Deco dive with divers on different back gas

[FF / EMT]

One swallow doesn’t make a summer.

To tox at 1.4 is “unusual” as there’s literally hundreds of thousands — millions? — of dives done to the old 1.4 & 1.6 rule — the DIR proponents advocate deco on 100% oxygen at 6m/20ft which is 1.6.

Is that doing ratio deco?

 

[Nick_Radov]

Except when it isn't... This woman never exceeded 1.4 and clearly toxed at 145ft on 24/25.

2007-07-14 Incident – IUCRR

iucrr.org

That doesn't seem clear at all. The victim's teammate reported symptoms consistent with a seizure. We don't know whether that seizure was caused by CNS oxygen toxicity or something else. The accident report is really thin: no medical history or toxicology report. Unless you're privy to more detailed non-public evidence?

There is a very good reason many folks dive with 1.2 on the bottom or even less.

Sure, that's a good practice for any significant dive.

 

[Wibble]

Sure, that's a good practice for any significant dive.

Would a 45m/150ft dive on backgas deco be considered a "significant" dive though?

Certainly a much deeper dive would warrant a much lower PPO2 of 1.2 or lower, but that's about mitigating the long dive duration of many hours and keeping the CNS/POT under control. Also it'll probably be on a rebreather where very high proportions of helium is possible to keep the narcosis at bay.

 

[crofrog]

One swallow doesn’t make a summer.

To tox at 1.4 is “unusual” as there’s literally hundreds of thousands — millions? — of dives done to the old 1.4 & 1.6 rule — the DIR proponents advocate deco on 100% oxygen at 6m/20ft which is 1.6.

The gas density thing you seem to dismiss and co2 retention increase the risk of cns oxygen toxicity.

On deco we should not be working which allows us to use a higher ppO2.

We all know that it’s statistics. CNS and POT (pulmonary oxygen toxicity) are well documented with oxygen clock tools.

Are they?

 

[rjack321]

That doesn't seem clear at all. The victim's teammate reported symptoms consistent with a seizure. We don't know whether that seizure was caused by CNS oxygen toxicity or something else. The accident report is really thin: no medical history or toxicology report. Unless you're privy to more detailed non-public evidence?

Sure, that's a good practice for any significant dive.

Ocram's razor. Or dive mix to 1.4 I really don't care. I won't and there's a couple decades of practices from early mix adopters which back up the practice

 

[Wibble]

The gas density thing you seem to dismiss and co2 retention increase the risk of cns oxygen toxicity.

Am not dismissing it, but also am not adopting it as a critical planning component.

Reasoning: it's subtle. It may be a big issue if working excessively hard or in a panic, but for most dives being a little more dense than the arbitrary threshold won't be an issue. The main issue with gas density is its impracticability on open circuit -- helium is bloody expensive now and it's simply not worth high concentrations on open circuit; $200 for an OC dive on doubles with 21/35

CCR on the other hand... Helium is cheap and easy to add high concentrations. If I'm planning any dive below 30m/100ft on my box, there will nearly always be helium in the diluent; why not as it's so cheap at under $10 per dive for a rich mix - 15/45 or even 15/60.

We all know that it’s statistics. CNS and POT (pulmonary oxygen toxicity) are well documented with oxygen clock tools.

Are they?

On all the technical courses I've ever done there's always mention of the various oxygen limits and timings.

 

[iliketopetsharks]

little more dense than the arbitrary threshold won't be an issue

It's anything but arbitrary... Density Discords: Understanding and Applying Gas Density Research

The main issue with gas density is its impracticability on open circuit

This is funny. Obviously it's "impractical", inconvenient even. Does that mean one should ignore it?
If you dive deep and often enough, get a ccr.
If not, why risk it?

 

[Blackcrusader]

Diver A has air. Diver B has EAN32. Both have 50% for deco.
The dive is 100 ft for 30 minutes. Let's say using air would require 16 minutes of deco and EAN32 requires 8 minutes.
The EAN32 diver agrees to put their computer in air mode or use air tables. Both divers follow the same deco schedule and do 16 minutes of deco.
If a gas share is required, they still assume the back gas is air.
Any downsides?

My own take on this.. I would prefer to do a dive with the same gas and same GF settings. I have good dive buddy and we both use Shearwaters. If I was the 32% diver I would use 32% in my DC and just stay with my buddy for his longer deco stop. as we dive the same GF we would have the same ceiling.

As both divers would have a stage bottle with 50% then there is no issue with not having enough gas.
NOt sure at what time during the dive you would need to share air. If before entering Deco on the 21% diver then no NDL exceeded can head straight to safety stop or surface.

One should not use 21% setting when diving 32% because if there is an emergency and you need a chamber you diving physician would look at your DC and not assume you were on 32%

Of course if you felt anxious at all then don't do the dive at all.

 

[Wibble]

It's anything but arbitrary... Density Discords: Understanding and Applying Gas Density Research

Digging into that link...

...recent research by Gavin Anthony and Simon J. Mitchell from the University of Auckland Department of Anaesthesiology (see link below) has cast gas density in a new light. Working with both open-circuit and rebreather divers, Anthony and Mitchell found that gas density near the 6 g/l mark significantly increased the risk of dangerous CO2 retention during dives, resulting in their test subjects failing more than half their attempted dives and experiencing issues at more than three times the rate of divers using gas even 1 g/l less dense.

...

View the Respiratory Physiology of Rebreather Diving research in full.

The linked paper is: Rebreathers and Scientific Diving - Workshop Proceedings


Is carbon dioxide retention as big an issue in open circuit diving compared with rebreather diving? One of the features of open circuit is each breath is from a clean source of "unbreathed" gas, so no CO2. Also OC generally has a very low work of breathing (assuming the second stage regulator is correctly used -- lever in breathe mode, cracking resistance correct, serviced, etc.).

Am not disrespecting the research at all, am just bringing it to the current topic of lightweight decompression on open circuit in the 45m/150ft range. For all of us diving beyond this level, often well beyond, then we'd be taking into account the gas densities and, as we're using rebreathers, high helium percentages are preferable.



BTW: great link, interesting paper.