100m dive

[leadduck]

Gf 60/85, 30mins at 150, 21/35 bottom gas and 100% for deco shows 19mins of stops before you get to 20’. That’s not insignificant.

good example, minimum gas (SAC 20l/min) would be OK for a twin 12l tank with EAN50 deco stage, but not with a 100% deco stage. For MG you need to feed both divers until the first stop.

 

[Beau640]

you're right i just ran the profiles myself. My bad it would be a big difference in MG. However it estimates you would need 44 cuft of 50% if you're doing that profile so a normal alum 40 isn't going to work for a team. I guess if you're willing to carry an 80 instead you could make it work. I mean realistically you can make any combination of back gas and deco gas work, even if they are weird custom mixes, as long as you have the right size tanks and follow the schedule. Still prefer to always have 100% O2 with me though.

 

[Trace Malinowski]

I came across some interesting studies about low dose oxygen in my lifeguard life as related to cardiac care. I wonder if future studies in diving will have us rethinking our oxygen choices like the US Navy Deep Stop Study found us questioning what we thought we knew about deep stops?

 

[RyanT]

I came across some interesting studies about low dose oxygen in my lifeguard life as related to cardiac care.

Can you post a link/copy for them?

 

[Trace Malinowski]

I'm looking for the ones from the UK or some other European country I had in mind. It's been a while, but Google returned these in the search results:

https://pdfs.semanticscholar.org/c505/cbf1c8ff8d5d1029a23bdc004baee8cb890b.pdf

How much oxygen in adult cardiac arrest?

I think the medical community may start looking at oxygen more closely in the future. More studies may either reinforce or call into question how oxygen is prescribed for various emergency or long-term uses. My hunch is the 80% O2 NJ wreck divers will have the last laugh on us 100% O2 Florida cave divers.

 

[Beau640]

If you go back to the originally question though when people were asking about "why not 80%," if you don't believe in carrying 100% with single bottle deco and are okay with 50%, where do you draw the line? Why not 80%? Why not 40%? Why not 60%? You can make any of those work with single bottle.

Decompression theory wise, there is a theoretical benefit to decoing at 1.6 pp02/essentially doing your entire last stops on 100% at 20ft rather than going 20 then 10. If won't change the time on your computer, but efficiency wise it's better (if you believe in the oxygen window) and theoretically would be safer in terms of chance of DCS hit (see Mark Powell Deco for Divers). You can't do that if you don't carry 100% O2

 

[boulderjohn]

If you go back to the originally question though when people were asking about "why not 80%," if you don't believe in carrying 100% with single bottle deco and are okay with 50%, where do you draw the line? Why not 80%? Why not 40%? Why not 60%? You can make any of those work with single bottle.

That's a good question for a tech diving class, In fact, I have started asking it, although not exactly in those terms. I have started asking trimix students to plan a dive with different deco gases, compare the results, and draw a conclusion as to which yields the best results for that dive.

 

[RyanT]

Thanks @Trace Malinowski. It's important to recognize that people who are experiencing cardiac problems experience some important changes in their physiology relative to healthy individuals. So examining the role of oxygen administration in sick versus healthy patients might not tell us very much. With that said, the short-term toxic effects of free radical production/oxidation that occur with the metabolism of oxygen are fairly well known. Our mitochondria have evolved to help control the damaging effects of oxidation during metabolism. But, they evolved to deal with this in a 1 bar world. So perhaps we shouldn't be surprised if we later discover longer-term negative effects of high oxygen exposures. I work with a respiratory therapist and I was telling him about the kind of oxygen exposures that tech divers are often exposed to; he was pretty shocked.

 

[Trace Malinowski]

I think when we look at everything holistically, we have to figure out where the true risk lies on every dive.

On some dives, where a diver may opt for one or two gas switches, carrying two bottles may significantly increase the amount of work a diver must do at depth. Ex. swimming into strong current. Could this harder work lead to a greater chance of bubble formation? If so, then do we lose some benefit of a two gas switch. If so, we have to ask ourselves, "If we ditch one bottle, which one?" Am I more at risk during the deeper portion of the dive? Would I be safer if I figured minimum gas from max depth to the Nx50 switch? Or, is my ascent route so easy that I have no doubt sharing gas to the 20 foot stop would be a breeze and I'm more worried about DCS efficiency?

Two good examples: The wreck of the JB King in Canada is in warm green water with lower visibility in the summer months. It's adrenaline junkie sideways skydiving in a current on steroids. Getting off the bottom safely with updrafts and downdrafts and twisted wreckage places the greater risk in an OOG scenario. I don't want to share gas to 20 feet, It's a long way in the crazy conditions. Meanwhile, 40 miles west in Lake Ontario, the cold water at depth may want you padding your risk in favor of reduced blood flow due to cold water in the 40 F range. Most wreck dives are down a line and up a line so ascent while sharing should be easy. Oxygen might be a better choice die to increased DCS risk.

I plan dives from practical experience.

 

[boulderjohn]

I'm not arguing Ken, but where's the data on gas switches being the biggest cause of death on technical dives. The last 10 tech deaths in the USA had nothing to do with a gas switch, unless you count Carlos, which was refusal to analyze.

I tell my students that probably 90% of the gas switch errors actually happen on shore, and Carlos is a good example of this. We are therefore quite anal about analyzing and marking tanks. Yes, there have been actual gas switch error casualties, but not many.