Redundancy Required for Decompression Diving?

[sea_otter]

You do a dive to 60 feet on air and have two computers, an Oceanic computer running DSAT and a Perdix running the medium conservation rec setting of 40/85. You start your ascent at 56 minutes when your Oceanic computer has 1 minute of NDL remaining. The Perdix gives you 7 minutes of deco at 10 feet. Is this a light deco dive or do you just blow off the Perdix deco, knowing that it is considerably more conservative than the Oceanic?

Personally I'd fill my tank with Nitrox and end with plenty of bottom time remaining on either computer. ;-) ...but I digress.

I don't think anyone is advocating "blowing it off", but rather, would you accept the risk of not using redundant gas and potentially needing to violate that ceiling in the event of a failure? What if you were separated from the team on ascent? Would you stop for 7 minutes at 10', or would you surface quicker to find them? Would you have discussed these plans of time? I can appreciate that some have different levels of risk tolerance here, which is fine, particularly when the person making that decision knows exactly what they're signing up for. For someone who is inexperienced or untrained (like me!), I see these as things that lead down a very slippery slope.

 

[Centrals]

In the example, the Perdix is telling him that, if he leaves the bottom then and stops for 7 minutes at 10', he will hit the surface with a GF99 of 85. If he understands what it is telling him and decides that he's okay with getting to the surface with a GF99 that is higher than 85, what is the problem? How does that equate to ignoring the information it is giving him?

If nothing else, the point of diving with two computers is so that you have a backup if one dies.

Two different computers each give slightly different ndl which one would you follow? The normal practice is to follow the conservative one and in this case it is the Perdix. And it should had never got into deco mode in the first place.
If one of the two computer had gone to deco then it is not much of a "back up".

 

[stuartv]

Two different computers each give slightly different ndl which one would you follow? The normal practice is to follow the conservative one and in this case it is the Perdix. And it should had never got into deco mode in the first place.
If one of the two computer had gone to deco then it is not much of a "back up".

You didn't answer my question.

And, if his primary computer failed and his Perdix was showing a deco obligation, it's still a perfectly fine back up. It's even still a perfectly fine backup if you choose to ride the GF99 display on the way up and hit the surface with a higher value than what the GF settings were set for, all the while ignoring the stops it tells you.

The stops a Perdix tells you are just a path to follow in order to hit the surface with the configured GF Hi. If you choose to get to the surface with a different value, the GF99 display certainly gives you the info you need in order to do that without following the actual stops the Perdix displays.

 

[Centrals]

You didn't answer my question.

And, if his primary computer failed and his Perdix was showing a deco obligation, it's still a perfectly fine back up. It's even still a perfectly fine backup if you choose to ride the GF99 display on the way up and hit the surface with a higher value than what the GF settings were set for, all the while ignoring the stops it tells you.

The stops a Perdix tells you are just a path to follow in order to hit the surface with the configured GF Hi. If you choose to get to the surface with a different value, the GF99 display certainly gives you the info you need in order to do that without following the actual stops the Perdix displays.

The primary computer is the one that has shorter ndl or more conservative among the two computers that you are using. To get the computer into deco mode is CARELESS.
You can set any GF or safety factor as you like on any two computer or two identical one but you still ascend when the first one shown the ndl is approaching zero.

I did not detect any question except poor practice or carelessness on ignoring the warning on the imminent deco obligation. I don't dive this way.

 

[Diving Dubai]

There's no "just semantics" in my mind. If a dive has a mandatory stop then, by definition, it is not a No Deco or No Stop or whatever you want to call it dive. It is not a recreational sport dive. It is a technical dive. And, in my opinion, any person or agency who attempts to suborn the definition of recreational diving to include dives with MANDATORY stops needs to be taken out behind the woodshed and re-educated.

Does that include BSAC? Non US agencies as previously discussed (in the other thread) teach Deco as part of their rec courses - In BSAC's case equivalent to AOW & Rescue.

Just wanted to correct slightly.

 

[Storker]

what I thought then (and still think now) is if sh** goes sideways and you have to make your egress from a different point to your ingress, you will have no deco gas, and depending on how big the wreck is or why sh** went sideways, it may be no simple matter to relocate them quickly.

Have you read "Shadow divers"? The scenario you're describing isn't entirely unlike what happened to Chris and Chrisy Rouse.

Does that include BSAC? Non US agencies as previously discussed (in the other thread) teach Deco as part of their rec courses - In BSAC's case equivalent to AOW & Rescue.

Just FTR, CMAS 3* (CMAS' version of PADI's DM and BSAC's DL) also includes staged decompression:

4.2 A CMAS Three Star Diver is qualified:
[...]
4.2.2 To conduct any sport diving activities including Stage Stop decompression diving

 

[Diving Dubai]

Just FTR, CMAS 3* (CMAS' version of PADI's DM and BSAC's DL) also includes staged decompression:

Thanks for that, I suspected that would be the case. I looked on the CMAS site for that and noted that 2* are taught it, but it's clearly stated that it's outside the qualification. I couldn't find 3* details. I didn't want to make an assumption

 

[Storker]

I couldn't find 3* details.

Hm. I found it. Anyway, for your perusal:

CMAS Three Star Diver Standards (PDF)
CMAS Three Star Diver Syllabus (PDF)

As always with CMAS, the national Associations (that make up CMAS) are free to impose tougher standards for their particular flavor of a CMAS cert. CMAS' standards are the minimum.

 

[Diving Dubai]

Hm. I found it. Anyway, for your perusal:

As always with CMAS, the national Associations (that make up CMAS) are free to impose tougher standards for their particular flavor of a CMAS cert. CMAS' standards are the minimum.

it was only a brief search...

Interesting reading - very similar to BSAC DL. We do benefit from much more detailed training this side of the pond

 

[Storker]

We do benefit from much more detailed training this side of the pond

At least those of us who don't go the PADI route

I PADI'd all the way up to RD, but I don't see any reason for me personally to continue further along PADI's track. Diving is a hobby for me, and "going pro" has never been an option. If I'm going to take further courses, it'll be either CMAS 3* (which should fully qualify me as a dive leader in a club environment), or maybe Fundies (to develop my diving skills). IIRC, crossing over from PADI to CMAS 3* requires RD and a lower number of logged dives than I have. Interestingly, many instructors up here don't just require a certain number of dives, but also a certain number of drysuit dives before you're allowed to take the class. Which kinda makes sense...