Reconsidering Deep Air?

[KenGordon]

What do you mean by time pressure? I thought a rebreather was more time consuming,

In the water on OC you have a hard time limit due to using the gas. Spend any significant extra time at depth or get in a tizzy such that your gas consumption goes up and you risk running out. On CCR you have hours of endurance and delay means more deco rather than drowning.

 

[Kay Dee]

What do you mean by time pressure? I thought a rebreather was more time consuming,

Ken has answered the 'time pressure' bit very succinctly, let me clarify your 'time consuming'. Yes, you do have to pay more attention than on OC where just knowing your breathing rate so well that you rarely need to look at your pressure gauge, or looking at your pressure gauge and depth gauge - well you shouldn't need to have to look at your depth gauge much as you can tell when your inadvertently descending or ascending, or should be able to anyway - at whatever intervals you fell comfortable with is about it. But with CCR you must, or you better, pay d%&n attention all / most all of the time! So that does distract somewhat from the u/w experience, but not so much that the trade off is not worth it. Besides now, with HUDs, and other 'alert' devices, etc, things are made a lot easier to pay attention to. Sure, I hear you say just more things that can fail / go wrong.....................but the trade off from OC if doing photography or deep dives - or both combined - especially is well worth it, or did I already say that?

 

[Celt]

In the water on OC you have a hard time limit due to using the gas. Spend any significant extra time at depth or get in a tizzy such that your gas consumption goes up and you risk running out. On CCR you have hours of endurance and delay means more deco rather than drowning.

Why would you spend more bottom time than planned, are you limited by your bailout on rebreather

 

[KenGordon]

Why would you spend more bottom time than planned, are you limited by your bailout on rebreather

Something goes wrong such as losing your buddy, getting lost, arguing with a lobster too long, the current being uncooperative or whatever but not a rebreather failure. Then you just have more deco. There is extra risk in case the rebreather also dies and you don’t enough gas for the ascent but for the case of just taking too long you are good.

I was very conscious of this doing blacked out mask exits from inside a wreck recently. I was quite deep and in a bit of a tizzy but able to stop and think “did the line go through here on the left or the right” for as long as I liked without any pressure to rush things and blunder forwards.

Of course with a twinset etc you ought to plan for plenty of gas, but I like that it is irrelevant.

 

[helodriver87]

Why would you spend more bottom time than planned, are you limited by your bailout on rebreather

Yes, you're limited by bailout, but only if you need to bailout. Take a dive to a bit below 200', for example. You're racking up deco at a 2:1 ratio. On OC, a significant delay for entanglement or getting lost inside a wreck or a cave could be fatal. You could run out of bottom gas, rack up too much deco to clear with your available gas, etc. Gas goes fast and deco accumulates fast at that depth. Now do that same dive on CCR. Say you've planned for 20 minutes bottom time but you end up with 40 thanks to a serious delay on the bottom. Now your hour dive is 2 hours. You may not have enough bailout deco gas to clear deco, but that's only if you have to bailout. A full 3L of O2 will last 6-8 hours depending on how fast you metabolize gas. A scrubber will last as long or longer, depending on the size, water temp, and your exertion level. So your dive has doubled in duration, but if the unit continues to operate normally, you're still only utilizing 1/3 to 1/4 of your resources after the delay. And you're still capable of dealing with additional failures. Let's say you get clear after this delay, only to have issues with dil addition (reg failure, ADV failure, etc). In open water, you don't need it, you're just going to ascend. Shut it down. Onboard O2 fails? Plug in your OC O2 deco gas and drive manually. You have the tools to survive a really bad day on CCR if you know how to use them effectively. So when people say CCRs are more dangerous, they're both right and wrong. They have a higher level of inherent risk, but they turn major issues on OC into minor inconveniences and offer additional tools to get out of tough situations if you're proficient and using the appropriate mindset.

 

[Celt]

Something goes wrong such as losing your buddy, getting lost, arguing with a lobster too long, the current being uncooperative or whatever but not a rebreather failure. Then you just have more deco. There is extra risk in case the rebreather also dies and you don’t enough gas for the ascent but for the case of just taking too long you are good.

