend to go out of tune and free-flow.
Increasing the work of breathing does not reduce air consumption. If you need less air, breathe less forcibly!
Should I put air in both my BC and my drysuit to maintain neutral buoyancy?
F Mireskandari, London
Get your weights right and you can keep your drysuit at constant volume and not need to adjust the air in your BC. Then you can just enjoy the dive! |
This is the sort of innocently asked question that can provoke harsh arguments on the divernet.com forum.
As you go deeper, increasing pressure crushes your wetsuit. It takes up less volume and displaces less water. Your weight has not changed, so you appear to get less buoyant. You can compensate by adding air to a BC.
Meanwhile, on an ordinary scuba rig, as your gas is exhaled into the water you lose the weight of it. A thousand litres of air weighs about 1kg, so you can become more buoyant as the dive progresses.
Every diver should start off neutrally buoyant near the surface by getting his or her weights right, but the BC has made us lazy.
We put on too much lead in the belief that we can always add some air to our BC to compensate if we have to.
The same rules apply to a drysuit, except that it does not become smaller and therefore less buoyant as you go deeper because the air you add to prevent suit squeeze should maintain its volume. The only reason to use a BC is for buoyancy should your drysuit fail, and you should need to add air to your drysuit only while under water.
In the case of divers using multiple steel tanks, many think they have no hope of attaining neutral buoyancy near the surface without putting air into a BC as well, and they may use this as an argument for using a redundant BC as well as the first BC, plus their drysuit.
Other diving regimes insist that only aluminium tanks should be used, for this reason.
I think it foolhardy to rely on bubbles of air in primitive plastic bladders, which is essentially what a BC is, to keep you afloat. I used to dive with a 38kg motion-picture camera which I made neutrally buoyant by attaching high-density foam panels. These don't crush under pressure. I still do this with my heavy still camera rig, because I like it to be neutrally buoyant.
You can do the same with your multiple steel tanks if you can't make your whole rig and yourself neutral when just submerged.
So, at the risk of receiving dozens of protesting letters, the habit of adding air to a BC while wearing a drysuit indicates a poor initial decision about weighting. Wear the right amount of lead, keep your drysuit at a constant volume during the dive, and you will not need to put air in your BC.
How big is an 80cu ft tank in litres?
Jim Breakell by e-mail
A litre equals 0.03531467cu ft, but the answer is not as simple as you might imagine (hence, perhaps, the question).
In most of the world we have tanks marked with their fixed volume and, to calculate how much gas they contain, we multiply this fixed volume by the pressure of the gas within.
So a 12 litre tank filled to 200 bar has 2400 litres of gas in it. That equals around 85cu ft. At 30 bar, a 12 litre tank has only 360 litres in it.
In the USA, however, the expression 80cu ft says nothing about the actual size of the tank. It indicates how much gas is in the tank when it is filled to its maximum working pressure of, say, 3000lb/sq in. So to know how big it is, you need to ask: "How many cu ft at what pressure?"
Often Americans use lower working pressures than we do. The metric system makes things simpler!
Should I carry my regulator and its integrated computer as hand-luggage so as not to subject it to the reduced pressures at altitude encountered in an aircraft's hold?
Peter Burgess by e-mail
We keep getting asked this one. Look at the cross-section of an airliner from the front. It's a long cylindrical tube, with a floor running through the middle. The top half is the passenger-cabin and the bottom half the baggage-hold. If there was a pressure difference, the passenger-cabin floor would be in danger of collapsing.
Check your gear into the hold and stop worrying. It all gets depressurised to the equivalent of about 3000m of altitude.
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