High O2 deco stops

[vjongene]

My son and I are going to Bikini Atoll in July to do some wreck diving. All of the dives are deep (over 40 m) and require long deco stops. The stops are being done at a surface-supplied deco station, with regs dispensing 75-80% O2. The times and depths of the stops will be calculated using air tables (or, in most cases, a computer set for diving on air), to increase the safety margin.

My question relates to the potential risks associated with breathing O2 for extended periods during the deco stops. At the 9 m stop, the pO2 will be about 1.5, i.e. higher than the commonly recommended max of 1.4. Total deco times will be in the 30-40 min range for each dive. Ox tox seems to be a potential worry.

Any thoughts on this? Any recommendations on safety precautions, other than watching each other carefully for convulsions or blackouts? One of the members of our club recommended trying to breathe normobaric O2 for 30-40 min before going on the trip, to see if either of us reacted badly.

Victor J.

 

[svidlano]

My son and I are going to Bikini Atoll in July to do some wreck diving. All of the dives are deep (over 40 m) and require long deco stops. The stops are being done at a surface-supplied deco station, with regs dispensing 75-80% O2. The times and depths of the stops will be calculated using air tables (or, in most cases, a computer set for diving on air), to increase the safety margin.

My question relates to the potential risks associated with breathing O2 for extended periods during the deco stops. At the 9 m stop, the pO2 will be about 1.5, i.e. higher than the commonly recommended max of 1.4. Total deco times will be in the 30-40 min range for each dive. Ox tox seems to be a potential worry.

Any thoughts on this? Any recommendations on safety precautions, other than watching each other carefully for convulsions or blackouts? One of the members of our club recommended trying to breathe normobaric O2 for 30-40 min before going on the trip, to see if either of us reacted badly.

Victor J.

First:
If you're going to be under 40 please rethink your idea of using air.

Second:
You'll calculate your deco obligation as if you are using air, and then you'll switch to O2. This is not added safety. It's simply WRONG!!!

Why? Because air would give you 15 min at x meters, but on O2 you need only 5 min (numbers here are arbitrary). So 10 additional minutes on O2 are not added safety, they are added risk!!! Do your risk management properly!

Please use some way of calculating deco using the gas mixes you'll actually breathe. Safety factor can be adjusted by using conservatism factor of your choice.

 

[Wreckie]

First:
If you're going to be under 40 please rethink your idea of using air.

Second:
You'll calculate your deco obligation as if you are using air, and then you'll switch to O2. This is not added safety. It's simply WRONG!!!

Why? Because air would give you 15 min at x meters, but on O2 you need only 5 min (numbers here are arbitrary). So 10 additional minutes on O2 are not added safety, they are added risk!!! Do your risk management properly!

Please use some way of calculating deco using the gas mixes you'll actually breathe. Safety factor can be adjusted by using conservatism factor of your choice.

First: whats the problem of going past 40 on air? O2 tox kicks in at 60m (based on 1.4ppo2). Could this be based on acceptable levels of narcosis? If properly dived up and prepared this also wont be huge problem (lets be honest they are going to be diving in what is basically a large deep swimming pool vis wise).

Second: it is common practice to use high % nitrox to give you a buffer, an extra safety cushen, on air stops. This is exactly what BSAC's advanced nitrox course allows you to do (not cut down the stops). Another common option is to use nitrox on the dive but do air stops. Again increased safety buffers.

As for upping your o2 risk on the stops the HSE recomend 1.6 bar as the max safe level of ppo2, so your 1.5 is still ok by that standard. Just track carefully your o2 tox and build in air breaks if your going to be doing long stops.

How close is the nearest chamber, think i'd rather kick the arse out of o2 that risk getting bent if the pot is a fair distance away.

Mike

 

[vjongene]

First:
If you're going to be under 40 please rethink your idea of using air.

I cannot rethink here. The dive op on Bikini does not supply Trimix. Period.

Second:
You'll calculate your deco obligation as if you are using air, and then you'll switch to O2. This is not added safety. It's simply WRONG!!!

Why? Because air would give you 15 min at x meters, but on O2 you need only 5 min (numbers here are arbitrary). So 10 additional minutes on O2 are not added safety, they are added risk!!! Do your risk management properly!

Please use some way of calculating deco using the gas mixes you'll actually breathe. Safety factor can be adjusted by using conservatism factor of your choice.

