Kit Q&A August 2009
My reg seems to tolerate nitrox
Ive just found out that my Cressi regulator is incompatible with nitrox, even though I have been using it with nitrox without problem for some time. Yet an Atomic T2 titanium regulator is said to be good for nitrox 40 straight from its box - how can this be
New European laws require a regulator for nitrox use to complete a test in pure oxygen, even if it will be used only with nitrox up to 40% O2. Manufacturers often use normal silicon lubricant on their regulators, which is perfect for normal nitrox use of up to 40% oxygen, but not for pure oxygen use, so it is not possible to certify these regs for nitrox use.
From a purely practical point of view these regulators can be used with nitrox up to 40% oxygen without any problem or need for adjustment - its just that they are not officially certified for this use.
The presence of some small titanium components inside the second stage of certain regulators does not in any way affect their use with nitrox.
Some incompatibility between oxygen and titanium has been found, but it occurs only at very high pressure, at high temperatures (over 80°C) and with a very high oxygen percentage in the mixture (90-100%).
So it could be a problem only with a titanium first stage that used pure oxygen in water too hot for a human to endure!
Cleaning the sensor is a delicate procedure.
DIY sensor swab
After some six happy months with my digital single lens reflex camera, Im now getting foreign bodies in my pictures, and I dont mean French sunbathers. They are always more or less in the same place in every picture. Do I need to send my camera back to the manufacturer
It sounds as if you have an accumulation of dirt on your cameras sensor. This will always be a hazard when using a camera with interchangeable lenses, especially in the tough conditions often encountered at diving locations.
Because sensor areas are so small, a foreign body will appear very large by comparison, even though you might be unable to see it with the naked eye.
If you follow the instructions carefully, you can use a proprietary sensor-cleaning kit, several of which are offered by the company Just, including Dust-Aid (www.cameraclean.co.uk).
If you use a swab and cleaning-fluid system, you must use the fluid sparingly, and make a bold single sweep with the swab across the sensor in one direction, rotating the swab if you make a second pass back the other way.
To access the sensor of your DSLR, remove the lens and set the camera to shutter priority with a time of 30 seconds. This should give you time to make a clean
pass, with no danger of the mirror coming down on the swab.
Check the result by shooting a blank, even area such as your computer screen displaying nothing but white, downloading the picture to examine the results.
Rolling and tumbling
Im told that when mixing nitrox by adding air to a partially filled cylinder of oxygen (partial-pressure blending) its important to mix the gas well in the cylinder by rolling it, or youll get a wrong percentage mixture. Is that true
I have seen gas blenders in Third World countries rolling nitrox tanks to mix the gas. They probably believe the old wives tale about the tank of air that was left standing for a year and all the oxygen settled to the bottom.
It was OK for the first part of the dive, when the diver descended head down, because the oxygen tipped up towards the top, but the diver passed out during the ascent, when all he got was nitrogen!
Gases have molecules in what is known as free association, so they diffuse into one mix almost instantly.
Any discrepancies that occur are due to pressure/temperature differences when mixing by partial pressure.
Cold gas may be decanted into a cylinder filled with another gas that is hot from being recently compressed, or vice versa, so wait until the temperatures have reached ambient before final analysis.
As every diver knows, gas pressures are affected by temperature, so mixing gases in large volumes is best done by the molecular-weight method.
When it comes to mixing helium with denser gases such as oxygen and nitrogen, there might be an argument for tumbling a cylinder - but only if this is done and the contents analysed immediately before use. Dont leave it standing.
This mask is connected directly to the divers hood.
Old-style with a twist
Im constantly being told to stop wearing my mask on the top of my head between dives, yet I often notice pictures of you with your mask perched up there. Why is it OK for you to do it, and not me
Jacques Cousteau and Hans Hass were often photographed the same way, along with a lot of lesser mortals like me.
So why should you not do it Because if youre wearing a hood, you might forget about the mask when you whip the hood off, and lose it over the side of the boat.
PADI has also decreed that a mask on the forehead is a sign of a panicking diver - although there are usually other, more kinetic indications of that.
How do I get away with it I cheat, of course! I use the Mask Connection System, which attaches the mask directly to your hood, such that the hood becomes part of the mask strap. If one side-strap should break (its never happened), the other side would still be attached.
When over-enthusiastic young divemasters shout at me to take my mask off my head, I throw it carelessly over the back of my hood, and the more observant ones get the message. Its part of my hood.
The Mask Connection System is easily fitted to both mask and hood and unclips completely when you want it to. Get it from Aquatec in the UK (www.aquatec.co.uk).
Dont go with the flow
My Apeks octopus rig has developed a fault after years of being totally reliable.
It now gently free-flows unless I unscrew the front cover a little. I am concerned that a loose front cover might go missing during a dive. What should I do
Get it serviced. Octopus rigs are subject to a lot of unnecessary abuse. I often see them being dragged in the sand and colliding with hard objects both above and under water. This battering has to have a detrimental effect.
It sounds as if the second-stage valve mechanism has revolved a little inside the main housing, making one side of the pressure-sensing lever stick up proud and touch the front diaphragm, depressing it slightly. This opens the valve enough for a trickle of air to escape.
A plastic part moulded within the second stage locates the mechanism, but over the years this can wear just enough to cause this problem.
Unscrew the front cover, and carefully remove the retaining ring and soft diaphragm. Does one side of the valve lever look to be more proud than the other
Your dive shop can swiftly put this right by loosening the retaining nut and resettling the whole mechanism where it should be within the second-stage body.
Weight of water
I often hear people talk about a flooded drysuit dragging you down, but how can water in your suit make you sink, if you are surrounded by water
You're right to question this often misunderstood part of diving physics. Of course, a drysuit filled with water has no extra weight until you climb out of the water into the air, but if you are using a drysuit properly, you will be keeping it at constant volume (and hence constant buoyancy) by adding air to it as you go deeper.
If you flood your suit completely, it is right to assume that you will lose the air that is keeping you neutrally buoyant.
You will therefore need a redundant buoyancy device such as a BC to save yourself from sinking deeper.
If you merely get a partial flood, the lower part of your suit will fill with water and the air will stay at the highest point, continuing to maintain your buoyancy.
However, remember that it is the air in your suit, within the structure of your undersuit fibres, that prevents the loss of heat that would otherwise occur. If you lose this airspace to heat-conducting water in cold conditions, you might well suffer from the disabling effects of hypothermia.
When commercial divers work in flooded suits, these are intentionally flooded with warm water supplied from the surface.