Deco planning

[MonkSeal]

Hi !

Here's the dive: Tx 21/35 42m, 15min, O2 deco at 6m.

Buhlman (80/30) gives stops:
18 m 1 min
15m 1 min
12m 1 min
9m 2 min
6m 7 min

or if deco is done on back gas:
18 m 1 min
15m 1 min
12m 1 min
9m 2 min
6m 23 min

RGBM (0-nominal) gives stops:
18 m 1 min
15m 2 min
12m 2 min
9m 4 min
6m 7 min

or if deco is done on back gas:
21 m 1 min
18 m 1 min
15m 2 min
12m 3 min
9m 5 min
6m 27 min

I understand the diferences between those two algorithms. But when using Buhlman algorithm, deep stops remain the same weather I use O2 or back gas for deco i.e. only last stop is longer. When using RGBM complete deco schedule is different depending on deco gas used.

Usualy when I plan a deco dive I plan enough quantity of back gas to make a full deco if something is wrong with deco gas (lost tank or some mailfunction). If RGBM is used and if I find out at 6m that something is wrong with my deco bottle I can't simply add some time to deco schedule because I've already missed to spend some time at deeper stops or even I've completely missed some stops.

Be so kind to comment
MonkSeal

 

[DepartureDiver]

That's a good question. The models are doing what they are supposed to be doing. But I don't have an answer on that one since your stops will assume your O2 will work, but then if it doesn't ... what to do?

 

[MikeFerrara]

I can't go into a lot of detail on RGBM but Buhlman or any Haldanian model uses the leading compartments "M" value or critical tension to determin the ceiling. The M value isn't effected by breathing gas. The inert gas PP in the inspired gas is really only used to figure the rate that the tissue inert gas PP changes. Meaning that it will let you move up sooner but the tissue desolved gas PP at which it allows you to move up doesn't change.

Again the M value itself isn't changed to compensate for the inert gas PP of the inspired gas that will be breathed at the next stop.

A bubble model is trying to look at more of the things that can make a bubble grow. It's easy to imagine that breathing a different gas and changing the diffusion rate would change more than just the length of time at that stop (rate of change of the desolved gas PP). After all is it really the ambient pressure that determins what desolved gas in the tissues wants to do? I don't think so especially when you're changing the fraction of inert gas in the inspired gas.

When you switch to O2 and have no inert gas at all it stands to reason that you're getting rid of inert gas faster right?

Getting to a shallow stop faster means that you'll not only have more total inert gas but maybe more in bubbles.

Doesn't it also stand to reason that having that same desolved and free gas load without the help of the decompression gas to get rid of it is less than ideal?

I sounds like RGBM says that if you're going be off gassing slower (without the deco gas) that you're better off getting rid of more while you're deeper.

Also remember that while the desolved gas gradient (offgassing) is maximized by going shallow, the bubble gradient (terminology) all else being equal is maximized by doing the opposits and staying deeper.

Sheck out the articles on wwwdecompression.org/mainken

The GAP site has some really good articles also.

 

[MonkSeal]

Sheck out the articles on wwwdecompression.org/mainken

The GAP site has some really good articles also.

Thanks Mike, I'm OK with your explanation and I'm familiar with background of both algorithms (and RGBM advantages). My questions is how to make a backup plan for lost deco bottle if you stick to RGBM.

 

[MikeFerrara]

All you can di is what you did I think.

I don't think the difference in the deeper stops are going to be that drastic. You could always pad the shallow stops a little more too in the case of lost gas.

 

[MonkSeal]

All you can di is what you did I think.

I don't think the difference in the deeper stops are going to be that drastic. You could always pad the shallow stops a little more too in the case of lost gas.

Thanks Mike.

 

[DepartureDiver]

Again the M value itself isn't changed to compensate for the inert gas PP of the inspired gas that will be breathed at the next stop.

Hi Mike. Nice post. I just thought I would try to give a little more insight here. Bubble models like VPM don’t use M-values … which is why they can be harder to understand. The allowable surface tension (i.e. M-value) is dynamic and changes upon the dive parameters. Most are used to thinking that fast compartments allow a greater M-value than a slower compartment, but this can actually be the opposite and reversed in a bubble model like VPM. It just depends on the dive and profile.

To understand a bubble model like the VPM, divers need to change the way they view decompression. Each stop and time is directly linked to all of the other stops and times. So, the deep stops know that O2 is gong to be breathed at 20’. Therefore, they can allow greater tensions when leaving the deeper stops based upon the idea that brief periods of higher tensions can be tolerated with it being addressed at the shallower stops. So this is a good question raised in this thread … what do you do with these models that operate this way if you ascend as if your going to have O2 and then you don’t. Keep in mind once you start increasing the times of the stops at the deeper depths, since they are linked to all other stops, the times at the shallower stops will also possibly increase. So if one were to take a profile with the O2 at 20’ and then add in (write in) more time at the deeper stops, this will now affect the time at the 20’ stop and require that it now be increased. If this is not taken into account, it could create a problem. So the question presented in this thread is a good one. And while the profile presented was mild, the difference can be more noticeable in more extreme profiles.

 

[ianr33]

<<<<<RGBM (0-nominal) gives stops:
18 m 1 min
15m 2 min
12m 2 min
9m 4 min
6m 7 min

or if deco is done on back gas:
21 m 1 min
18 m 1 min
15m 2 min
12m 3 min
9m 5 min
6m 27 min
>>>>>>>>>

Could you plan this as a multi level dive ,putting in the depths and times required for deco on back gas into the software ??? The program would then give you a correct time for the 6m stop on O2???
Not very elegant I agree but it might be a workaround

 

[Spectre]

Sheck out the articles on wwwdecompression.org/mainken

What's that link supposed to be?

 

[DepartureDiver]

What's that link supposed to be?

It's http://www.decompression.org/maiken/home.htm