Scenario Ascending fromDepth

[Charlie99]

not going on with a bunch of complaints saying that you shouldn't be in this situation:

I think your question can be rephrased as "What is the best use of a less than appropriate amount of gas?"

If you play with GAP or any other deco software, there is a typical deco curve. More time is always spent shallow. In addition, if you do run out of gas, it is a lot better to be shallow than deep.

The problem with a very rapid ascent from 140' all the way to 15' or to the surface is that this sort of rapid ascent seems to cause serious neurological problems / Type II DCS.

Somehow you need to achieve a balance.

My vote would be for 60fpm or even 120fpm up to 50', then a one minute stop at 50'. That gets your ascent rate under control. The 50' stop gives your fast tissues a chance to offload a bit. And you are within easy ESA distance of the surface.

At that point, reanalyze the situation, slow your breathing and plan the rest of your abbreviated stops.


DAN Europe studies have shown that instantaneous ascent rate is NOT correlated with DCS, while total ascent time is.

Clearly, you should be using both your gas AND your buddies. Obviously, it is better to avoid this sort of situation, as I'm sure others will tell you.

This is free advice. It's worth what you paid for it.

 

[zeN||]

Tha'ts along the line I was thinking, better to make a quick ascent to say 60 feet, where outgassing is going to be minimal, then use your last remaining air to get a small deco in the last atmospheres.
But I have to chuckle, divers are so mired in the dogma of the dos, don'ts they just can't discuss diving scenarios without preaching-go to church if you want to be a preacher! But thanx for the informative section of your posts zeN

 

[H2OHead]

While we’re all using our imagination, let’s say I’m at 140’ on an old Al65 tank, no spg, and a reg with no mouthpiece when my cell phone rings and it’s the wife calling to tell me I just won the lottery. Should I hang up and ditch all of my gear since it’s all junk anyway and make a b-line for the convenience store or have her call the shop I got gas at to see how much he put in the tank? Why are we always out of air in the what-ifs? I’m a little brighter than I look and manage to actually plan my dive and dive my plan (I heard that somewhere) all while watching my spg (kind of like walking and chewing gum). Of course, I may just be lucky.

 

[saying]

I'm going to make sure I always dive with divers to whom accidents requiring hypothetical solutions being hammered out beforehand never occur. Seems I'll always make it to the surface safely.

Of course the only thing I'll ever see will likely be my spg and watch.

 

[zeN||]

I'll take a diver who is independent and smart enough to forgo this petty preaching and has worked thru potential problems and is not ignorant enough to think that if he keeps on diving according to his 'plan' he is going to outsmart Mr. Murphy zeN||:mean:

 

[Dryglove]

I have never run low on air or had an OOA at a depth of 140ft. Hell the deepest i have gone is 120ft. If your gonna dive that deep it is imperative to have a good dive plan using the rule of thirds. The rule of 1/3 is to use 1/3 to your destination and 1/3 from your destination back to the surface. The other 1/3 is reserved for emergencies. The first person to reach 1/3 of your gas supply signals the other to turn around and start heading back using the other 1/3. You should return to the surface with 1/3 of your gas supply left. It may seem like a waste of good air but is very important in case of an OOA situation.The problem with diving to a depth of 140ft especially with deco obligations is that you may not have a second chance to make it to the surface so it is important to plan your deep dive properly in case mr. murphy shows his ugly face. With proper planning and good buddy skills you can defeat mr.murphy in all but the most serious situations.

There are probably other ways to dive planning but this is the method i was taught by my instuctor for deep diving.I should also say that i have never done a planned deco dive and stay within the NDL limits when dive planning for deep dives.Plan your dive and dive your plan.

Forgot to mention that extra 1/3 can be real handy at monastery beach for getting back through the surf on to the beach at times

 

[Cave Diver]

zeN|| once bubbled...
This is a hypothetical case I'm working on: say you push your limits to 140 feet, whether you are solo or partnered up is irrelevant for this scenario, you are alone, or you can't get to your partner Gas is air. I can't be as particular about some details because I haven't done the math just yet. lets say your total dive time is c. 30-45 minutes, you will need a deco stop, you have several hundred pounds in your tank-would you attempt to ascend to say, 60 feet quickly, knowing the pressure changes between 140 ft and 60 ft will be less than the last 2 atmospheres, and ascend the last 60 feet slower, with a small deco stop, to allow outgassing- or would you stick to the book and ascend at 1 ft/sec , which in my scenario would leave you no time for a deco stop? This is a purely hypothetical case. Take a slow deep breath, (getting your pantys in a bunch over what you did wrong or what you should have done different is not allowed (it's hypothetical)); this is about dealing with an event you could find yourself in, and what choice you would make) Thanks for replies) zeN||

