Disabled deco gas on backup computer.

[jstrang]

It also depends on how deco you are doing. On my last dive I had an hour of deco with 2 deco gases - lying to my computer and telling them i only had one gas would have added another hour of deco. And I would have run out of gas.

If you only have 15 to 20 minutes of deco and you lose a gas it wont be the end of the world. You can just hang longer on back gas. But as the deco obligation increases it just becomes unrealistic to mislead the backup computer.

I do think thinking through the scenarios and coming up with a backup plan is a good idea. Knowing that you have carried enough gas to cover a lost deco gas, or surface support to give you extra gas, or extra team bailouts or hanging extra gas etc all would be a possible solutions - i dont really see any benefit to misleading the computer.

Plan your dive - dive that plan. That works really well.

 

[CptTightPants21]

Carrying another deco gas is not complicated. Really, 2 deco gases is hardly a blip on the radar of complexity nor effort. If a diver is so cavalier about their technical diving that they're willing to ignore planning and peg their runtime to their available backgas deco, 1) they probably shouldn't be doing decompression dives, and 2) might as well carry a backup since losing their deco gas is their big concern rather than actually doing it safely. If a technical diver can't handle two deco bottles they shouldn't be doing technical dives. We tell divers all the time that carrying a redundant source of gas is a good idea. In this case, they're carrying a redundant source of deco gas.

I'm not saying it's super complicated. It's like every member of a dive team bringing a 400' primary reel into a cave - it's unnecessary. Are you telling me that you bring two deco bottles on every deco dive?

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

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.

We are kinda stretching the field here....this conversation was about a single gas deco dive. But running some numbers

GF 50/80, 240ft, 25 min. Rock Bottom is 135cuft with 3 deco gas.

With that run time and depth, I would be looking at 3 deco gases plus a bottom stage. I haven't done a dive this big, but I might debate putting the 35/25 deco gas in an al40. Using only two deco gases really ups the rock bottom calculations.

No matter what option you choose, a buddy team can absolutely handle a double major failure.

At max penetration, magic, buddies entire gas supply disappears. You use rock bottom gas to get to first gas switch. Boom, first deco gas is also magically gone.

Now at this point, there are a couple of options.

1. In this scenario dive plan above, there isn't that much deco on that third deco gas. There is 1 min each at 120-100 and 2 min each at 90-80 ft stops. I would ascend as far as my computers tell me I could. Depending on that depth and time, there could also be a discussion with the buddy about moving up to 70' immediately and/or switching to 2nd deco gas briefly at a slightly deeper depth. Now before this is decided, we also need to remember that the min gas calculation is inherently conservative because it assumes a 10ft per min ascent rate--a normal 30ft ascent rate with from 240 with 3 min for diagnose and switch built in would consume roughly half of your rock bottom gas and still leave you with ~65 cuft of gas that could be used for deco and breathed in the event of a 2nd major failure. (I'm not even counting the potential gas that is left in the bottom stage--again depending on how you wanted to run your reserves)

2. Buddy breathe the 3rd deco gas until you can ascend to 2nd gas switch.

I'm curious, how would you plan this dive so that it couldn't handle a double failure?

 

[rjack321]

@Zi55ou
You realize that with if you lose your one (and only) deco gas your TTS basically doubles? (not precisely but close enough).

My biggest concern with your plan is that you are: 1) solo 2) relying on these two computers a lot. You also shouldn't be getting ascent warnings regardless of which gas you are breathing.

 

[rjack321]

I'm curious, how would you plan this dive so that it couldn't handle a double failure?

This is a 2 deco gas dive with a bottom stage and I have never planned for 2 majors. But let's walk through this...

Using your example and 35/25, I get about 7 mins on 35/25. vs about 12mins on 15/55 so basically there's almost no need to use that deco gas for this short of a bottom time. I wouldn't bring it. 240ft for 25mins is not a 3 deco gas dive for anyone I know. It is doubles and stage territory as you surmised. My RB is not 135cf though.

