Nitrogen loading and DCS risk based of depth

[rubixcube]

(Note: I am sure this has already answered somewhere on scubaboard but no matter how I worded it I simply could not find a thread, apologies if this is a commonly discussed topic)

I know that different types of tissue on/off gas at different rates. My question is whether a shallow dive (40') or a deeper dive (140') both made to the NDL's (200 minutes and 10 minutes respectively) will have different levels of DCS risk, deserved or undeserved. Will the longer dive load more varieties of tissue and will the different off gassing rates increase DCS risk? Or is there no real difference in risk between the two?

 

[TSandM]

In theory (and remember, as far as publicly accessible information goes, no one has put probes into organs to see if our assumptions match the reality) at the end of a dive, as you begin your ascent, there is a "controlling compartment". As you ascend, the identity of this compartment may change. For most programs, the first controlling compartment is one which has a very short half-time -- but those compartments also tend to be assumed to tolerate much higher oversaturation values. What this means is that, on ascent from a short, deep dive, you can go up further before you stop, and the compartments that are most saturated are the ones that will offgass fastest.

For a moderate depth, longer dive, you begin to negotiate with compartments which have longer half-times. They take longer to fill up, but longer to empty, and they are thought to tolerate much less oversaturation. The effect of this is that you will have to ascend more slowly and pay more attention to ceilings.

In recreational diving, where gas consumption is a serious limiting factor for dive duration, moderate depth, long dives are probably the most worrisome in terms of DCS.

 

[beertiki]

The longer dive will saturate more compartments. Look at the difference in rep groups and RNT after each dive. The long shallow dive is very risky.

 

[gcbryan]

I would guess (I don't know) that the literal answer to your question is that it doesn't matter. To answer the question one has to have data and there isn't data about undeserved hits or it wouldn't be undeserved. When the NDL "lines" were drawn I would imagine that they would be drawn so there is the same risk of DCS for each depth as well.

The practical answer and the way I conduct my own diving is in line with the answer TS&M gave above. Many times someone doing a short but deep dive is also not ascending directly to the surface but rather ascending to a shallower depth to look around for a while. So in many cases those dives have built in "deco stops".

If someone stays at 60 fsw for 50-60 minutes ascending a little isn't nearly as helpful both because of the limited time available and because the gradient is less.

The other practical aspect to this question is that in many cases the surface interval is set (on a dive charter for instance) so the nitrogen loading on a subsequent dive will be higher when the first dive was for a longer but shallower dive than for a deeper shorter dive.

If the surface interval was adjusted to account for that however the risks should be the same.

As mentioned the faster compartments can handle much more supersaturation. It's interesting to look at the model (ZHL-16 for example) and to look at what depth for each compartment one could go to and ascend directly to the surface regardless of bottom time.

For the fastest compartment as I recall it's more than 100 fsw. So that (theoretical) compartment can handle going directly to the surface from 100 fsw even after a stay of an hour/day/week etc. The next faster compartment has it's limit in the 90 fsw range I believe and it progresses through the slower compartments until at the slowest compartment that depth is roughly 30 fsw or so.

Of course even theoretically one compartment can't come to the surface without them all coming to the surface but it's interesting to look at things this way sometimes

 

[rubixcube]

There seems to be some conflict in the answers.

The effect of this is that you will have to ascend more slowly and pay more attention to ceilings.

This makes sense, more nitrogen means being a bit more careful right?

I would guess (I don't know) that the literal answer to your question is that it doesn't matter....When the NDL "lines" were drawn I would imagine that they would be drawn so there is the same risk of DCS for each depth as well.

This also makes sense, after all if both are set as guidelines for avoiding DCS both must have similar levels of risk.

The long shallow dive is very risky.

How risky is "very risky" exactly? Do you mean that there is a high likelihood of DCS? I suppose my question now is if there is a difference in risk between the dives how great is that difference? Is it a negligible difference that simply requires being mindful of ascent rate and stops? Or is it, as beertiki put it "very risky" and of significantly higher danger levels than the deeper dive.