(no I actually don't reccomend solo-diving but I recognize some may accept the additional risks)

A 30 minute dive at 140' on air would incur somewhere just over an hour of deco obligation, with your first stop occuring in the 70' range. Based on my consumption rate, I would use 88 cft of air while still at depth before ever starting my ascent. Therefore, using a standard AL80 (this is a recreational dive after all) I would run out of air before ever leaving the bottom.

Here's a little more realistic scenario: 140' for 20 minutes on air, using a single AL80.

Based on my working SAC rate I would use 60cft of air during this dive. Suddenly at the 20 minute mark, I look at my gauge and it's only showing that I have 750 psi left to complete my dive and get to the surface. :wacko: OH SPIT!!!

Due to my depth and time, I have an estimated 40 minutes of deco to complete and only 20 cft of air left to do it with. I would start a controlled ascent to 70'. Since I have roughly 700 psi of air left, and I am at 70', I would try to do deco stops at 10' intervals for 100 psi.

So I would start my 70' stop at 700 psi, and move to 60' @ 600
50' @ 500, etc.

Using this method I should be staying progressively (tho only slightly) longer at each stop, as you use less air as you get shallower.

This should have me hitting my 20' stop with 200 psi of air remaining. I would stay at my 20' stop until I actually ran out of air, then make a direct ascent to the surface. Again, based on my SAC rate, I should be able to spend at least 20 minutes at 20' before running out of air. From 20' almost anyone should be able to make a controlled ascent on a single breath of air, and I think this method would offer me the least risk of taking a hit than any other scenario I could come up with, without getting additional air from another source.

DISCLAIMER: This example is for illustrative purposes in the course of discussion only. This in no way implies an acceptable method of diving or figuring decompression obligations. Attempting to dive beyond recreational limits without the proper training greatly increases your chances of DCI.

 

[Dryglove]

Then once back on the surface start looking for signs of DCS and get your DAN card handy along with the O2. With the way my luck runs i would be one bent dude......lol.

 

[zeN||]

I was referring to 30-45 min of total dive time, not bottom time at 140 feet.
Try this on: You are diving a new site with your buddy (because No one should ever solo dive), you plan to dive to 130 feet for the max no deco limit. Viz is getting poor, you feel the current increase. Lo and behold, you lose your buddy (But damm, I was following my dive plan?). You start searching for your buddy, circling your position. In the momentarily frantic moment, you check your dive computer, your alarm is sounding, lights flashing, you have earned a penalty by descending below rec dive limits, and several minutes beyond your dive plan and you are now in deco mode. (Shyte-but I was following my dive plan!)
You start to make your ascent-lo and behold, Mr. Murphy is punishing you without mercy-you spy a large ____ shark circling you (fill in your personal fave, the one that makes you pee in your wetsuit). You decide to hang toward the bottom. 10 minutes go by, but they feel like hours. You start your ascent. HE'S STILL OUT THERE. But up you go. Your anxiety increases, which increases your rate of gas consumption. You don't have enough gas for your DIVE PLAN! Hence the hypothetical example zeN||

 

[Iruka]

realizing the questioner is probably aware of the means to AVOID that situation. That being said, I'd rather make a fast ascent than drown, and rather make the "slowest" part of the ascent the last 20' or so. I have enough dives to have a pretty good "feel" for how long I could stretch out the remaining air, but it's late (here in Guam) and I'm too tired to elaborate on how many minutes at such & such depth. I DO know that I'd try to keep that last 20' ascent slow, and suck out every last molecule (well, I know you can't make a vacuum in your tank, even at the surface, but I'd sure as hell try!) of air to prolong that last ascent.
(4400+ dives & no OOA emergencies....but then, if it's a depth I can comfortably free dive to, I don't consider a voluntary low air situation to be an emergency)(voluntary means I've done my safety stop an am just enjoying watching the fish & stuff until I have to get back on the boat...since I'd have no trouble ascending WITHOUT gear, I'm not too concerned about having a bunch of air left to dry my dustcap or destroy boat captains' hearing.)