Is there a Nitrogen Loading Equilibrium Point?

At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing, and are there variable factors that have relevance. Can this depth be incorporated for planning in extended diving i.e. liveaboards where doing 5 tanks in a day isn't unheard of.-Thanks

Some could say that it's 33 fsw. Because that is where the pressure gradient is at a 2:1 ratio, and there is no - NDL for less than 33 feet. However. Offgassing will occur at deeper depths, depending on how much nitrogen you've accumulated, and the depths at which you've accumulated them.

However... I think what you're asking is - is it safe to do 5 dives a day on air? In short. Yeah. You can do that no problem. Do you have a dive computer?

Hillmorton Scubie:

At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing, and are there variable factors that have relevance. Can this depth be incorporated for planning in extended diving i.e. liveaboards where doing 5 tanks in a day isn't unheard of.-Thanks

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I think you're confusing some concepts here...

You're at equilibrium at the surface - your tissues are completely saturated. When you go to 5m you'll be ongassing, but in practise the rate will be such that it won't require you to decompress when surfacing from any exposure allowed by scuba. You'll be offgassing at 5m only if you're ascending from deeper.

Most tables and models will allow you stay almost indefinitely above 9m. However, this doesn't mean that you can completely ignore shallow dives, especially if you've done any previous deeper dives.

Staying well hydrated, diving nitrox and avoiding bouncing will all help to stack odds in your favor.

//LN

Yeah, I think I understand your question.

As you may already know, in the Haldane model, you have a number of "compartments", usually 12 but as many as 16 depending on the equation the manufacturer is using.

Each compartment a few controlling factors.

The first one is a half-time, which determines how fast your on/off gas. The shorter the 1/2 time the faster that compartment ongasses or offgasses. This is why you sometimes hear people talk about fast/slow compartments or tissues.

another is called an M-value. This value is applied to the equation as a factor of how much *underpressure* the compartment can tolerate without bubbling happening. There are really two types of M values. Surfacing M values, which are used to calculate NDL and decompression M values that are used to calcuate decompression ceilings. M values are generally viewed on the slope of a linear equation (a graph with a straight line) and compared to an "ambient" line as compared to the inert gas loading in the given compartment to calculate if the compartment is experiencing sufficient over/underpressure and therefore if it is on-gassing or off-gassing at that moment.

That's the square-blocks-on-white-board version. There's a lot more than can be said about this.

R..

HowardE:

Some could say that it's 33 fsw. Because that is where the pressure gradient is at a 2:1 ratio, and there is no - NDL for less than 33 feet. However. Offgassing will occur at deeper depths, depending on how much nitrogen you've accumulated, and the depths at which you've accumulated them.

However... I think what you're asking is - is it safe to do 5 dives a day on air? In short. Yeah. You can do that no problem. Do you have a dive computer?

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I think most sources have settled on 1.6 and not 2, actually.

In layman's terms, one can extend any deco stop at 6 meters (+/- 20ft) and shallower for as long as you like without additional ongassing concerns. I think this might actually be the short answer to the OP's question.

R..

Hillmorton Scubie:

At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing, and are there variable factors that have relevance. Can this depth be incorporated for planning in extended diving i.e. liveaboards where doing 5 tanks in a day isn't unheard of.-Thanks

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If you are underwater you are on gassing (in some compartment) unless you stay at one depth for so long that you are saturated. In a Haldanean world it's really just a matter of which compartment is controlling at that moment. That is a function of depth and time (and of the half-time of that compartment).

There is a depth where the controlling compartment is at equilibrium at that moment. It's a moving target however both as to depth and as to which compartment is controlling.

These are some of the factors that your dive computer is using to answer the questions that you are raising.

You can also use desk top decompression planners such as V-Planner to plan such a dive trip or to simulate dives to answer these questions.

You could also just use your dive tables to do the same thing.

By the way, the depth that you mention (equilibrium depth) doesn't tell you enough to plan any dives. It does suggest a level below which safety or decompression stops would not be advised or effective.

Hillmorton Scubie:

At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing

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You will only be offgassing nitrogen at 15 feet AFTER a dive . (You are getting rid of the nitrogen that has been absorbed during the dive)

When you first drop down to 15 feet at the start of a dive you will be ONgassing nitrogen slightly. ( The pressure of nitrogen is greater than it was at the surface )

V-Planner gives the depth at which offgassing starts. It varies with the depth and time spent there. e.g after 20 minutes at 100 feet on air offgassing starts at 69 feet.

This offgassing is only from the leading (most heavily loaded) compartment. Stay at this depth too long and a slower compartment will absorb enough gas to become the leading compartment. At that point you start ongassing again.
This is all closely related to deep stops. You need to get a bit above the offgassing depth (typically around 30 feet above) to produce a mild offgassing gradient, but only stay there for a minute or 2.