 

[Kiwi303]

My understandign of long shallow dives being dangerous is that there is less depth of pressure gradients, with a deep dive you can come up to a level high enough above the previous pressure to off-gas, but still deep enough you don't fizz over like a shaken coke when you pop the tab. When shallow you have less margin to go up to off-gas, yet still remain low enough to be at pressure enough to avoid oversaturation/fizz/DCS.

You have more flexibility at depth with a partially filled control compartment and more ability to rise slowly, than to be shallow with a completely filled control compartment.

 

[gcbryan]

Long shallow dives within the NDL aren't "very" risky. It just takes longer to off gas when the controlling compartment is a slower compartment. This can affect a second dive but if you wait the appropriate time (surface interval) that is taken care of as well.

As TS&M mentioned running out of gas and having a deco obligation is the main concern since by the time you realize you have that obligation (we're talking about mistakes in planning having to be made for this to happen in the first place in recreational diving) you probably don't have a lot of gas left to fulfill that obligation.

A long shallow 20 fsw dive for instance isn't "risky" at all other than drowning, running out of gas, or freezing to death since you can always go directly to the surface regardless of the length of the dive

Of course it's the longer 60 fsw dive vs the shorter 100 fsw dive that we've been discussing. The shorter 100 fsw dive can come closer to completely off gassing before you get out of the water than is generally the case with a longer 60 fsw dive.

The best of both worlds (to me) is a short deeper start of the dive and a longer shallow end to the dive...100fsw start and 30/20/10 fsw end.

 

[popje]

(Note: I am sure this has already answered somewhere on scubaboard but no matter how I worded it I simply could not find a thread, apologies if this is a commonly discussed topic)

I know that different types of tissue on/off gas at different rates. My question is whether a shallow dive (40') or a deeper dive (140') both made to the NDL's (200 minutes and 10 minutes respectively) will have different levels of DCS risk, deserved or undeserved. Will the longer dive load more varieties of tissue and will the different off gassing rates increase DCS risk? Or is there no real difference in risk between the two?

I think the risk of an actual DCS hit depends more on physiological factors such as age, BMI, dehydration etc rather than depth and time. I would also consider the severity of the hit as opposed to just a DCS risk. It has been shown that deeper dives result in more CNS and neurological DCS whereas the shallower dives more cutaneous type DCS. Now, I would be more concerned about which type of DCS would be more risky to my life!

 

[Mr Carcharodon]

I think the risk of an actual DCS hit depends more on physiological factors such as age, BMI, dehydration etc rather than depth and time. rislife!

Why do you suppose that all tables and dive computers use depth and time and none use age, BMI, dehydration etc?

For the OP from what I've read of Workman and Buhlman I think the risk is the same regardless of depth. Of course what they were doing was counting bends without trying to classify them into groups with different severities of outcome. The previous poster has a good point in that regard.

The bubble models suggest why the risk should be similar. They say the volume of dissolved gas converted in bubbles is a function of the volume of dissolved gas present, the force acting on that gas (pressure gradient) and the length of time the force acts. For shallow dives you can stay longer and get closer to saturation for that depth, but saturation at 40 feet is a lesser volume of gas than at 160 feet. The deeper dives have a shorter time limit but the volume of gas dissolved during the bottom portion of the dive is similar.

 

[ianr33]

Its a really good question.

My belief is that the long,shallow dive carries more risk for the following reasons:

On a short,deep dive only the fast compartments become significantly loaded.Upon surfacing the nitrogen may just be eliminated by the lungs. or,maybe due to exertion,some of it goes "sideways" into a longer half time compartment. No problem as that compartment has plenty of room in it.
Compare that with the long shallow dive. It would be quite possible to surface from that dive with many (all?) compartments heavily loaded. Any nitrogen going "sideways" can cause oversaturation and possibly DCS.

100 feet for 15-20 minutes is a profile done 1000's times a day at 100's of dive resorts every day around the world. Easily done on an 80 with a decent SAC rate.
The PADI NDL for a 40 foot dive is 140 minutes. I've never done that. It can't be done on an 80. It's an extremely unusual dive for which there is probably very little data.

Finally, if you do a good 5 minute safety stop after a 100 feet/20 minute dive that's going to reduce N2 loading significantly. A 5 minute stop after 40 for 140 will do next to nothing for the slow compartments.