check my logic?

[CD_in_Chitown]

Running a 100' for 30 minutes plan (Milwaukee Car Ferry - standard beginning tech diver plan around here ) I require 72 cubic feet of backgas and 6 cuft of deco gas to execute. Run it through the *1.5 1/3rds factor and 108 and 9 are my magic numbers. but even an RMV of 1 x 10 minutes on deco for 2 divers = 20 cuft deco mix. Half the volume I'm carrying and theoretically the least volume I would carry if I tried to get two such dives on the same 50/50 fill.

 

[grazie42]

Like someone said...can you think of a scenario where someone loses both backgas and decogas? How likely is that scenario? What level of risk are you willing to accept?

If you feel that total loss of one divers gas is something you want to plan for, then carry on... I´d think your time would be better spent thinking about other kinds of failiure and how to prevent/respond to those...

ymmv...

 

[limeyx]

could you elaborate? that's what i'm trying to think through

i figure that if worse comes to worse, you want to get through deco and
get your buddy through deco, so just double your deco gas.

that way, you'll be covered no matter when the emergency happens

Here's (roughly) how I would do it:

1) Back gas. You need enough gas reserved to get two stressed divers, one OOA up to your "next gas" (either the surface or deco bottle). This is why diving 50% deco bottle is so nice -- you can get on it deeper than 100% so you dont need to reserve as much gas. this gas reserve needs to include all deco stops up to the gas switch.

2) Deco gas. Opinion is split here.
Some people reserve 1.5x deco gas.
Some reserve some deco gas for if you mess up (spend longer, go deeper etc) and then plan to share the deco gas if there's a problem.

Also, remember you are planning for only 1 "major" failure,so you wont plan for lost back gas and lost deco gas.

So my planning would be:

say 25 min dive to 150 gas = 21/35 back and 50% deco in an AL40

Back gas -- enough gas is reserved to get two divers from 150 feet to 70 feet, including 1 min stops at 100, 90, 80 and gas switch at 70 feet. This works out somewhere around 50-64 cft depending on how conservative you want to be.

Deco - enough for 5 mins extra.

So what can go wrong? We have two main problems that can occur.

1) Back gas -- worst case is as you surmised. One diver goes OOA at their rock bottom. -- No problem. Share gas and ascend to 70 feet, doing all deco needed and then you are on your deco gas to the surface.

2) Loss of deco gas. You get to 70 feet and one diver loses all their deco gas.
Let's assume we have 3 min stop at 70 and 60. For 50%, these are the critical stops (although many people might argue with that)

So the plan is: The diver with his deco gas does 3 mins on deco bottle at 70 and 60, 3 mins on back gas (giving the deco gas to the other diver).

Then you do the rest of the deco either sharing at each stop or one diver on back gas and the other on deco gas (extending the stops somewhat for back gas).

So think about how much gas is available for deco like this:

Diver 1 with no failures has 40 cft in deco bottle and the gas remaining from his RB (this should be around 3/4 of RB gas since you only had one non OOA diver ascending from 150 to 70 feet)

So the normal deco plan would be 2 guys on AL40 = 80 cft
Now you have 40 cft (one deco bottle) + 2 * 36 cft

So you have enough gas to extend your deco and share the 50% on the deeper stops.

yes, if both deco bottles go, or you go OOA and lose a deco bottle, you are in trouble, but you can only plan for so many failures.

If the crud really hit the spinny thing, it's good to have a "disaster plan" In this case, my training says that doing the deeper deco is a good idea as that will clean out the faster tissues and hopefully reduce the chance of a typeII DCS hit.
So I'd do the 100,90,80,70,60 stops then ascend to 20 feet (slowly) and burn whatever gas was left.

 

[limeyx]

Running a 100' for 30 minutes plan (Milwaukee Car Ferry - standard beginning tech diver plan around here ) I require 72 cubic feet of backgas and 6 cuft of deco gas to execute. Run it through the *1.5 1/3rds factor and 108 and 9 are my magic numbers. but even an RMV of 1 x 10 minutes on deco for 2 divers = 20 cuft deco mix. Half the volume I'm carrying and theoretically the least volume I would carry if I tried to get two such dives on the same 50/50 fill.

