For the benefit of my students, can you give me a simple explanation as to why they can't re-pack the scrubber canister of a closed-circuit rebreather with partially used material?
When a person breathes, there is a by-product in the form of a waste gas. For every hundred molecules of air inspired, four molecules of carbon dioxide (CO2) and one of water in the form of vapour are produced. The build-up of CO2 is the main signal to the brain to cause the lungs to exhale. In effect, it is poisonous.
It is vital for safety that a rebreather's scrubber canister be packed tightly.
As the diver goes deeper, denser gas is inhaled and exhaled but the percentage of CO2 and water vapour remains the same.
The chemicals in a proprietary CO scrubber material such as Sodasorb or Sofnolime have an affinity for CO2. Passing molecules of the gas attach themselves to form a soluble carbonate, together with water and some heat. Each time this chemical reaction takes place, molecules of scrubber material can be said to be 'used up'.
The granules of material are stacked one behind the other in the scrubber canister. The first to make contact with the CO2 are the first to become affected and no longer active.
The next molecules of CO2 pass by these inactive scrubber molecules and form a chemical reaction with the active ones immediately behind. As the scrubber molecules get used up, the reaction passes further and further through the scrubber stack.
Think of the scrubber granules as lines of defending soldiers on a medieval battlefield and the CO2 as the attacking enemy. The enemy has a never-ending supply of attackers, but as more defenders are lost there are reinforcements waiting behind.
If a scrubber is inadequately packed and 'settlement' or 'channelling' occurs, some of the CO2 can take a route that passes through the scrubber stack without coming into contact with any active scrubber material. The enemy can find a route through, passing only dead soldiers.
The position of these dead soldiers relative to active soldiers is crucially important. If a partially used scrubber is emptied out of a canister and the same scrubber material replaced, it would not be possible to arrange the granules in exactly the same way so that all the active material was positioned behind the inactive used-up material. Routes would form that would allow poisonous CO2 to pass through the scrubber.
This is why scrubbers must be packed tightly, so that all granules maintain their positions relative to one another. A filled scrubber canister should not rattle when shaken. Repacking one with partially used material could be fatal.
If I fill my cylinder at my local dive centre and do not use it because of unfavourable weather conditions, how long can the air remain in the cylinder and still be breathable? Will air go stale and have to be refreshed after a period of time?
Most of the air we breathe is millions of years old or older. It doesn't go off. Air compressed for breathing is filtered and dried by having the water squeezed out of it.
However, if moisture has entered your cylinder through a wet fill (which should not be allowed to happen with a properly managed compressor), a steel cylinder might rust, using up some of the oxygen in the air. The fact that a cylinder must be emptied for inspection every 30 months removes this risk.
An air-sharing exercise - two divers should have no problem breathing off the same first stage if both second stages are of good quality
In the light of a recent report (HSE Warns Against Octopuses Used With Cheap First Stages, News, June), why haven't manufacturers tested their regulator first stages to the possibility of supplying two divers?
There's been a lot of fuss about this HSE report, which tells us that some regulators are not up to the job of supplying two divers breathing in phase at one time. When carries out in-water regulator tests at depth, we always rigorously check for this, and in recent years have not found a single regulator that could not be used safely with two second stages at the same time. This, of course, assumes that the regulator in question is well-maintained.
That said, many divers are inclined to buy a new high-performance regulator but relegate an old second stage to the octopus rig. This is where problems may lie. The octopus rig should be as good as your second stage, even if it is detuned a little to avoid unwanted free-flows.
If you want to be sure, you can always do what is common practice in the rest of Europe and use an H-valve.
This allows you to run two completely independent regulators from one tank and even turn one off independently.
When I did my drysuit training, the dump valve was on the cuff. The one on my suit is on the shoulder, and I am not sure what setting to have it on when I go diving.
Thousands of words are written on forums about cuff dumps and auto-dumps for drysuits. Auto-dumps are more correctly called 'constant-volume' dump valves, because they are used to keep the volume of air in your drysuit, and with it your buoyancy, constant during an ascent.
Open a drysuit shoulder dump all the way on ascent
Single-cylinder divers who are neutrally buoyant at the start of the dive will only have to keep the volume of the suit constant as they descend to maintain that neutral buoyancy. Open the auto-dump valve all the way (anti-clockwise) to let excess air out as you leave the surface; then just close it a notch or two as you proceed with the dive. Put air in your suit as necessary to maintain the volume of air in it.
When you ascend, it is safest to open the valve all the way, but be sure that it is positioned at the highest point in your suit. Some suits demand that you do a sort of Roman salute to achieve this, because the valve is not perfectly sited.
When you are at the surface you may wish to add more air for comfort and warmth, so this is when you can shut the valve down by turning it clockwise all the way. Then no air escapes through the valve.
I get specks of dust on the CCD [Charge Coupled Device] of my digital SLR camera and these show up on the pictures I take. What can I do about it? Also, can I remove small scratches from the polycarbonate dome port of my underwater camera housing?
It's a fact of life that when you change lenses on your digital SLR, dust from the atmosphere is attracted to the CCD or CMOS by the electrical charge. It will get in, and it usually shows up in blue areas of your underwater pictures.
This can be retouched in Photoshop using the 'healing tool', but it would be nice if it were not there in the first place.
Because the picture is so greatly enlarged, don't bother to try to see it on the CCD. Use a proprietary cleaning swab and solution as recommended by the camera manufacturer.
Never leave a camera without a lens in place, and when you change lenses, dust off the outside of the camera first. Inverting it and allowing gravity to take effect also helps.
Check for dust by photographing an area of white sky, underexposing it to a mid grey, and then downloading the picture to your PC for close scrutiny.
Scratches on polycarbonate are easier to deal with. First, you must cut back the surrounding area to match, using a very fine abrasive as used in car paint-shops and often called 'wet 'n' dry'. Its effect is quite dramatic, so don't overdo it.
Then polish the area back to transparency, using a proprietary silver polish with cotton wadding such as Silvo.
There are also scratch-removers available for the plastic windshields of light aircraft. Either way, it takes time and a lot of elbow grease.