Question on extended stops at gas change

[kramynot2000]

I sure hope this question doesn't cause too much controversy, but I have always been curious as to why stops in a decompression profile seem to get longer when you get shallower. For instance, you may get something like this (I just made this up):

70 1 50%
60 1
50 2
40 5
30 7
20 10 O2
10 15

It seems to my very, very, very non-scientific mind that it should be sorta inverse with the most time spent at the gas change depths since this is where the gradient is greatest, so more time at 70 and more at 20. So I must be missing something.

Can one of you deco experts provide me with a dummy'd down explanation of what I'm missing?

Thanks

 

[chrisch]

I think the general view is that at depth you are still on-gassing so the deeper stuff is for specific tissue groups and bubble control.

What software are you using? I find with V Planner that extending the stop at which you switch to deco gas "loses" some of the mid range stops, although with the same time to surface (no doubt for the gradient you mention). The software treats it as a "multi level" dive, so you need to revert to the "old" method to calculate lost gas deco requirements.

Chris

 

[RTodd]

I sure hope this question doesn't cause too much controversy, but I have always been curious as to why stops in a decompression profile seem to get longer when you get shallower. For instance, you may get something like this (I just made this up):

70 1 50%
60 1
50 2
40 5
30 7
20 10 O2
10 15

Thanks

The deco on this profile barly warrants carrying 50% and doesn't quite "look" right. As a gross generalization, assuming the right deco mixes, your deco will look like a fibonacci (sp) sequence, i.e. add the times from the previous two stops to get a rough estimate of what the next stop should be.

In general, you add a few extra minutes to your gas switch depth and then shave time (or keep it if you are not comfortable) at later depths where you are getting less gradient advantage from your gas. Once you are on oxygen, there is no need to do a 10' stop. The window is already fully open. If I had this amount of 20 & 10 foot deco, I would spend 15-20 on O2 at 20 with a short back gas break in the middle. Then, I would spend a few minutes moving to 15, then 10, then surface. If you first model stop is really at 70, you should be starting 1 minute stops (basically regulating a really slow ascent) much deeper.

 

[RTodd]

I think the general view is that at depth you are still on-gassing so the deeper stuff is for specific tissue groups and bubble control.

What software are you using? I find with V Planner that extending the stop at which you switch to deco gas "loses" some of the mid range stops, although with the same time to surface (no doubt for the gradient you mention). The software treats it as a "multi level" dive, so you need to revert to the "old" method to calculate lost gas deco requirements.

Chris

The race to the shallowest stop or "bend" and treat theory of the old navy tables does not work as well as proper deep stops and a slow regulated ascent.

 

[Diver0001]

I sure hope this question doesn't cause too much controversy, but I have always been curious as to why stops in a decompression profile seem to get longer when you get shallower. For instance, you may get something like this (I just made this up):

70 1 50%
60 1
50 2
40 5
30 7
20 10 O2
10 15

It seems to my very, very, very non-scientific mind that it should be sorta inverse with the most time spent at the gas change depths since this is where the gradient is greatest, so more time at 70 and more at 20. So I must be missing something.

Can one of you deco experts provide me with a dummy'd down explanation of what I'm missing?

Thanks

Let's see if I can get this across without making a mess of it...

The deeper part of the ascent is a balancing act. Go too fast and you'll bubble and go too slow and you will either not off-gas or even continue to on-gas. Both bubbling and taking on extra gas will actually increase your deco obligation because bubbles are harder for the body to eliminate than gas in dissolved form and extra gas is.... well.... extra.

I think Your confusion is stemming from the thought that the gradient is greatest at your deep stops. In fact the gradient for offgassing increases as you go shallower or when you change to a breathing gas with a lower percentage of the inert element in it.

So....Stops get longer as you go shallower because (a) you need to either go shallow or change your mix or both in order to keep the gradient increasing so you can keep the offgassing running and (b) you need to avoid further on-gassing, which you can only do shallow.

That's the main line of thinking.

To my way of thinking the deep stops are not there for deco purposes per se. It's more of a mechanism to keep the ascent slow so bubbles don't start to form (or that the ones that do form stay small). The bubble models cook the deep stops right into the recipe so it looks like part of the deco but it's really part of the ascent leading up to the deco..... If you get my drift.

Having said that your instinct that you can extend a deep stop in combination with a gas-switch is correct within limits. The gas-switch gives you a little spike in the gradient that you can use to stay at the stop a little longer. However it's not something you can just do arbitrarily because the gradient spike is short and if you wait too long your offgassing will slow again and you'll end up with extra deco in the shallow zone. On the other hand, a little extension of the 70ft stop after the switch may allow you to actually shave a few min off the deco later on in the curve. To my way of thinking, however, it's not worth trying unless you really know your stuff because of the risk of extra ongassing. It's better to just follow the ascent rate you calculated with and extend a shallow stop instead. It's harder to screw up extending a shallow stop and a little more deco on the shallow stops can't hurt.

Does that make any sense?
R..

 

[RTodd]

The gas-switch gives you a little spike in the gradient that you can use to stay at the stop a little longer. However it's not something you can just do arbitrarily because the gradient spike is short and if you wait too long your offgassing will slow again and you'll end up with extra deco in the shallow zone. Does that make any sense?
R..

The biggest reason for extending switch depths in the example provide is it takes a minute or two for the new gas to get in your system. You won't actually get any benefit of 50% O2 if you only spend 1 minute at 70'. So, you spend 3 minutes there for example.

 

[Diver0001]

The biggest reason for extending switch depths in the example provide is it takes a minute or two for the new gas to get in your system. You won't actually get any benefit of 50% O2 if you only spend 1 minute at 70'. So, you spend 3 minutes there for example.

The blood circulation thing?

R..

 

[RTodd]

The blood circulation thing?

R..

yes

 

[Diver0001]

yes

Gotcha. Thanks.

R..

 

[DepartureDiver]

Diver0001 is right on. To off-gas there must be a gradient, i.e. the difference between the tissue gas tension and the partial pressure of the inert gas being inspired. The greater this difference the faster and more efficient the off-gassing will be. This occurs by two mechanisms. One is by being at the shallow depths where the partial pressure of the inert gas inspired is lower and the second is by using a deco gas such as EAN50 or oxygen (which also reduces the partial pressure of the inert gas) ... and of course the oxygen percentage can also be increased at the shallower depths. With this being said, you can't rush to the shallow stops or "too much" bubbling can occur. Once this happens, the gas is no longer in solution for efficient elimination. Instead, the bubble must be absorbed ... which is a more time consuming process ... and if too many result then a hit can occur. But you also don’t want to stay too long at the deeper stops either since on-gassing can still occur.