Faith diving and magic?

[fdog]

The OP requested this be a learning experience; in that vein I'll add this in the hopes it opens someone's eyes.

Folks who dive a lot are familiar with Rock Bottom. For those not familiar with it, the bucket of ice water from seeing these numbers is often enough to change diving habits for the better:

This is using the most common definition for Rock Bottom: Gas needed to get 2 divers to the surface in an emergency.

The assumptions are:

• 30 fpm ascent rate
• Emergency RVM of 1 cu. ft. per minute per diver
• Air share
• 1 minute on the bottom to start the air share
• Aluminum 80

Min Deco is built around...well...Min Deco, and typical field calculations in your head.



Recreational is built around the following profile:

• Emergency
• Air share (1 minute)
• 30 fpm ascent to 15 fsw
• 3 minutes @ 15 fsw
• 30 fpm to surface

Hope this helps.


All the best, James

 

[MauiScubaSteve]

I think a 60 feet per minute ascent rate is appropriate for emergencies, at the very least below say 50 feet deep, and if the recreational diver is making "no stop" dives the 3 minute SS is not really necessary in an emergency, IMHO.

 

[lowviz]

Back again to the original question: How much do I count on my dive computer?...........................Before I enter the water I have a "max depth" and "max time" decided upon, that is more conservative than what my computer allows. I refer to my computer, but also keep an eye on my analog dive watch as well............................

If it is more conservative than your computer, you are diving tables with a computer as backup. Do you really end the dive before your computer tells you to?

If so, you are diving tables with a computer as backup. My guess is you aren't. Been there myself for quite a while. Started to wonder why I always calculated tables then ignored them because my computer said I'm still OK. At that point, the calculations became completely useless. Now I calculate the worst possible dive that gets me out in the nick of time. If my computer craps, I have something to follow that is a whole lot more useful than "you should be on the surface now".

The OP requested this be a learning experience; in that vein I'll add this in the hopes it opens someone's eyes.

Folks who dive a lot are familiar with Rock Bottom. ..............

This is the fundamental essence of the buddy system. Say what you want, you need to reserve a minimum amount of gas to save both if either of you suffers a total loss of gas. Rock Bottom represents a level of dive awareness that includes your buddy.

I like to know three things: my dive computer, Rock Bottom, and when the last horse leaves Dodge.

 

[MXGratefulDiver]

BTW, if it's and consolation to your wife, most of my dive buddies have sucked surface O2 multiple times. Nothing to be embarrassed about. Just being cautious.

Really?! I've only seen that a couple of times, before shipping them to the chambers. Ran out both times too.

Breathing oxygen on the boat is rather common among some technical divers ... and there is no stigma to it whatsoever. The O2 you're breathing is what remains in your deco bottle ... because if you've got it, why waste it ... and it can help "clean" your system of excess N2.

Recreational diving mentality is different ... perhaps because most recreational divers don't really have an O2 bottle readily accessible except for the one in the green case, which is there for emergencies.

I think a 60 feet per minute ascent rate is appropriate for emergencies, at the very least below say 50 feet deep, and if the recreational diver is making "no stop" dives the 3 minute SS is not really necessary in an emergency, IMHO.

This thought process seems to me to be potentially more harmful than helpful, in that if your emergency is that you ran out of air, that implies you were pushing your NDL. In most cases, that means your fast tissues are going to be on the high end of saturation. Increasing your ascent rate AND skipping a safety stop may be a great way to get out of the water with the minimal amount of gas usage ... but you're pushing the risks of a Type II hit pretty high.

I'd say you really have to base your ascent rate on the amount of available gas at that point, and don't push your risks any higher than you have to. If you have the gas, keep your ascent rate within recommended limits, and do your safety stop. If you don't ... well ... even a Type II hit is better than drowning ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

This is the fundamental essence of the buddy system. Say what you want, you need to reserve a minimum amount of gas to save both if either of you suffers a total loss of gas. Rock Bottom represents a level of dive awareness that includes your buddy.

I like to know three things: my dive computer, Rock Bottom, and when the last horse leaves Dodge.

