Decompression issues

[lacdenis]

Hi,

I am a relatively new diver so I figured rather than answering a question for my friend I would turn to people with more experience. He is going diving in a lake this Friday. He plans to start diving at 8:00am and stop diving by 6:00pm (a total time of 5 1/2 hours under water). The lake is 30' deep. He is planning on flying on Saturday at 10:00am. I consulted the charts and I know the recommended wait time between diving and flying of 24 hours, but I have broken that rule on shallow dives myself. I told him that I thought it would be okay, but that I would consult someone more experienced. Any thoughts?

 

[Dirty-Dog]

There are good reasons why that recommended NoFly time exists. Can you get away with ignoring it? Sure. Sometimes. You can also put one round in a .357 magnum, spin the cylinder, slap it closed, put the barrel to your head and pull the trigger and not get hurt. Sometimes.

 

[pasley]

Frankly, I think someone is doing what we call trolling here. I sincerly doubt your "friend" would get anywhere near 5.5 hours of time underwater. But what the heck, I will play along.

Dirty Dog about summs it up. The suggested dive profile is gambling plain and simple. True it is a shallow dive of 30 feet or less and at that depth you can stay down a very long time, but not that long. Assuming even in warm water you would A) Prune up like crazy and B) very likely get cold and C) yes you still take on nitrogen. To accomplish 5.5 hours underwater between 08:00 am and 6:00 p.m., a period of 10 hours, The dives would have to be just under 1 hour long with 1 hour surface intervals (the reommended minimum SI). A very agressive schedule. Lets see how that plays. Dives less than 40 feet are counted as 40 feet (NAUI, 35 Feet for PADI) Assuming no dives in the previous 24 hours. Dive 1, 60 min 30 feet= Group G (NAUI tables) SI 1hr= Group F, Dive 2 60 min 60 feet plus RNT 61=121 TNT= Group K. Dive 3 SI 1 hour= Group I, Max adjusted dive time is 29 minutes STOP, already this misison is a fail. After dive 3 you are off the dive table reservation.

The rules were established based on studies of past incidents, medical research and science. Even following the rule people sometimes get bent. The Rule is simple, 1 dive to 60 feet or less, no fly 12 hours (24 hrs recommended), 2 or more dives, no fly for 24 hours. Short answer is why risk it? What is the point?

So to sum it up, the plan as stted by the OP is to exceed the limits of the dive table and to break the 24 hour multi-dive no fly rule. Hey what could possibly go wrong? And heeeers your sign.:fail:

 

[lacdenis]

Well, the situation is that he is doing four 1 hour and 15 minuteish dives over the span of 12 hours. The dives are only to 30' and not to 60'. The flight is 16 hours later. I know the recommendation for multiple dives, but you can pretty much dive without stopping for a freakin' eternity at 30'. As such, you can almost categorize this as one dive. I'm not "trolling" here. I am serious. I don't see the fault in this logic. I would totally understand if it were at deeper depths, but 30' doesn't even register on my dive watch at all! There is no max time according to it. As such, classifying it as one dive or multiple dives seems pointless. Also, no decompression stop is needed. In addition, at that depth there seems to be no real limit. Am I just totally missing the point here? I'm a new diver and maybe I have the wrong opinion on this. I don't know. I know he is dead set on going either way regardless of what I say, but I just don't see compelling evidence that he really shouldn't go. Studies that I've read show the incidence of issue for flying on multiple dives between 12-24 hours is .004%. I'd like to think that the majority of those issues happens to be with individuals diving deeper than 30'. Oh, and suggesting you won't get that kind of bottom time on 4 110 tanks is crazy at 30'. I think a 300lb man could get that amount of bottom time at that depth. When I do shark tooth diving in Florida I hit 30' for 1 1/2 hours and have 500-600 left when I surface.

 

[Dirty-Dog]

Well, the situation is that he is doing four 1 hour and 15 minuteish dives over the span of 12 hours. The dives are only to 30' and not to 60'. The flight is 16 hours later. I know the recommendation for multiple dives, but you can pretty much dive without stopping for a freakin' eternity at 30'.

Irrelevant, since you're not coming up from 30' to 0', you're going to 8000 feet (assuming the plane is pressurized).

As such, you can almost categorize this as one dive. I'm not "trolling" here. I am serious. I don't see the fault in this logic. I would totally understand if it were at deeper depths, but 30' doesn't even register on my dive watch at all! There is no max time according to it. As such, classifying it as one dive or multiple dives seems pointless. Also, no decompression stop is needed. In addition, at that depth there seems to be no real limit. Am I just totally missing the point here? I'm a new diver and maybe I have the wrong opinion on this. I don't know. I know he is dead set on going either way regardless of what I say, but I just don't see compelling evidence that he really shouldn't go. Studies that I've read show the incidence of issue for flying on multiple dives between 12-24 hours is .004%. I'd like to think that the majority of those issues happens to be with individuals diving deeper than 30'.

You'd like to think it, but you don't actually have any reason to think it. Your friend is about to do something stupid. With less than 24 dives, you're saying that because you don't understand why these limits are in place (feel free to dig through the research, if you'd like), the limits don't need to be followed.

Maybe you'd like two bullets for that .357 magnum? One for each of you?

Oh, and suggesting you won't get that kind of bottom time on 4 110 tanks is crazy at 30'. I think a 300lb man could get that amount of bottom time at that depth. When I do shark tooth diving in Florida I hit 30' for 1 1/2 hours and have 500-600 left when I surface.

So you come up empty after 90 minutes. You DO know that 500PSI is considered empty, by most, right? Or are you also in the habit of draining tanks lower than that before you head form the airport?

