Planned deco on a recreational dive?

[MXGratefulDiver]

This thread has made me do some thinking and calculating.

... my work is done here ...

... Bob (Grateful Diver)

 

[gfaith]

No Bob, there are lots of starfish to throw back..
Steve Lewis

 

[BRT]

So you used 400 psi for a normal ascent from that depth. That's reasonable. So if your dive buddy needed to ascend while sharing your tank, your assumption is that would take double that amount. Logical, but not likely. Consider that if one of you is OOA, that one's going to be stressed, and breathing harder than normal. Also consider that unless you've practiced it regularly, ascending while holding onto each other isn't going to go as smoothly as ascending alone ... which you do practice regularly. That will tend to put some stress on you, which will result in an increase in your breathing rate as well. That 800 psi you expect to use could easily exceed the 1200 you're allocating before you reach the surface. You can, of course, just go faster and skip your safety stop ... but you're just completing a dive from 110 feet, so it's not a recommended way to end that dive.

It's all about risk mitigation. Maybe you can do it, maybe not ... would recommend you practice it with somewhat more gas than you expect to need before deciding that you can. We're all different. And for 12 years I taught this stuff and took individual measurements from students in both normal and stressed conditions. What I saw is that the variations were all over the map, due to differences in mental and physical makeup of the students ... but on average, a stressed diver will roughly double their normal consumption rate. Some barely registered a difference, while others tripled or higher their consumption rate when placed in stressful circumstances.

... Bob (Grateful Diver)

I'm confident that we can get safely to the surface in less than 3 minutes. That is not a rate I would choose to ascend at but in an emergency I'd think nothing of it. 2200# in an AL80 gives us an hour or more at an average depth deeper than 50 feet. That is 37 psi per minute. Twice that for 2 people is 74 psi per minute. Double that for stress and we have 150 psi per minute. 800 divided by 150 is 5 and 1/3 minutes to empty. I think we will get a safety stop. This is not a situation that I would choose but there is risk in diving and this does not strike me as the worst risk we take. You are assuming total gas loss instantly and I think you will have a hard time doing that. After all this happens, everything goes wrong, total gas loss at the worst possible time, my wife and I each have that useless 3cf in our spare airs. I don't ever count on them, but there they are. Another minute, or 2 or 3.

There are the calcs, but I don't need them because I already know I am ok from 110' on 400 psi.

 

[BRT]

... knowing that they have the gas to go to a certain depth for a certain amount of time. I didn't say they weren't watching their gauges, I said they'd get distracted at some point in the dive. Making a plan makes it easier to stick to one than just watching your gauge and deciding to turn around when it hits a certain point .... that's why, in your OW class, they taught you to "plan your dive and dive your plan". Where they obviously failed was teaching you how to do it ...

... Bob (Grateful Diver)

But if they distracted they don't know how long they have been down????

 

[Kevrumbo]

This thread has made me do some thinking and calculating. Such as, a diver with RMV .6 diving to 110 feet on air AL80 after 16 minutes at depth NDL is near or at zero. Diver used 4.86 cuft during 3 minute descent, 42.24 at depth and now has 30.3 cuft left from a 77.4 cuft tank (about 1174psi). In other words, he/she has to head up anyway or go into deco because NDL is zero (still more than 700psi).

Is this enough for me and my buddy under stress??? She/buddy said "nope we are only going to be at depth for 12 minutes". I say yes ma'am. Now we have used 4.86 + 31.68 and have 40.86 cuft left 1583psi. She is fairly happy.. Why only 12 minutes? She wants to have NDL time in case we need to skip SS.

Just some thoughts..

Another method (new GUE):

Using a slightly higher RMV of 0.75 and allowing one minute at 110' to get into gas sharing emergency contingency mode with your buddy, and then a slow and sane constant 10 fpm ascent rate to surface --which would be 11 minutes to transit. Average depth is 2.65 ATA (55fsw) -->

2 divers x 0.75 rmv x 2.65 ATA x 12 minutes = 47.7 cuft.

UTD's method for two novice divers both stressed to 1.0 rmv in an emergency gas sharing contingency, a "Rock Bottom" amount of 42.4 cuft would cover a quicker ascent of 30 fpm from 110' to 50', and then a slowing "minimum deco" ascent rate of 10 fpm from 50' to the surface -->

2 divers x 1.0 rmv x 2.65 ATA x 8 minutes = 42.4 cuft.

So the point is you're gonna have to reserve more than half of the 77.4 cuft in an AL80 for an emergency air sharing contingency from 110'. If you need more usable air at depth to cover the planned nominal 12min BT (15min NDL time) at 110', then obviously you will need a larger volume cylinder(s). . . [hp steel 130 or double AL80's tanks for example].

 

[Diver-Drex]

The post I answered was the proof of the inability to two people to surface from 110' on 800 psi in an AL100. Absolute calculations that showed you couldn't do it. And absolute nonsense. I don't have to calculate it I know I can do it.

Yes. If my wife comes up to me OOA at 110" and I have 800# we will have air to go up.

You get that number by leaving 110 feet with 1200 lb and finding yourself back on the boat with 800 after doing your safety stop. I don't want to leave the bottom at 110 high on nitrogen with 700 lb but I have.

Possible does not constitute a good plan. I did a free ascent from 40fsw during my OW cert. I know I can do it if my life depended on it, but I plan to avoid that. I think we agree that makes sense.

 

[gfaith]

@Kevrumbo yes I agree we need more that 1/2 tank using a AL80. By the way, I still use your rock_bottom_revA.xls to check my plans. Appreciate it.. I would also use larger tanks.

@BRT a full secondary hose leak can evacuate a AL80 in 81 seconds as per Curt Bowen tests.
Life Ending Seconds • ADVANCED DIVER MAGAZINE • By Curt Bowen

 

[Diver-Drex]

So back to the original question... Decompression dives, whether you call them tec or rec, require planning. Planning to determine deco obligation and planning to determine your gas need and adequate reserve. If you don't know how to do this go get some training to learn how.

 

[BRT]

@Kevrumbo yes I agree we need more that 1/2 tank using a AL80. By the way, I still use your rock_bottom_revA.xls to check my plans. Appreciate it.. I would also use larger tanks.

@BRT a full secondary hose leak can evacuate a AL80 in 81 seconds as per Curt Bowen tests.
Life Ending Seconds • ADVANCED DIVER MAGAZINE • By Curt Bowen

Yes it can, but not instantly.

 

[oldschoolto]

You guys are making a mountain out of a mole hill.. Just came back from lionfish hunting tonight... 148' max depth and end up with 15 of Deco to do. I knew how much gas I had and turned the dive to get back to the exit point with the gas needed to do my Deco.. plus had my 19 cu pony if needed... Total dive time was 69 minutes with the Deco stop.. It's not rocket science.. The Diver should be able to make calculated ,Deco, Time and depth with gas left and usage on the fly.. if they can't... They should not be at a depth that REAL Deco is likely... Again, Deco needs to be a thing that you understand and can figure out on the fly in your head without a computer... You need to get the tables set in your brain... Dive with in your limits..

Jim...