From 240ft (GFs of 50/80) its 15mins up to 70ft at an average ATA of about 4. 4ata * 0.75cf/min * 15min = 45cf * 2 divers = 90cf. If you are doing this kind of diving you can't be getting all stressed with a jacked up consumption rate of 1+ from a little air share.

But if you lose the 50% you are in a world of hurt since you have about 30mins of 50% time and without that gas you have 84mins of time between 70ft and 30ft on 15/55. Average ATA is about 2.5ata * 0.5cf/min * 84min = 105cf of 15/55 to complete the 50% stops on backgas while you only (realistically) have somewhere in the 50-100cf range. Having 105cf of 15/55 still available as you arrive at 70ft means that you needed to leave the bottom with about 195cf. Oi that's a chunk but maybe we could?

But wait we were doing 25mins at 240ft (8.25 ata). At a decent working consumption rate of 0.6cf/min * 25min * 8.25ata = 123cf. So we need to start the dive with 195 + 123 = 318cf. Double jacked up 104s and a stage and you <might> squeak this out. If you're in the tropics and diving a wetsuit you're now talking about double al80s + 3 bottom stages and the 2 deco gases - seven 80s each and you're pretty much planned yourself out of this dive if that's what you need.

Everyone I know and dive with plans for 1 major failure. OOA(backgas) / out of deco gas / bailout from CCR etc. While in some usually less than worst case scenarios you can get out with 2 or even 3 major failures its not something we plan for.

 

[Zi55ou]

@Zi55ou
You realize that with if you lose your one (and only) deco gas your TTS basically doubles? (not precisely but close enough).

My biggest concern with your plan is that you are: 1) solo 2) relying on these two computers a lot. You also shouldn't be getting ascent warnings regardless of which gas you are breathing.

I can't state enough that the plan will only be shortened by this set-up that worst case is still worst case. What we're talking about here is max 15min accelerated up to and hour air deco (also while not taken into account drop tanks are available)

 

[doctormike]

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.

Aha! Got it. I had misunderstood your scenario.

That makes more sense, and I understand that you aren't continuing a dive after losing your deco gas. I guess the only thing I would say would be that whatever your maximum tolerable back gas deco should be something that you determine ahead of time, so that it's built into your dive plan and you don't need to be making that call during the dive.

Don't know if you have the latest firmware upgrade, but SurfGF is a nice piece of information to have if you are ever considering cutting your deco time short for operational reasons, and want to know how that will actually impact your decompression stress.

 

[CptTightPants21]

This is a 2 deco gas dive with a bottom stage and I have never planned for 2 majors. But let's walk through this...

Using your example and 35/25, I get about 7 mins on 35/25. vs about 12mins on 15/55 so basically there's almost no need to use that deco gas for this short of a bottom time. I wouldn't bring it. 240ft for 25mins is not a 3 deco gas dive for anyone I know. It is doubles and stage territory as you surmised. My RB is not 135cf though.

From 240ft (GFs of 50/80) its 15mins up to 70ft at an average ATA of about 4. 4ata * 0.75cf/min * 15min = 45cf * 2 divers = 90cf. If you are doing this kind of diving you can't be getting all stressed with a jacked up consumption rate of 1+ from a little air share.

But if you lose the 50% you are in a world of hurt since you have about 30mins of 50% time and without that gas you have 84mins of time between 70ft and 30ft on 15/55. Average ATA is about 2.5ata * 0.5cf/min * 84min = 105cf of 15/55 to complete the 50% stops on backgas while you only (realistically) have somewhere in the 50-100cf range. Having 105cf of 15/55 still available as you arrive at 70ft means that you needed to leave the bottom with about 195cf. Oi that's a chunk but maybe we could?

But wait we were doing 25mins at 240ft (8.25 ata). At a decent working consumption rate of 0.6cf/min * 25min * 8.25ata = 123cf. So we need to start the dive with 195 + 123 = 318cf. Double jacked up 104s and a stage and you <might> squeak this out. If you're in the tropics and diving a wetsuit you're now talking about double al80s + 3 bottom stages and the 2 deco gases - seven 80s each and you're pretty much planned yourself out of this dive if that's what you need.

