Tracking Nitrogen in Tec Diving

[MyronGanes]

How do you manually track nitrogen when deco is involved and especially when deco using different gases is involved?

In no-stop recreational diving you can track nitrogen accumulation by letter groups on the tables. So now if we are going into decompression how do we manually calculate how much time we need to decompress if we aren't using a dive computer and we have accidentally over-stayed our bottom time as a recreational diver?

Getting into tec diving, how do you calculate how much time you need to remain at a deco stop with 21% O2 versus 100% O2?

I'm studying for my tec courses now but so far the only numerical calculations involve short and long term O2 exposure. Are we to rely on a dive computer to track nitrogen when decompression is involved? Maybe I am brain farting somewhere here but it feels like I'm missing a piece of information.

 

[KevinNM]

You normally decide on a dive profile and then you use a computer program to produce a deco schedule. You ensure you have plenty of spare gas for this plan and then you write the plan down and take it with you. You usually have a couple of alternates in case something happens and you overstay your plan. But diving your plan is really important, as is doing the stops at the depth and time your plan says.

There are some tools you can use to modify the schedule on the fly too.

Alternately people use multiple sophisticated dive computers, set them up with the algorithm and conservatism they want, then follow the guidance they produce. But your computer has no idea how much gas you have, so if you screw up and stay too long you can be presented with a deco schedule that will drown you.

This will all be talked about in great detail in a class.

 

[bl6394]

You would create custom tables (dive profiles) depending upon the back gas and deco mixes you were planning to use. Vplanner is the tool that I use for this. (V-Planner VPM & VPM-B & VPM-B/E dive decompression software for technical divers)

You could also look at one of the other deco planners (multi-deco, etc.)

To get a better understanding - I would suggest reading Deco for Divers (https://www.amazon.com/Deco-Divers-...g_0?ie=UTF8&psc=1&refRID=375JMX2KMZCPKMGB5XYP) and working with your Tec instructor.

 

[Centrals]

Don't forget to track the O2 as well.

 

[MyronGanes]

I understand using the tables and the software. The discrepancy I see is that we are calculating oxygen exposure but not nitrogen exposure. For that we are relying on computers and preconceived data. How do I mathematically manually calculate nitrogen exposure? From all the reading I've done it seems like the"ghost" of all things.

If I don't have a dive computer, dive tables or dive software how do I calculate my my nitrogen accumulation based on partial pressure N2 and time and how do I determine how much deco I need to do at a certain depth using just math? Yes custom dive tables can be used which just involves EADs.

Maybe I am asking too much in that I need to peer into the discrete deco algorithms. Some things are so precisely calculated inside and out and other things seem ethereal.

Yeah I think I'm in over my head as I'm basically asking how to reproduce the dive tables manually.

But is there a way to use the existing tables to derive deco plans?

 

[DevonDiver]

You can use tables to plan technical, decompression, dives - but they are very limiting. Obviously many/most recreational diving tables don't include depths/bottom times that mandate decompression stops; they are no-stop tables. But you can use tables from technical agencies and the U.S.N. You can Google these (image search is often successful) and take a look.

Because of the limits of formal tables, most technical divers use software to 'cut' their own tables. These are specific to the dive being planned. There is a range of software available for PC, Android and IOS that allow decompression diving planning. The most popular of which are; MultiDeco, V-Planner, GAP, Pasto Deco etc

When cutting software tables, the diver inputs the depth, bottom time and gasses to be used. They will also populate the software with their SAC/RMV rate and may opt to amend certain algorithm parameters, depending of the decompression model/algorithm used in the software. The software then calculates an ascent stop profile, gas requirements and O2 management (CNS/OTUs) for the dive.

These pre-cut tables are taken on the dive, often transcribed onto a slate or into a waterproof notebook, and adhered to using a simple bottom timer, which gives depth and time.

Last, but not least, technical divers will use technical diving computers. These typically adopt the same algorithms used by software planners and track the dive in real time, as do recreational diving computers.

As bl6394 has mentioned, it is very worthwhile to read Mark Powell's book; 'Deco for Divers' in preparation for technical diving training. It is an excellent book, explaining the key facets of decompression science in an understandable and logical way..

 

[PfcAJ]

Let's back up and consider how you'd reproduce the NDL tables.

You'd have to do all the algorithm math by hand. Incredibly time consuming and most normal folk would cock it up somewhere.

All dive tables are calculated via algorithm. The algorithm makes certain assumptions (nitrogen/helium uptake and off gassing rates), and certain inputs (time, depth, gas) give certain outputs (ascent schedule).