... this is where the buddy system typically breaks down ... if you just ran out of air because you overstayed your welcome, how much air do you think your buddy's gonna have ???

... Bob (Grateful Diver)

 

[Bubbledup]

... this is where the buddy system typically breaks down ... if you just ran out of air because you overstayed your welcome, how much air do you think your buddy's gonna have ???

Good point; I don't think there is any valid excuse for running out of air.

 

[lowviz]

... this is where the buddy system typically breaks down ... if you just ran out of air because you overstayed your welcome, how much air do you think your buddy's gonna have ???

... Bob (Grateful Diver)

No argument at all.

You enter the water with a dive plan and follow it with your computer (or bottom timer and tables) and constant awareness of Rock Bottom. If something starts to go wrong, a glance at where the gory end resides will help prioritize your available choices, nothing more.

If a diver is likely to go OOA from inattention, he should probably skip all of the dive planning too as it is just a waste of time and mental effort.

 

[LeadTurn_SD]

If it is more conservative than your computer, you are diving tables with a computer as backup. Do you really end the dive before your computer tells you to?

If so, you are diving tables with a computer as backup. My guess is you aren't. Been there myself for quite a while. Started to wonder why I always calculated tables then ignored them because my computer said I'm still OK. At that point, the calculations became completely useless. Now I calculate the worst possible dive that gets me out in the nick of time. If my computer craps, I have something to follow that is a whole lot more useful than "you should be on the surface now".



.......

I like to know three things: my dive computer, Rock Bottom, and when the last horse leaves Dodge.

Hi lowviz,

Good questions. The majority of my dives are "gas-limited", not NDL-limited. So really, I could dive them with a good depth guage and SPG and be fine

What I meant was I use the tables as a "sanity check" when looking at what my computer tells me at depth.... I know what the "square profile" max-time-at-max-depth is before I enter the water, and stay well away from that limit at max depth... but that does not mean I have "shortened" the dive, it just results in very short stays at max depth.... I believe on a recent dive max depth was 89 feet, total dive time 1 hour and 12 minutes..... obviously a multilevel dive with lots of time admiring the fishies in very shallow reef at the end of the dive and just a few minutes at max depth.

So while it sounds like a contradiction, I'm not ignoring the tables, or riding the NDL-curve on the computer.

Best wishes.

 

[lowviz]

Hi lowviz,

Good questions. The majority of my dives are "gas-limited", not NDL-limited. So really, I could dive them with a good depth guage and SPG and be fine

What I meant was I use the tables as a "sanity check" when looking at what my computer tells me at depth.... I know what the "square profile" max-time-at-max-depth is before I enter the water, and stay well away from that limit at max depth... but that does not mean I have "shortened" dive, it just results in very short stays at max depth.... I believe on a recent dive max depth was 89 feet, total dive time 1 hour and 12 minutes..... obviously a multilevel dive with lots of time admiring the fishies in very shallow reef at the end of the dive and just a few minutes at max depth.

So while it sounds like a contradiction, I'm not ignoring the tables, or riding the NDL-curve on the computer.

Best wishes.

OK, now I understand. It is information you like to have with you during the dive.

Exactly the same as me wanting to know the very last moment that I can escape undrowned or unbent. I'm never planning to use that information, but plans go to pieces at times. So basically, you're saying that if your computer tells you something stupid you have something to compare "stupid" to. You compare "stupid" to a square profile that is diveable, I compare "stupid" to the worst dive that I can survive.

Two different ways of staying on course, what's the diff?

Best wishes.

 

[D_B]

I found something in a friends dive bag that got me to thinking about Tables, Deco, and Planning
It is a well worn Navy Deco Table printed on vinyl with a snap link attached .. I know that she used to have deco times and contingency deco times written on her slate, so I know that was not used for that , but it looked to me like a contingency, contingency table, for after a shat hits fan scenario that fall outside of all the ones she had printed up .. so I asked, and that's exactly what it was for ...

I like that

for my purely recreationl depths diving I use Lamont's Rock Bottom "rule of thumb" ...easy to use/remember/do calculations on the fly, ... It's nothing for me to recalculate Rock Bottom if my depth changes from my planned depth