How much cushion do you leave..

[TSandM]

My advice is to buy Mark Powell's book, Deco for Divers, and read it. The more you understand about the theory of nitrogen dynamics in the body, the more intelligent your choices underwater can be.

If I am doing a square profile dive, where I'm going to make a direct ascent at the end with nothing to look at, I'm not going to push no-deco limits at all, unless I plan to do it and have the equipment, gas, and thermal units to do some deco on the way up. And yes, I do "deco" on no-deco dives, because all dives involve offgassing, and spending some time in shallow water (and make a VERY slow ascent from my last stop) make me feel better.

If I'm doing a terrain-based dive, where I'm going to meander upslope and have the possibility to spend a lot of time at 30 feet or shallower, looking at something interesting, I'll push the limits a lot further in the deep part of the dive. After all, there is nothing "magical" about going past deco limits; it just means you have to do deco. If you're going to do it anyway, because it's fun, there's not really any reason to be miserly with the bottom time in the early part of the dive.

Of course, if you push limits, you are making some assumptions: One is that you have the gas, after that deep time, to putz around in the shallows for a good while. Another is that you are not going to lose control of your ascent or have buoyancy problems at the end of the dive. If you are a new diver, and not real steady with your ascents yet, it's best not to push ANYTHING, and in fact, to keep dives shallow enough so that gas will limit your dive before decompression does.

 

[rhwestfall]

This is where it becomes "soapbox" time. Planning a dive using tables (or now the fancy graphics based computer programs) had a couple good "visuals" involved. We were taught to back off two pressure groups for various reasons, and had those times right there for planning (or in the case of the computer analysis, the colored NDL graph or tissue loading)... The visual of where it put you in terms of dive time I think really did something for me...

I do like the software DiveNav linked you to.....

This doesn't answer your question directly, but hints at your need for further investigation into dive planning......

Just flying a screen isn't a good practice

 

[Mr Carcharodon]

... as can the amount of sleep you got the night before the dive. ... Bob (Grateful Diver)

How does that work? I guess I do not understand what the mechanism behind this would be. I am not disagreenig just trying to understand and see if there is experimenatal support, or if this is gut feel.

For the OP it is worth understanding that the limits are intended to capture the tail of the curve since of course the goal is to have safe dives for nearly everyone nearly all the time. Gas loading, and unloading, is thought to be driven by perfusion (blood flow) and that in turn is known to depend on exertion and body temperature. Also of course 15 minutes pad at 60 feet and 120 feet is not at all the same thing since different tissues are driving the limits at the different depths. So using a fixed minute pad does not seem resonable. One way to pad is to spend more time decompressing on the way back up. If you are cold or spent a lot of time swimming hard doubling the amount of time spent between 30 feet and the surface makes a lot of sense. And as has already been mentioned you really do want to slow down the last 10 or 15 feet of the ascent.

 

[DivemasterDennis]

Here's a different perspective to consider, answering the poster's initial question. I do not factor in a cushion absent stressful conditions or extreme depth. The recreational dive tables are very conservative, as is the programming in nearly all computers. If you honor the rdp no decompression limits, or those of you computer, you will be fine. So I am with EthanOln on this one- 1 minute for a buffer, because when a computer puts you in deco, it's a pain, and one does report it to the dive boat if diving with a public concession, and they may limit the diving or yell at you. Candidly, as I approach 1000 dives, I can truthfully say this was an issue only once, diving off a liveaboard near St. Kitts. Diving with the liveaboard's "diving director" we got interested in a burr fish and "barely went into deco." But once was enough for me.
DivemasterDennis

 

[Mr Carcharodon]

it's a pain, and one does report it to the dive boat if diving with a public concession, a

You do? That's a new one. Maybe just a local thing, that does not apply to my locality.

 

[MauiScubaSteve]

I havent done too many dives, so maybe im hanging with the wrong crowd, but I have had maybe 3 or 4 divers that seem to be "Ideal" physique-wise

If your batting average is that good this early, I'm seriously recommending moving to a resort area and becoming an instructor.

Anyway, I dive a Suunto Viper; same puc for the last 9 years. Even though it is renowned as conservative, I feel it gives fairly robust first dives. When diving as a guide, I follow my own briefings; don't let the big number in the middle get below 5 min. When diving for fun, I have seen 0 a few times. On dives where I pretty much just sit on a scooter for over an hour, I have had 8 minutes of mandatory, but after dozens of minutes at 40-60' depths that usually goes away.

Here is a dive that followed a boat briefing....

http://www.scubaboard.com/forums/hawaii-ohana/243718-molokini-kayak-dive.html

 

[ScubaBB]

I have a Suunto Gekko. It's on standard setting. Normally I start slowly ascending when it displays 5min NDL and then I keep it on 5 min or more. This always worked fine for me.

 

[Diver0001]

Yeah I don't the idea of that because everyone's nitrogen absorption is different, so the computer may have an NDL that was calibrated by someone who absorbed nitrogen very slowly, and you do not

But that's just me.

I haven't read the other responses yet so forgive me if I'm repeating something.

Black, you have a good point but this is the reason for the safety stop. That's your cushion.

What many divers don't seem to realise is that the safety stop is a MINIMUM of 3 minutes, but there is no maximum. You can extend any stop from 6 metres to the surface indefinitely without incurring additional penalties.

Using the PADI tables, in fact, the bottom time stops accumulating at the bottom when you initiate an uninterrupted ascent to the safety stop. Since "uninterrupted" is a bit hard for some new divers to measure on the fly, I tell students to stop counting bottom time at the safety stop and to take as much time as they want for that. If there's something interesting at 5m then there's no reason to hurry to the surface.

For my own diving (and I'll preface this by saying that this approach is well within the limits of my training and experience), I seldom look at the NDL number on my computer. I do, however, plan my dives such that I know how much deco time I'm going to accumulate during the dive and how I'm going to work it off. I do make at least 1/2 of my dives within the NDL's but if I'm planning on going over, I seldom accumulate more than about 10 minutes of stops and under proper conditions so the time can be worked off normally during the ascent. If I want to go further over the NDL's I do it differently. Personally I don't like dives with more stop time than bottom time so I don't do that. That makes me a guppy compared to some of the hairy chested tekkies here but I would need some really compelling reason to do more than about 30min of stops.

The only time I'm really reluctant to dive over the NDL's is when I'm making a lot of repetitive dives in a short time (like 20+ dives in a week). When I do that I make most of the dives well within the NDL's and spend a LOT of time shallow. Most of my dives on diving vacations are 60-75 min and often times we do 30 min of that time shallow, partly for the "cushion", and partly because the most interesting stuff can often be found in the first 10 metres.

R..

 

[Diver0001]

How does that work? I guess I do not understand what the mechanism behind this would be. I am not disagreenig just trying to understand and see if there is experimenatal support, or if this is gut feel.

.

Any physiological process than can affect quality of respiration or circulation of blood in your body will have an effect on decompression.

From what I've been told, dehydration, working hard on the bottom (or harder than normal, which may be caused by fatigue) and lifting heavy weight after the dive are big factors that can affect your body negatively with respect to decompression.

R..

 

[vladimir]

Another way you can increase the margin of safety against DCS, so far unmentioned, is to increase your surface interval between dives. I always have at least an hour between dives and more for deeper dives, or ones with any exertion, ragged ascent, or anything else that might enhance risk. And I monitor my body's response to the dive--any sense of fatigue and I extend the interval or skip a dive or two. A single dive within NDLs already has a huge cushion, assuming proper ascent procedure.