Everyone I know and dive with plans for 1 major failure. OOA(backgas) / out of deco gas / bailout from CCR etc. While in some usually less than worst case scenarios you can get out with 2 or even 3 major failures its not something we plan for.

Correct me if I am wrong, but average ATA for rock bottom is 5.7 ATA (240+70/2) not 4.4. Also, min gas time calc is 240-70 = 170 (17 min) +2 min = 19 min plus the deco time that would be required before getting to 70ft.I mean, the min gas calculation is a bitch with the first switch at 70ft--you have gas to do a lost deco. Your min gas just covers a real 30ft/min ascent plus the 12 min of deco between 120 - 70 ft. (3.8ATA x 12 x 1.5). I guess in your example this is a one failure dive, although buddy breathing the 50% bottle is still an option--however much that would suck.

We will have to see how I feel when I start doing these dives, but all my stage caving plans, all my ccr buddies carry two al80s, all planning has been for double failure. I agree with your sentiment about not planning yourself out of a dive--I have a couple of dive buddies that love to throw some wacky scenarios out there, but I always was able to demonstrate that their crazy failure was completely covered by the plan. When I dive, I don't really have much stress or worry. The third bottle on this profile relieves a whole host of stress off the dive plan at multiple levels.

 

[rjack321]

Correct me if I am wrong, but average ATA for rock bottom is 5.7 ATA (240+70/2) not 4.4. Also, min gas time calc is 240-70 = 170 (17 min) +2 min = 19 min plus the deco time that would be required before getting to 70ft.I mean, the min gas calculation is a bitch with the first switch at 70ft--you have gas to do a lost deco. Your min gas just covers a real 30ft/min ascent plus the 12 min of deco between 120 - 70 ft. (3.8ATA x 12 x 1.5). I guess in your example this is a one failure dive, although buddy breathing the 50% bottle is still an option--however much that would suck.

We will have to see how I feel when I start doing these dives, but all my stage caving plans, all my ccr buddies carry two al80s, all planning has been for double failure. I agree with your sentiment about not planning yourself out of a dive--I have a couple of dive buddies that love to throw some wacky scenarios out there, but I always was able to demonstrate that their crazy failure was completely covered by the plan. When I dive, I don't really have much stress or worry. The third bottle on this profile relieves a whole host of stress off the dive plan at multiple levels.

coming up from 240 on 15/55 stops are:
240 = 1 min to sort yourselves
110ft up at 30ft/min = 3 (I rounded down)
130 : 30sec (will probably blow off before you arrive but its a "stop" nevertheless
120: 1min
110: 1
100: 2
90: 3
80: 4

Total time = about 15mins
the 1/2 way point of that time is 7.5mins which is about 95ft and I rounded that up to 4ata

Suit yourself on the 3rd deco gas. I will say you're going to be in the extreme minority using that on a 25min wreck dive. And the drag of that 4th bottle plus pita of rotating on a drifting deco is not insignificant.

Sometimes I use 2x AL80s as BO too. Last week in the great lakes I had an 80 of 15/55 and an 80 of 50%. I could not lose the CCR and lose the 50% (or the 15/55 either for that matter). To have enough gas for 2 majors would require 4 al80s. Basically a spare of each.

In caves with 2x lp85s jacked up to 3600 I plan on having all 200cf of that BO available. Its not a 100cf with 100cf of spare BO.

 

[Zi55ou]

i guess the only thing I would say would be that whatever your maximum tolerable back gas deco should be something that you determine ahead of time, so that it's built into your dive plan and you don't need to be making that call during the dive.

Absolutely, and its something that's been calculated, but what we calculate for a worst case isn't always what we would want to do given the choice and armed with more information that would allow us to make a decision to cut the dive short can't be a bad thing. This only works in a limited set of circumstances and it has to be considered within the gas choices. With more than one deco gas this is a bad idea and I wouldn't advocate it outside of what I have described and there's a definite case to be made of gigo...