OOA assessment time in gas planning

[2airishuman]

I've been doing some gas planning exercises, calculating the maximum bottom time for various scenarios.

In calculating rock bottom, the assessment time in an ooa emergency can be the major driver in the amount of reserve air required, particularly as depths increase. With two divers, elevated air consumption rates due to stress, and a presumption that the emergency takes place at the deepest point in the dive, air is consumed rapidly.

There's more than assessment going on during this time, of course. It includes identification of the emergency, switching regulators, regaining physiological and mental control, and dealing with any other aspects of the emergency that must be addressed before ascent.

I've seen times from one to three minutes used, although some packaged OOA contingency products (e.g. Spare Air) are sized such that the underlying assumption must be that the assessment time is zero.

Reading through accident narratives, the OOA situations that result in fatalities are often complicated by other things going wrong at the same time: entanglement, medical problems, equipment problems, trauma.

What assessment time do you use in your planning? Why?

 

[tomfcrist]

I usually expect 1-2 minutes. An immediate ascent while doing all those things is possible if solo and nothing is holding you down there( overhead, entanglement etc. I almost always have twice as much gas as I need for a given dive when solo, so time doesn't become a critical issue for me. Worst case scenario for me would be a manifold failure, and being too deep to make it to my deep deco gas MOD prior to "being forced" to switch. That is why I don't do really deep stuff without a teammate....preferably one that brings as much excess back gas as I do.

 

[dumpsterDiver]

ZERO time. I freedive, when it is time to go up, it is time to go up.

When you engage in recreational scuba, you are generally planning for one and only one failure during an incident/accident. If you begin to plan for more than one independent accident/failure then it starts to become too complicated.

So.. don't plan on a diver running out of air AND being tangled in a giant net from which they need to be cut out - AFTER all their air has been expended. What if their regulator fails, an earthquake causes them to lose their mask and they get a reverse squeeze on the way up and they can not ascend at a reasonable pace?

It is great to make plans and prepare for emergencies, but sometimes it just doesn't make sense to plan for a raging forest fire to erupt in the middle of the night - every time you set up your tent in the forest.

 

[uncfnp]

You use the time appropriate to you and your dive. I plan for solo recreational dives with near zero chance of entanglement. I use time to assess, switch to redundent if needed and preferably shoot a bag. I count about a minute.

 

[Jim Lapenta]

Depends on the dive. Simple recreational dives with known buddies? 1 minute. Unknown buddies? 2 minutes.

Just a note, IMO a spare air is not a reliable viable resource for any OOA situation unless you're in a swimming pool. Even then I wouldn't trust it. It's an escape tool. When your helo ditches and the water starts to fill the cockpit you take it and stick in it your mouth as you are punching the release on your harness.

 

[dumpsterDiver]

I'm sure most people will disagree with my assumption of just ascending immediately, but the question really IS important.

The time you spend dealing with a problem on the bottom, while the victim is close to panic and breathing fast, will make a big difference in the calculation. You can do the math, but 30 seconds on the bottom at 100 feet breathing like crazy is probably equivalent to 5 minutes on a safety stop, when everyone has regained their composure.

 

[lerpy]

It is great that you are taking gas planning seriously, many early in their diving ( including me ) do not recognize the importance of gas planning and that includes contingency planning. Considering these things will potential help you out someday when things go sideways.

You hit on that you need to consider time, and of course your consumption rate is going to increase under stress. When I was doing some planning in a course I think I ended up saying I would double my SAC rate in the initial moments of dealing with a situation, this will hopefully drop back as you manage and calm down, but it could remain high until you exit the water as you have dealt with a situation, and now you are ending your dive under stress, and now you are making your assent under stress, possibly continuing to deal with a situation.

Although ideally only one things goes wrong and you end your dive and surface, but the there is always potential for additional issues, the domino affect can put a diver in serious trouble starting with something minor, so managing your situation is critical, and knowing you have enough gas to deal with situations is the first step in managing it and the stress. The rule of thirds used in cave diving is not a bad rule even in recreational diving, it means you should always have a decent reserve to deal with incidents. When I dive recreationally on boats in the south and I hear dive master say turn your dive at 700lbs, that concerns me a little. Depending on depth that is not a lot of air to get back to the surface especially if something goes wrong. Think of this scenario, AL80, 90 foot of depth, you turn at 700lbs, and that's when your buddy goes OOA, now you are sharing 700lbs in a stress situation at 90 feet. I make it habit to never let my air drop below 1000lbs before decent, at 1000lbs I am at the line and starting up, unless very shallow.

As for Spare Airs, I have seen them a lot on boats, and I always feel they are potentially a false sense of security, many with argue. Here is my feeling on them, the largest one is 3cu/ft, and the company says you can get 57 breaths out of it at the surface. I will assume that is at the surface, not in gear, not under stress. Now take your 3cu/ft down 3 atmospheres, so you are at 1/4 the volume, have an out of air situation and the stress that it will add. Your not going to have many breaths on a spare air.

I personal would recommend a completely redundant system, such as a AL40 with reg as your back up, and when I was diving singles, that is what I would carry for my contingency.

As well I would recommend getting a good book, might be more a tec book, that will discuss gas planning. I just finished reading Steve Lewis' book, Staying Alive, and it was great for going through a lot of aspects of dive planning and contingency planning to keep you safe.

 

[decompression]

Most training agencies that teach Rock bottom use 1 minute for the emergency, they also advocate the use of a team, well trained. It's a good baseline.


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[DevonDiver]

2 minutes is adequate. Any more than that and I'd be grabbing them by the scruff of the neck anyway..

 

[boat sju]

Think of this scenario, AL80, 90 foot of depth, you turn at 700lbs, and that's when your buddy goes OOA, now you are sharing 700lbs in a stress situation at 90 feet. I make it habit to never let my air drop below 1000lbs before decent, at 1000lbs I am at the line and starting up, unless very shallow.

This is even a violation of the most simple rock bottom rule of thumb calculations which say 80 ft max depth on an 80 cuft tank and leave the bottom at (90x10 plus 300) 1200 psi. And for a beginner I would add 500 instead of the 300.