100' for 30 is a no deco dive on 32%
You just need standard rock bottom, including gas for stops at 50, 40, 30, 20, 10 (but a direct ascent is possible)

Here regular rock bottom rules apply.

 

[JimC]

what he said

 

[limeyx]

so in that case, you would use 1.6 for yourself and .3 for your buddy? (that is,
they can loose half their total deco gas need (either backgas or stage) and
still be able to complete deco with your reserve.

so.... assuming 100 cf dive, that is 160 cf to get me through deco,
and 190 cf. to get me and my buddy to deco should he lose half his
backgas or deco bottle?


btw, if i could be diving right now, i would be... can't, so am here instead

I think planning multipliers for back gas is going to end in trouble.
just use the regular RB rules as extended for deco diving and get the right numbers.
The multiplier on deco gas can be fine (since you extend the deco if you have to do some of it on back gas) but here you need to include some back gas as available deco gas or you wont have enough unless you carry a big deco bottle and that has other issues.

 

[CD_in_Chitown]

100' for 30 is a no deco dive on 32%

But it is on 29, who dives a 1.3 running Po2 anymore? You can't go below 100 without He without being a spoke, use whatever numbers suit your fancy the theory is valid n'est pas? Or am I confusing RB and 1/3 rules?

 

[limeyx]

But it is on 29, who dives a 1.3 running Po2 anymore? You can't go below 100 without He without being a spoke, use whatever numbers suit your fancy the theory is valid n'est pas? Or am I confusing RB and 1/3 rules?

1.3 PPo2 may be a tad high (Gue I think officially says you ought to limit 32% to an average of 90 feet depth). Maybe I'm just a rebel and like to push the envelope

if i was really concerned (i.e. it was going to be a stressful dive) I'd take 30/30.
I could also consider using 30% (gasp) and diving it like 32%. (Now I'm really in for it)

 

[T-Dog]

As you state up front you're just looking for a quick "rule of thumb", but figuring out your deco gas requirements isn't much more difficult than multiplying your bottom gas by a fudge factor.

A common profile for me & my buddy is a 100' dive on 32% O2 for 60 min. The NOAA 32% table shows 25 min @ 10' for deco for this profile. In practice we will do a "deep" stop @ 60', we'll program our computers to go on 100% O2 @20', and then move to 10', but let's assume the deco tanks are missing.

My buddy & I have identical SAC rates - (0.7cfm, 0.5cfm @ deco). So our gas requirements, using rectangular profiles and rounding up to be conservative, are the following:

100' (~4ata) x 0.7 x 60 = 168 ft3 - call it 170 ft3
60' (~3ata) x 0.7 x 3 = 6.3 ft3 - call it 10 ft3
20' (1.6ata) x 0.5 x 3 = 2.4 ft3 - call it 3 ft3
10' (~1.3ata) x 0.5 x 25 = 16.3 ft3 - call it 17 ft3

Total gas requirements 200 ft3. In this case a multiplier of 1.2 or 1.3 times your bottom gas would actually be more conservative.

We use 112ft3 LP doubles pumped to 3600psi which is 305ft3 of gas, call it 300ft3.
So if some drunk redneck steals our deco tanks we can deco on back gas.
BTW I've met 2 people on separate occasions that this has happened to!

However, if the O2 bottles are there when we return, then bonus, we can get out of the water in about half the deco time, based on computer times. I carry an 80ft3 O2 tank, and my buddy carries a 40ft3 O2 tank, and either one is enough for both of us to deco on if we’re only missing one tank.

 

[limeyx]

...

We use 112ft3 LP doubles pumped to 3600psi which is 305ft3 of gas, call it 300ft3.
So if some drunk redneck steals our deco tanks we can deco on back gas.
BTW I've met 2 people on separate occasions that this has happened to!

However, if the O2 bottles are there when we return, then bonus, we can get out of the water in about half the deco time, based on computer times. I carry an 80ft3 O2 tank, and my buddy carries a 40ft3 O2 tank, and either one is enough for both of us to deco on if we’re only missing one tank.

Wow, you must dive some ferocious places if some redneck is going to swoop down on you at 100 feet and cut your O2 bottles off you