Deco Gas

I've heard a little about "deco gas" being more oxygen-rich than whatever the diver is normally/otherwise using. Is that because they are normally diving deeper than is safe for nitrox toxicity levels so they dive air then use a high O2 gas for their stops? Or is there some other reason?

The higher concentrations of oxgyen help flush the nitrogen that has been absorbed out of the tissues. Percentage of deco gasses used varies by divers, agency and dive plans, but typically a deep dive will use 50% oxygen mix starting at their 70' stop and switch to 100% oxygen at 20'.

As you go deeper, the oxgyen toxicity becomes a concern. Deeper dives are more commonly being carried out on trimix, a mixture of helium, nitrogen and oxygen. The helium is used to displace some of the oxygen molecules, reducing the oxtox risk and to displace nitrogen, reducing the effects of narcosis.

The more oxygen you can add into a mix, the more you can speed up the offgassing of nitrogen from the tissues. However this has to be balanced against the risk of oxtox and is the reason for the use of multiple gasses.

Just to add to Cave Divers post. Carrying high O2 deco bottles brings significant extra risks. Accidentally switching to 100% O2 at depth (say 200 feet) will be pretty much instantly fatal. Switching to 100% O2 at 70 feet would be really really bad (although you might survive) Emphasis on might!

ianr33:

Just to add to Cave Divers post. Carrying high O2 deco bottles brings significant extra risks. Accidentally switching to 100% O2 at depth (say 200 feet) will be pretty much instantly fatal. Switching to 100% O2 at 70 feet would be really really bad (although you might survive) Emphasis on might!

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Thanks for the addition Ian.

As indicated by him, there is significant planning and training that goes into making a dive with a mandatory decompression obligation using multiple mixed gasses. Many things have to be taken into account to safely complete it, emergency and contingency planning such as lost/unavailable gas supply.

One really needs to understand the potential risks in this type of diving by getting appropriate training and building experience and skill level before attempting.

openhelix:

I've heard a little about "deco gas" being more oxygen-rich than whatever the diver is normally/otherwise using. Is that because they are normally diving deeper than is safe for nitrox toxicity levels so they dive air then use a high O2 gas for their stops? Or is there some other reason?

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Decompression diving is a form of technical diving and takes training to do it safely. For 'information only' we can post some general information, but experienced technical divers are usually cautious about giving too much info on a technical subject like this, and they should be. If you're interested in actually doing decompression diving, please concider taking a course, say the deco/extended range course from TDI and any pre-req's for that you haven't taken.

ianr33:

Just to add to Cave Divers post. Carrying high O2 deco bottles brings significant extra risks. Accidentally switching to 100% O2 at depth (say 200 feet) will be pretty much instantly fatal. Switching to 100% O2 at 70 feet would be really really bad (although you might survive) Emphasis on might!

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This is the reason that cave divers leave their O2 one stop below it's MOD instead of carrying it with them. On wreck dives in the open ocean you have to carry all your cylinders with you in case you get 'blown off' the anchor line and have to 'shoot a bag' and deco while you drift.

So, for the record, no plans on deco diving anytime soon. Just wanted to understand a bit more when others were talking about it. Thanks!

openhelix:

I've heard a little about "deco gas" being more oxygen-rich than whatever the diver is normally/otherwise using. Is that because they are normally diving deeper than is safe for nitrox toxicity levels so they dive air then use a high O2 gas for their stops? Or is there some other reason?

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Actually...Its not really about the O2. Its about the gas being Nitrogen and Helium poor. O2 has to take their place because we can metabolize that gas.

Then the body tries to equalize the concentration of the breathing gas and your body.

Thus...flushing out inert gas.

The question has been pretty much answered already...

Decompression gas mixes (typically 40%+ nitrox) are used to accelerate decompression and shorten your stop times. They accelerate decompression because higher O2 percentages create a greater gradient between the pressure of gas in your tissues and the gas in your lungs. The higher the gradient, the quicker that those gases will be inclined to equalize.

Accelerated Decompression, using rich nitrox mixtures, is a technique used by technical divers to reduce the amount of time hanging around in the water at their stops. The use of rich gas nitrox mixes demands specific training, discipline and excellent bouyancy control....for the following reasons...

1. Switiching to an incorrect mix below its maximum operating depth. When dealing with multiple cylinders, an error in choosing the correct tank for a given depth is likely to mean that you exceed your maximum PPO2...causing oxygen toxicity and drowning. This form of human error is the primary killer of technical divers.

2. Poor bouyancy control may mean you sink deeper that permitted for a given nitrox mixture. Depending on the % blend being used...the margins between maximum operating depth and the planned deco stop depth can be very precise and with very little room for error.

I cannot stress strongly enough the need for proper training and procedures if you are going to conduct accelerated decompression using mixed gases.

If you are interested in learning more, Deco for Divers by Mark Powell is an excellent source ...

... Bob (Grateful Diver)

Cave Diver:

The higher concentrations of oxgyen help flush the nitrogen that has been absorbed out of the tissues. Percentage of deco gasses used varies by divers, agency and dive plans, but typically a deep dive will use 50% oxygen mix starting at their 70' stop and switch to 100% oxygen at 20'.

As you go deeper, the oxgyen toxicity becomes a concern. Deeper dives are more commonly being carried out on trimix, a mixture of helium, nitrogen and oxygen. The helium is used to displace some of the oxygen molecules, reducing the oxtox risk and to displace nitrogen, reducing the effects of narcosis.

The more oxygen you can add into a mix, the more you can speed up the offgassing of nitrogen from the tissues. However this has to be balanced against the risk of oxtox and is the reason for the use of multiple gasses.

Click to expand...

Cave Diver said it all. For reference, there is a small variation on the limits of oxygen diferent divers would consider when planning their blends, specially for botton gas.

The toxicity is measured by the partial pressure of Oxygen, which is basically the % of oxygen in the blend multiplyed by the ambient pressure in ATA. I use 1.4 ATA max ppO2 in the botton and 1.6 max during deco, there are some divers that use also 1.6 in the botton and others more conservatively use even less that 1.4, if you expect a more stressfull dive in terms of task load, physical exercise, you should consider lower ATA limit for ppO2 in the botton, that's why also, in deco you use it higher, you are relaxed, in this case I believe the vast majority of divers use 1.6 ATA. Relating that to your question in the OP, this is the reason of more than one bottle, as Cave Diver explained, 50% Oxygen would give you 1.6 ATA at 22 meters and 100% oxygen give you 1.6 ATA at 6 meters.

Needless to say and already posted here the importance of good cylinder markings and training in gas changing protocols to prevent one from breathing the wrong gas, putting you way above oxygen toxicity limit, what could cause convulsion and death by drowning.