Disabled deco gas on backup computer.

[rjack321]

Right, I understand that. We all do lost gas scenario, which means that we can complete all required deco assuming that we have to deco on our backgas at the end of the dive, with the maximum inert gas loading of the planned bottom time. My point is that lost gas is a contingency for when you ascend fully loaded and THEN find out that you don't have the option of accelerated deco. It's less than ideal, and it involves an increased risk if anything happens to your back gas (yes, I know, two failures).

But the OPs scenario was losing your deco gas during the dive, and then making the decision to KEEP on loading inert gas. So you are voluntarily increasing your deco obligation while already in a failure mode and working on contingency. I wouldn't do that, no matter how comfortable I was doing a longer deco on my back gas. There is no reason not to end the dive at that point.

If you were in a cave with two lights, and one failed, would you continue on with one, to the point of maximum penetration that you felt comfortable doing with only one light, or would you turn the dive and exit? How is this different?

He's just doing a backgas deco dive - its not the end of the world if the deco obligation is short or manageable.​

Just to clarify, in a buddy dive there is a contingency against two major failures--failed manifold and lost deco, or two lost deco, etc and those care covered by buddy's deco gas, your 2nd deco gas if dive requires it, and the 3rd of backgas/min gas reserve. A solo dive can handle one major failure, maybe 1.5 depending on time and circumstances

Rock bottom is customarily calculated as enough gas to get you and an OOA buddy to the next gas source. That is either the surface (in recreational diving) or a deco gas.

So no you can't have an OOA buddy and lose a deco gas too. That's two majors. You can try to plan for those but you are going to rapidly plan yourself out of diving at all. If you are doing this (successfully?) I'd love to see your 2 majors gas plan for a 240ft 25min dive. Use whatever consumption rates you want.

 

[Zi55ou]

So I'm wondering why you would continue a dive once you had realized that you no longer had the capability to complete you dive plan and now were in contingency mode? Personally, if that happened to me (say with a blown LP hose on my deco bottle), I would just end the dive at that point.

But the OPs scenario was losing your deco gas during the dive, and then making the decision to KEEP on loading inert gas.

Nowhere did I say that I would continue the dive having lost my deco gas. This is simply a "were I to lose my deco gas at this point what would the deco picture look like" all this within the confines of an already existing plan.

If you were in a cave with two lights, and one failed, would you continue on with one, to the point of maximum penetration that you felt comfortable doing with only one light, or would you turn the dive and exit? How is this different?

And many people bring two (edit: backup) torches with them that doesn't say that the dive should continue if your primary fails.

 

[rjack321]

Nowhere did I say that I would continue the dive having lost my deco gas. This is simply a "were I to lose my deco gas at this point what would the deco picture look like" all this within the confines of an already existing plan..

So how much deco are you willing to do on backgas? (assuming unlimited supplies)

You have two curves on a collision course. The amount of deco you would (in theory) have to do on backgas and the dwindling amount of backgas available since you're breathing it. How are you calculating rock bottom here?

 

[doctormike]

Nowhere did I say that I would continue the dive having lost my deco gas. This is simply a "were I to lose my deco gas at this point what would the deco picture look like" all this within the confines of an already existing plan

OK, then I don't understand your question at all. You wrote:

were I to lose my deco gas I am aware of how much air deco I've incurred and can monitor this air deco throughout the dive.

It seems to me that you were saying that were you to lose your deco gas, you would use the information on your backup computer to decide what to do next - how long a dive to do. Is that not what you were saying? If I misunderstood you, I'm sorry.

If I planned a dive with staged decompression, planning to switch to a deco mix on ascent, and if I were to lose that deco mix, I would personally just end the dive at that point. I wouldn't make a decision of how long to continue diving based on my TTS on back gas. Are you also saying that if you lost your deco gas you would end the dive? If so, why not just ascend and do the deco that your computer tells you to do.

I believe Shearwater TTS assumes using all of your active gasses, but you won't clear deco until you do the back gas ascent plan if you never switch to the richer mix. You will just get an alert that there is a better mix to use...

 

[doctormike]

He's just doing a backgas deco dive - its not the end of the world if the deco obligation is short or manageable.​

It might not be the end of the world, but you are now choosing to voluntarily increase your inert gas loading when you are in contingency mode (i.e. you have no alternative but to blow off deco if you have a problem with your back gas). He didn't plan a backgas deco dive, he planned a dive with a rich mix for deco, and now because of a failure that plan is no longer doable.

It's not the same as planning for two failures, because the decision tree has changed. And sure, at some point, your backgas deco requirement is so low that blowing it off doesn't really give you much more of a risk of getting bent, but that is what you are trading off.

I wouldn't do it.

 

[rjack321]

It might not be the end of the world, but you are now choosing to voluntarily increase your inert gas loading when you are in contingency mode (i.e. you have no alternative but to blow off deco if you have a problem with your back gas). He didn't plan a backgas deco dive, he planned a dive with a rich mix for deco, and now because of a failure that plan is no longer doable.