Here's a discussion point for you all (not sure I agree with the story, but here's what I heard):

The dive captain / instructer on the Molokini trip I did in Hawaii said that when he was a DM in the Virgin Islands, he knew of an instructor that got bent in 20 feet of water, due to the fact he conducted five OW check out classes in one day with several students each class. I asked him why, and he said it was because of the numerous ascents made with each student when the did the CESA at the end of each class with each student.

I found it hard to believe, but I suppose that it will put a strain on your body to some degree.

I don't know the details, but I assume that he was underwater pretty much all day long (four to five hours bottom time with minimal SI?), which is not unheard of with resort instructors. Proves that you definitely ongas at very shallow depths. Maybe the depth was minimized for the story - maybe he was in closer to 25 to 30 feet, but still, that is an interesting story.

You can reach equilibrium at any depth, and no, I don't think it's wise in general to use saturation to plan liveaboard diving.

However, according to some theory (neo-Haldanean/Buhlman), there is a minimum pressure gradient required to produce bubbles. Some consider that to be 2:1, others consider it to be 1.58:1.

So, if you accept those models as close to reality, and if you accept that you will eventually reach equilibrium with the surrounding pressure, I suppose you could use that gradient to plan your dives. So long as you never get deeper than 1.58ATM, you'll never exceed a 1.58:1 pressure gradient in any compartment regardless of bottom time (assuming you surface at sea level).

Hillmorton Scubie:

At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing, and are there variable factors that have relevance. Can this depth be incorporated for planning in extended diving i.e. liveaboards where doing 5 tanks in a day isn't unheard of.-Thanks

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When we first descend we are ongassing from the time we leave the surface until the time we reach our maximum depth. Once we begin our ascent, some of our tissues will start offgassing while others will continue ongassing. The variable factors will be the tissue type (although mathematical models will divide these into "compartments" there are actually a more complex set of relationships than the compartments will accommodate), depth, amount of time spent at that depth, and the makeup of the gas you are breathing.

For my classes I use the following analogy to explain the ongassing/offgassing process ...

Think of decompression as a series of sinks ... all of which are connected to a common trough. Each sink represents a type of tissue in your body. The trough represents your lungs. Each sink has a hole in it that allows water to drain between it and the trough. Since different tissues ongas and offgas at different rates, each sink has a different size hole that represents the rate at which that particular tissue ongasses and offgasses ... slower tissues have smaller holes, while larger ones have bigger holes.

The trough is filled with water. That represents taking a breath. Each sink will receive water from the trough at the rate that it's able to pass through the hole. When you are on the surface you are at equilibrium ... all of the sinks are at the same level as the trough.

When you begin a dive, you effectively put more water in the trough. Since the water level is higher in the trough than in the sinks, each sink will begin to fill with more water through the hole ... each at a rate proportional to the size of the hole. As you go deeper, more water goes into the trough. The greater the difference between the water level in the sink and trough, the more "pressure" is applied to the water flowing through the hole.

At any given point, the water level in all the sinks will vary because some will be filling up faster than others. The sinks with the bigger holes ... which represent faster tissues ... will fill up faster than those with smaller holes.

At some point you will begin to come up from the deepest part of your dive. When this happens, the water level in the trough goes down ... which represents what happens in your lungs when the pressure of the gas you are breathing goes down. When it reaches a point where the level in the trough is less than the level in a sink, water will begin to flow from that sink into trough. That represents "offgassing". But because the sinks are all at different levels, some will start to "offgas" sooner than others. By the same token, during an ascent your faster tissues (mostly your circulatory and nervous systems) will begin to offgas while your slower tissues (muscular and skeletal systems) will continue ongassing ... because the level of their "sinks" are still lower than that of the trough. Each sink will only start to flow back into the trough when its level exceeds the level of the trough.

So as you can see, since there are literally dozens, if not hundreds of "sinks" in your body, the point at which each will begin to offgas depends on the rate at which it ongassed during the descent part of the dive, as well as the amount of time you spent at a given depth.

There are additional factors which impact this process ... which vary from person to person and even from day to day. These include such things as hydration, blood pressure, overall health, personal physiology (i.e. fat to muscle ratios), stress, fatigue, effectiveness of your exposure equipment, and host of other variables.

Which is why decompression is more an art than a science ... there are simply too many variables to be able to predict its effect with anything resembling accuracy. So we use mathematical models based on "risk factors" and an idealized body, add some safety factors, and come up with a decompression algorithm that works most of the time.

So there really is no hard answer to your question ... my best advice for a 5-dive a day liveaboard is to use nitrox (if it's an option), limit the time you spend deep, spend a longer than normal time in the shallows on each dive, drink lots of water or other hydrating fluids, limit your alcohol and caffeine intake, and get a good night's sleep between those 5-dive days.

... Bob (Grateful Diver)