I understand your arguments perfectly. But in this case I am dependent on (1) the procedures implemented by the dive op and (2) the equipment available to me. They DO encourage divers to bring computers that allow gas switches during the dive (e.g. the Suunto Vytec or the DiveRite Nitek 3). I do not have one of these, and do not want to buy two expensive computers for a single trip. So we will be diving Aladdin Pro's, and following the deco stops calculated by them. This practice is proposed by the dive op as an alternative to gas-switch computers. What they will NOT allow is to come out of the water with a computer that still shows a deco obligation. The last alternative is to dive tables only, but these assume square profiles. So the deco times indicated by the tables are likely to be longer than those calculated by the computer.

There IS in fact some safety built into the procedure, in that N2 degassing is greatly accelerated by the O2. The problem, as I indicated originally, is with possible ox tox.

If you have a simple solution to propose to this dilemma, I'd be glad to hear it. If you have a couple of Vytec's to loan to us, I'd be positively delighted!

 

[svidlano]

First: whats the problem of going past 40 on air? O2 tox kicks in at 60m (based on 1.4ppo2). Could this be based on acceptable levels of narcosis? If properly dived up and prepared this also wont be huge problem (lets be honest they are going to be diving in what is basically a large deep swimming pool vis wise).

40 on air? It's OK (sort of). I've done it. Lots of people have done it. It can be done. Doe's it make it right?
The question is "why do you dive"? I dive for the pleasure of it. I enjoy 3D environment and enjoy looking at underwater creatures, and sceneries. So, is there a point (for me) in going deep on air if I miss all the things that are important to me?
Due to nitrogen narcosis, there are fewer things I see and even fewer that I can remember. Haven't you seen pictures from a dive that you can't recognize? Where was that fish? I pointed it to you? You must be kidding!
So if I take a dive trip and dive on some wreck, then I'll definitely want to see it and remember that dive.
A safety point aside, that is why I would reconsider using air.

 

[svidlano]

I cannot rethink here. The dive op on Bikini does not supply Trimix. Period.

There IS in fact some safety built into the procedure, in that N2 degassing is greatly accelerated by the O2. The problem, as I indicated originally, is with possible ox tox.

If you have a simple solution to propose to this dilemma, I'd be glad to hear it. If you have a couple of Vytec's to loan to us, I'd be positively delighted!

OK you are limitid by the procedures of your dive OP. I don't like them, but you've decided to go by them. And I'm sorry to say, I can't give you a Vytec, because I don't have any. :sad:

Wish you nice warm weather, clear water, great vis, and (most of all) safe diving!

 

[ninjamuzo]

There are two issues here.

1. The point of doing the deco on O2 is to shorten your deco stops. If you are going to be doing the stops as per the air timings then don't bother taking the O2. If you are going to take the O2 then dive on the tables for that mix. If you are forced to do both then you put yourself in danger for the O2 exposure.

2. Extended exposure to O2 will build up you exposure. One of the two factors tracked is cumulative over time the other clears out over night. Make sure you track your O2 exposure for the multi dives. This is a real risk I know of people that have has problems due to this.

James - Technical Instructor.

 

[svidlano]

There IS in fact some safety built into the procedure, in that N2 degassing is greatly accelerated by the O2. The problem, as I indicated originally, is with possible ox tox.

My idea is – the greatest risk factor in diving is being underwater. So it's wise to get out of it as fast as possible without getting injured. Staying there longer than necessary (after finishing your task) is just adding (unnecessary) risk.

So if I can do my deco in 5 min and for some reasons stay 10 or 15 it does not contribute to my safety, but compromises it.

You'll definitely have lesser nitrogen load upon staying longer, but is it worth it?

Since you've explained your situation (your dive OP's standards) you can only go by it or stay home.

 

[vjongene]

There are two issues here.

1. The point of doing the deco on O2 is to shorten your deco stops. If you are going to be doing the stops as per the air timings then don't bother taking the O2. If you are going to take the O2 then dive on the tables for that mix. If you are forced to do both then you put yourself in danger for the O2 exposure.

Do you know where I could get tables for multi-gas dives? This would actually be very useful.

2. Extended exposure to O2 will build up you exposure. One of the two factors tracked is cumulative over time the other clears out over night. Make sure you track your O2 exposure for the multi dives. This is a real risk I know of people that have has problems due to this.

Yes, this is a good suggestion (also made by Wreckie). I will be tracking cumulative ox tox for these dives. In any case, I doubt that we will be able to make all of the dives in the program (12 in 6 days), and this can be one factor in deciding which ones to skip.

 

[svidlano]

Do you know where I could get tables for multi-gas dives? This would actually be very useful.

Have you tried http://www.gap-software.com/? I like it very much.

Don't forget - it's just another piece of software, you are the one who is doing a dive!