It's not the same as planning for two failures, because the decision tree has changed. And sure, at some point, your backgas deco requirement is so low that blowing it off doesn't really give you much more of a risk of getting bent, but that is what you are trading off.

I wouldn't do it.

In theory the way to plan backgas deco would be to have enough of a reserve that you could lose 1/2 of your remaining backgas and you're still good (ie shutting down your isolator). I don't think this is what the OP is doing, but I can't tell for sure. You end up with a 10-15min max backgas deco plan if you think about it this though. If you only have a few mins on backgas (5-12 etc), bringing a deco gas starts to seem like more work than its worth. If you have 25-40mins of deco gas deco to do then doing backgas deco is unwise.

Not something I recommend either. My approach would be to:
1) plan with a buddy
2) plan the dive using all deco gases available, if none are available then rock bottom is going to be high and max deco times are going to be short
3) bring enough deco gas to allow for sharing should me or my buddy lose one
4) not plan for "2 majors"

I believe Shearwater TTS assumes using all of your active gasses, but you won't clear deco until you do the back gas ascent plan if you never switch to the richer mix. You will just get an alert that there is a better mix to use...

This is the way they implement it, but you could/can turn off your deco gas which is what he's doing on his "backup". Your backup is going to be bent if you follow the deco gas plan on your primary though.​

 

[doctormike]

This is the way they implement it, but you could/can turn off your deco gas which is what he's doing on his "backup". Your backup is going to be bent if you follow the deco gas plan on your primary though.​

Right. I think that you can't get into system setup during a dive, but I can't remember if you can turn off a gas that is on during a dive. Doesn't really matter - if you have to backgas deco, it will still track your deco correctly even if it is recommending that you switch to the richer mix that you told it you were carrying.

 

[Zi55ou]

I may not be explaining myself clearly, my bad. Can't right now but will write it out clearer tomorrow.

 

[rjack321]

Right. I think that you can't get into system setup during a dive, but I can't remember if you can turn off a gas that is on during a dive. Doesn't really matter - if you have to backgas deco, it will still track your deco correctly even if it is recommending that you switch to the richer mix that you told it you were carrying.

I mostly use system setup too, which is inaccessible underwater. And follow shearwater's recommendation to have all gasses I'm carrying turned on...

But you can turn gases on/off underwater through the dive setup menus as well - those are accessible during the dive.
Starts on page 38
https://www.shearwater.com/wp-content/uploads/2012/08/PetrelManual_DocRevD.pdf

 

[Zi55ou]

Okay so I don't think I've been explaining what I'm trying to get across very well.

So this only applies to dives with one deco gas as I can't see it working with more than two geses. And I'm coming at this from the point of view of the diving I tend to do (twin 12l air with 7l stage of 50-80% 30 - 50m).

So you plan your dive with contingencies considered. Lost deco, just deeper, just longer, worst case, lost buddy. I can't reiterate this enough the plan will not change in the following scenarios.

How a dive would usually go:
Do the dive with both gases set and enabled on both computers.
Complete the working portion of the dive and on reaching the switch depth switch gas and then switch both computers to the deco mix.

How a dive usually goes with one gas disabled on the backup computer;
Do the dive with both gases set and enabled on primary computer and both gases set but only back gas active on secondary computer.
Complete the working portion of the dive and on reaching the switch depth switch gas and then switch both computers to the deco mix. This is trivial to achieve as both my ostc and perdix allow gases to be toggled to active during the dive.

How a dive would happen should the deco gas be lost with both computers set and enabled with both gases:
Immediately begin ascent knowing deco will have to be completed with back gas, this isn't an unforeseen issue as it's within the realm of what's been planned.
On ascent ignore warnings to switch to non available gas and disable deco gas on both computers to give correct deco information, again all within the contingencies of the original plan.

How a dive would happen should the deco gas be lost with primary computer set and enabled with both gases while backup only has back gas enabled:
Immediately begin ascent knowing deco will have to be completed with back gas, this isn't an unforeseen issue as it's within the realm of what's been planned.
On ascent ignore warnings to switch to non available gas and disable deco gas on one computers to give correct deco information, again all within the contingencies of the original plan. But having a clear idea of deco obligations at all times.

How a dive might go with one gas disabled on the backup computer;
Do the dive with both gases set and enabled on primary computer and both gases set but only back gas active on secondary computer.
It's a small wreck I've seen what I came to see and I'm feeling a bit cold given the choice I might stay for a few more minutes but I can see how much deco I "would have to do were I to lose my deco gas" and I think to myself that's about as much time I want to spend doing deco so I'm going to call it and start heading up now and on reaching the switch depth switch gas and then switch both computers to the deco mix. I've only used this information to cut the dive short for a scenario that hasn't happened and for which is already an integral part of the plan.

Hope that explains it a bit better. The plan hasn't changed only potentially shortened woth more margin added.