Knowing what I wrote in paragraph 2, all divers (even rec divers) base their dives on the validity of these algorithms. The dcs rate for your normal padi dive tables is really low. Generally the same holds true for tech diving decompression planning software. We have some faith in it backed up by formal testing combined with practical day to day usage.

One thing to be clear about is that no algorithm is "right". They're models that try and simplify an extremely complex process. The ZHL-16 algorithm makes an assumption that there are 16 different "tissues" that ongas and off gas nitrogen at different rate. A real human, however, does not have 16 tissues. But the model creates ascent schedules that works well in real life.

So to hit back on the main point, decompression divers track their ongassing offgassing, and create ascent schedules using software that makes these algorithms easy to use. Just as an ow diver has no means to calculate his NDL without tables (short of memorizing them) a tech diver cannot for tech dives, either.

 

[bl6394]

Using a method called Ratio Deco is perhaps the closest thing to what you are describing - i.e. being able to calculate rough decompression schedules on the fly - during the dive - without a computer. I don't personally know anyone who uses ratio deco - and can't recommend it. But it is described here:

Ratio decompression - Wikipedia, the free encyclopedia

In general - divers use multiple computers for redundancy to calculate a decompression schedule based upon an estimate of the inert gas that has been absorbed in your tissues as a function of your dive profile and gas choices using an algorithm of your choice. Alternatively, you could create a profile that would do the same - which could be backed up by a computer.

 

[DevonDiver]

If I don't have a dive computer, dive tables or dive software how do I calculate my my nitrogen accumulation based on partial pressure N2 and time and how do I determine how much deco I need to do at a certain depth using just math? Yes custom dive tables can be used which just involves EADs.

In a nutshell... you cannot manually calculate saturation. It is way beyond the scope of mental arithmetic unless you're Rain Man. Here's a basic example:

For a start, there's a myriad of different algorithms used. So you first need to pick one of those. Some models calculate based on inert gasses saturating into/out from varied tissue compartments at pre-designated speeds. These are called 'dissolved gas models'... and this concept is what most recreational divers are familiar with from scuba courses. An example of this is Buhlmann ZHL16.

Other models also add factors/calculations that attempt to limit the growth of micro-emboli (micro bubbles) on ascent.... these are called 'dual phase models'... as they track nitrogen but also apply some heavy math to limit 'critical bubble radii' based on the mechanics of bubble formation, growth and collapse. Examples of these are VPM-B and RGBM.

Assuming a (more straightforward) dissolved gas model,you need to apply some mathematics for each tissue compartment (i.e. 16 compartments with varying saturation half-times).

Such a formula for saturation level might be:

Buhlmann Saturation Level

Pcomp = Pbegin + [Pgas - Pbegin) x [1-2 -te/tht]

Where:
Pcomp = Inert gas pressure in the compartment before exposure time (bar)
Pbegin = Inert gas pressure in the compartment after exposure time (bar)
Pgas = Inert gas pressure in the mixture being breathe (bar)
tc = Length of exposure time (minutes)
tht = Half time of the compartment​

Once you know your saturation level/s, you need to calculate ascent ceiling/s.

Pamb.tol = (Pcomp - a) x b

Where:
Pcomp = Inert gas pressure in the compartment
Pamb.tol = Pressure you could drop to
a and b = a and b values for that compartment and the gas in question​

This will give a ceiling for each tissue compartment. The deepest ceiling is where the stop is conducted. The calculation is repeated to provide necessary stops until the surface is reached.

As has been mentioned, there is a system called Ratio Deco that allows dive planning using some basic maths. This is based on a ratio between depth/time and stops on ascent. The ratio creates an ascent profile that mimics profiles given by decompression algorithms.

Ratio Deco is taught by the agencies GUE and UTD (they both use different versions) and has a debatable level of success.

In reference to your question, however, Ratio Deco doesn't calculate nitrogen saturation... it simply gives a 'curve' to the ascent profile/stops.

 

[PfcAJ]

Using a method called Ratio Deco is perhaps the closest thing to what you are describing - i.e. being able to calculate rough decompression schedules on the fly - during the dive - without a computer. I don't personally know anyone who uses ratio deco - and can't recommend it. But it is described here:

Ratio decompression - Wikipedia, the free encyclopedia

In general - divers use multiple computers for redundancy to calculate a decompression schedule based upon an estimate of the inert gas that has been absorbed in your tissues as a function of your dive profile and gas choices using an algorithm of your choice. Alternatively, you could create a profile that would do the same - which could be backed up by a computer.

Ratio deco = your deco algorithm of choice. It's entire point is to produce the same ascent schedule. If it doesn't, it's wrong, and GUE makes that very clear in class.

It works well for a range of depths and times with certain gases.