I was very conscious of this doing blacked out mask exits from inside a wreck recently. I was quite deep and in a bit of a tizzy but able to stop and think “did the line go through here on the left or the right” for as long as I liked without any pressure to rush things and blunder forwards.

Of course with a twinset etc you ought to plan for plenty of gas, but I like that it is irrelevant.

Okay I see,what kind of bottom times would ye typically get on wrecks in 60 to 70 metres on a rebreather

 

[The Chairman]

I think divers who believe they are prone to narcosis should take measures to lessen it. But some divers do not suffer from it.

I can't trust any diver who thinks they are immune to narcosis. Going on feelings is a fool's errand. You really can't tell how it affects you until it's too late. Hal Watt's book was entitled "Managing Narcosis" for a reason.

I've done some deep air dives in my time, but I don't think this old fat body is up to doing them anymore.

 

[Celt]

Yes, you're limited by bailout, but only if you need to bailout. Take a dive to a bit below 200', for example. You're racking up deco at a 2:1 ratio. On OC, a significant delay for entanglement or getting lost inside a wreck or a cave could be fatal. You could run out of bottom gas, rack up too much deco to clear with your available gas, etc. Gas goes fast and deco accumulates fast at that depth. Now do that same dive on CCR. Say you've planned for 20 minutes bottom time but you end up with 40 thanks to a serious delay on the bottom. Now your hour dive is 2 hours. You may not have enough bailout deco gas to clear deco, but that's only if you have to bailout. A full 3L of O2 will last 6-8 hours depending on how fast you metabolize gas. A scrubber will last as long or longer, depending on the size, water temp, and your exertion level. So your dive has doubled in duration, but if the unit continues to operate normally, you're still only utilizing 1/3 to 1/4 of your resources after the delay. And you're still capable of dealing with additional failures. Let's say you get clear after this delay, only to have issues with dil addition (reg failure, ADV failure, etc). In open water, you don't need it, you're just going to ascend. Shut it down. Onboard O2 fails? Plug in your OC O2 deco gas and drive manually. You have the tools to survive a really bad day on CCR if you know how to use them effectively. So when people say CCRs are more dangerous, they're both right and wrong. They have a higher level of inherent risk, but they turn major issues on OC into minor inconveniences and offer additional tools to get out of tough situations if you're proficient and using the appropriate mindset.

Yep I understand, with 20 min @ 60m I could handle a 5min delay. A 10min delay and I’d have to do some deco on air before switching to 50% that I carry, a 15min delay and I’m definitely bent.

 

[KenGordon]

Okay what kind of bottom times would ye typically get on wrecks in 60 to 70 metres on a rebreather

Personally I am a wuss and would be doing 30 or maybe 35 minutes for a 90 or 100 minute runtime. I am looking at a log of the Leonatus in 57m, that would have taken 4450l of backgas OC by my usual OC planning with 18/45.

The bloke giving me the blacked out mask treatment (not a wuss) was giving a talk recently about a 120m dive with something like 20 minutes bt and a 4 hour run time.

The limiting factor is bailout. By the book you have to assume 45l/minute for a CO2 hit. With an hour of deco to do that is really more that two ali80s. However if you have a 90 minute plan and are delayed 10 minutes on the bottom so you end up with a 120 or 130 minute dive you still have plenty of everything. Clearly a CO2 hit and bail out on such a delayed dive would be bad, but that is two major failures. Worst case might be a co2 inside some wreck and kicking it up so your exit is slow. Don’t do that

These sorts of dives are not terribly expensive on gas, a 3l of 15/50 or so is £10 or £15 of helium and you’d have plenty left for an afternoon bimble. With a twinset you’d be looking at £70 to £100 so much less likely to actually use it.

For me a CCR is a toy, but it does make these gas limited dives much more enjoyable. Checking your ppO2 is less stressful than thinking “bugger I am 10 bar lower than I ought to be, better turn sooner” etc.

 

[Celt]

I can't trust any diver who thinks they are immune to narcosis. Going on feelings is a fool's errand. You really can't tell how it affects you until it's too late. Hal Watt's book was entitled "Managing Narcosis" for a reason.

I've done some deep air dives in my time, but I don't think this old fat body is up to doing them anymore.

Saying you can’t tell how narcosis affects you until it’s to late, is simply wrong.