200' on air for 5 min bottom time?

[DevonDiver]

As others have expressed, I don't feel ethically confident to contribute information to a thread that may lead to the conduct of unsafe diving practices. If the OP can assure me that his question is entirely theoretical, rather than practical, this might be otherwise..

 

[Kevrumbo]

Lets say I am diving to 200' on air and descend at 50 fpm (4 min) and then stay down for 5 min before ascending. At the moment I ascend what would my ceiling be?

(I assume 4 min at 50 fpm down then 5 min BT would put me into deco)

Thanks

Minimum Deco methodology of the Ratio Deco Algorithm prescribes 1 minute stops at half the max or half the average operational depth for a 5min bottom time at depth 130'/39m and anything deeper. So for 200'/60m for only 5min bottom time, ascend at 30'/min (same as 9m/min in metric) to 100'/30m depth and hold there for 30 seconds; then take 30 seconds to ascend the next 10'/3m and hold at 90'/27m for 30sec. Continue this 30sec move & hold at each 10'/3m interval & depth plateau until you ascend to a depth of 20'/6m: then do a slow 3'/min (same as 1m/min) to the surface.

Be advised that you"re going to need roughly 70 cuft/1980 litres of air to nominally complete the above profile with a Surface Consumption Rate of 0.75 cuft/min (same as 22litres/min); for an emergency air sharing contingency with an out-of-air buddy, you both are going to need approx 100 cuft/2800 litres to roughly complete the above deco profile together (a SCR of 1cuft/min, same as 28 litres/min). The minimum volume of backgas tank cylinders to use would be a manifolded twinset of steel HP100's or AL100's (12L or 13L twinsets in metric respectively).

Finally the point of the above exercise in this hypothetical dive to 200'/60m, is to illustrate that it would be extremely dangerous to do this even as a shorter bounce dive on single tank for a novice diver because of deep air narcosis, and a limited air supply to provide enough volume for the deco profile as well as a potential air sharing emergency contingency with a buddy.

 

[chrpai]

Air sharing with a buddy? On this dive?

I really hope the buddy is a tech diver using Doubles with Trimix.

 

[Tortuga68]

Safer not to take a buddy, then you won't need any extra gas

 

[Dive Bug Bit Me]

To the OP. I also believe you're trying to better understand the fatality that's been discussed here on SB.

I just ran the dive plan for the following dive on Buhlman with no GF.
Gas = air
no other deco gas.
Descent time = 3 minutes
Bottom time = a further 5 mins.
Depth =200 feet.

Results were:
Total dive time 18 mins.
CNS loading 8%. OTU of 15. (Not enough Oxygen exposure to worry any technical diver).
I had 5 minutes of compulsory stops on the way back.
My gas consumption is well above average, and I should end with 600 psi on an AL80 tank.

Would I ever do the dive? Hell No!

My equipment is well maintained and I've still had a free flow at 240 ft. With the equipment listed above I would be dead today.
I have had a free flow at 115 ft while on Nitrox. I admit my response was weaker than the one at 240ft while on Trimix. Narcosis is real.
I will not dive the Buhlman tables with no Gradient Factors, just beyond my risk appetite.

So the problems with the dive include;
inadequate training to deal with issues (if anything goes wrong you are dead);
inadequate equipment (if anything goes wrong you are dead);
aggressive deco plan (no means to adjust for any delays on surfacing - most likely any error means you are in a wheel chair);
Narcosis means if anything goes wrong you are very slow to react. Time is of the essence in fixing problems at depth.

As someone mentioned 99 of 100 of these probably go unnoticed. The other one ends up here on SB.

 

[Tortuga68]

Total dive time 18 mins

3 mins descent plus 5 mins at target depth = 10 minutes ascent/deco or 3 mins of stops

So basically you're saying it can be done as a rec dive with a 3 min 'safety stop' if you have enough gas

What conservatism setting are you using to get that??

 

[Colliam7]

I think the various caveats (optimal mix, training, gas supply, etc.) have been mentioned, emphasized, reiterated, etc. To offer one additional answer to your question:

Lets say I am diving to 200' on air and descend at 50 fpm (4 min) and then stay down for 5 min before ascending. At the moment I ascend what would my ceiling be?

Download vplanner . . . to your desktop or phone . . .

And, if you run V-Planner on your phone (as a theoretical exercise, of course), and set it for a VPM-B model with a conservatism of +2, it would indicate you should ascend at 30 ft/min., and do a stop for 1 minute at 20 feet, and another stop for 2 minutes at 10 feet.

And, dives exactly like this are done in warm tropical waters (frequently on an AL80) more often than we might like to acknowledge, without incident. Occasionally, however, . . .

 

[Dive Bug Bit Me]

3 mins descent plus 5 mins at target depth = 10 minutes ascent/deco or 3 mins of stops

What conservatism setting are you using to get that??

No gradient factors =no conservatism.

Normally I would dive 30 / 80 GF, but seeing as the rest of the plan for the above dive was somewhat aggressive the 100/100 GF would be the least of my worries.

 

[Tortuga68]

seeing as the rest of the plan for the above dive was somewhat aggressive

What plan?

I still don't see how you get an 18 min dive. Do you have a chamber frequent flyer card?

 

[BDSC]

I still don't see how you get an 18 min dive. Do you have a chamber frequent flyer card?

Doing the math quickly in my head, Colliam7's V-planner would also give an 18 minute dive.