 

[smellzlikefish]

I just worked out the proposed profiles on NOAA's air no-deco dive table. I then plugged that letter in to their ascent to altitude table to see if there was some wiggle room around the 24 hour rule. I assumed that he was doing 4x1:15 dives with 1:15 surface intervals at 40 feet because as you already mentioned, 30' isn't an option. Admittedly, I was rushing it a bit and my fingers have a tendency to wander whilst going round and round those tables, but your friend is going to come up from dive #4 as an "N" diver which requires 24 hours at sea level before ascent to 8000 feet (the pressure of a commercial aircraft). I am well known in my circle for pushing the limits and diving more liberally than most, but I would not do the dives your friend is proposing. Why is (s)he so hellbent on doing these dives anyway?

 

[pasley]

... Am I just totally missing the point here?

Yes you are.

... I'm a new diver and maybe I have the wrong opinion on this.

Yes you do. Go get your dive tables, re-read the chapter in your training manual on dive tables and repetive dives. Dives under 40 feet (NAUI) or 35 Feet are counted as dives to 40 or 35 feet respectivly and do have limits. Do the math and then come back and tell us how this is within the limits of his training and certification agency guidelines. Remember in SCUBA we round up, not down for safety when it comes to depth and bottom time and round down for remaining air (1950PSI is 1900 PSI)

...I'd like to think that the majority of those issues happens ...

Key word majority. That means there are people (the miniorty) that stuff happens to. Example, my daughter, who had been married 2 years at the time and going to college, was on the pill. The pill is 99.9% effective. That means there is a 0.1% chance of getting pregnant. That would put my daughter in the minority as the majority do not get pregnant, but she did get pregnant on the pill and is now a mother. So majority does not cut it, stuff happens to the minority and if you happen to the the minority, life can be cut short.

Oh, and suggesting you won't get that kind of bottom time on 4 110 tanks is crazy at 30'.

Not suggesting he can't get 1 hour 15 on a dive. I have done 90 miutes or more on AL80 on dives that went deeper than 40 feet. I am suggestesting that he might get bored (lake diving) and chilled and maybe tired and maybe actually remember his certification training and abordt after 3 or 4 dives. Frankly I think your friend is pulling your leg. No, you can't stop him, nor are you responsible for his decisions. Sometimes you have to remember that you can't fix stupid and the ocean does clean out the gene pool from time to time. You can do the right thing and not let him talk you into something stupid like joining him.

True these are shallow depths and if he is diving a dive computer he will be allowed more time under because he will be charged with his actual depth and time (guessing he is at less than 20 feet most of the dive) and the computer may allow it. But that still leaves the no-fly rule. Can he do it? Maybe. Is it smart? No, it is not. You have to weigh the potential cost. Unlikely as it may be, but there is the potential for injury going from -30 feet to +8000 feet in less than 24 hours. So what is the worse that could happen? Could range from nothing to skin bends to, paralysis for life or death. Just make sure he remembers or his next of kin knows that 1: he was trained not to do what he did 2: he intentionaly ignord his training 3:He knew the risk.

Bottom line, each of us make and are responsible for our own dive decisions. Would I do 2 dives to 30 feet with 1 hour 30 minutes between them and then fly in 16 hours? No, I would not. But then I am old and there is a reason I have survived this long.

 

[LowDrag]

What about the altitude of the lake? Is at sea level? If not, doesn't that have to be added into the equation as well? I am not up on altitude diving but I do know that when I did my lake dive, my computer turned on at about 3000' IIRC and said I had a 13 hour desat time. Another thing I don't understand is you come on here asking for advice from people who know better than us "new divers" and then balk at what they say. I think you should go back and re-read your OW manual and knock some sense into your friend...Just a thought. :blessing:

 

[divebunnie]

Hi,

I am a relatively new diver so I figured rather than answering a question for my friend I would turn to people with more experience. He is going diving in a lake this Friday. He plans to start diving at 8:00am and stop diving by 6:00pm (a total time of 5 1/2 hours under water). The lake is 30' deep. He is planning on flying on Saturday at 10:00am. I consulted the charts and I know the recommended wait time between diving and flying of 24 hours, but I have broken that rule on shallow dives myself. I told him that I thought it would be okay, but that I would consult someone more experienced. Any thoughts?

The problem with long shallow dives is that the slower tissues start to really get laden with nitrogen and these tissues take that much longer to release that nitrogen afterwards. Hence you can become a Z diver on the PADI tables only on longer, shallow dives as opposed to deeper dives, which are so short that the slower tissues will have only absorbed minimal levels of nitrogen.

It is quite possible that you can do several long, shallow dives with short surface intervals according to the PADI tables which actually do exceed the maximum allowable levels of microbubbles, hence they introduced the "w,x,y,z" rules which require you to do a minimum of one hour (if you find yourself in pressure group w or x) or three hour (y or z) surface intervals if you are planning on three or more dives in the day.

Yes the minimum fly time on multiple dives is 18 hours, however we always teach to work well within all limits, and when I last looked, 16 hours is actually less than 18, so your friend is not only planning an aggressive day's diving, they are also planning to ignore the minimum surface interval. You say the dives are shallow, but as I said, these are the dives where you can load dangerously large amounts of nitrogen if the rules are not followed.

Good luck to your friend...

 

[lacdenis]

I was thinking of joining him, but you guys have definitely changed my mind. I tend to bend rules as I know that charts tend to be conservative. That said, I've seen some accidents and only a few weeks ago in Venice Florida (also shallow dives for me) a man died from what we believe to be a heartattack under water. I spoke to my friend and provided some of the examples you have put here and he has decided to forgo the last dive which will squeeze him into the 18 hour range and also cut the amount of time he has been underwater.

Per the other divers question, he is diving close to sea level (less than 100' difference).