Planned deco on a recreational dive?

[oldschoolto]

All you need to do is talk with dive masters a tropical dive spots and you get the feeling that 60% or better have no clue how to dive.. They plan is to look at fish.. and that's the total planning that goes on .

And it's NOT their fault... It's the Training... It's the anyone can dive game...

Jim..

 

[boulderjohn]

I'll go through my OW book this week-end, I learned most of my diving outside of courses (forums are wonderful ), so I don't remember exactly what was in the book and what I found elsewhere at the time. Since OW divers should be able to plan their dive, I expect there is some level of gas management in it, even very light.

If PADI, it depends upon when you took the course. There is much more now than there was even a few years ago. A number of instructors, many active on ScubaBoard, made a concerted effort to bring that problem to PADI's attention, and when the next revision of the OW course came out, there was much more. On the final exam, for example, students must calculate turn pressure leaving a specific gas reserve and using the rule of thirds. They are still not required to know their SAC rates, but they are expected to be able to calculate how much faster they will go through their air at a deeper depth. (That was true years ago as well.)

 

[Diver-Drex]

I'd say it's safer for them to blindly follow typical guidelines, which are generally conservative, than to attempt to calculate gas consumption, for which they're not qualified to do anyway. Gas planning is NOT a good thing if done incorrectly.

Simpler, OK. But not safer. And what is good if it's not done correctly?
The universal gas plan is to be back on the boat with 500psi. Ok, how do we figure out how to do that? We'll swim around and be back at the up line at around 800psi. Here are two examples where this doesn't work.
- We're doing a 60fsw wall dive. On the NAUI table we have a 55 minute NDL, but decide to be conservative and do 50 minutes. Our plan is to go down the mourning line, swim into the current with the wall on our right for 25 minutes, turn and come back to the mourning. Following guidelines, planning a little and being conservative. The problem is we don't have the gas to do this. At 60fsw with a surface RMV of .7 we are breathing 1.97 cf/min. At 25 minutes we've breathed 49cf, we don't have enough gas to complete the dive. 30 seconds of math on a smart phone calculator solves this. Literally 4 math function - divide by 33, add 1, multiply by your RMV and then multiply by planned dive time.
- Now were diving the U352 at 110fsw, breathing 30%. Using the NAUI tables again with an EAD of 100fsw we stay conservative and decide on a 20 minute dive time. Again the guideline tells us to be back at the up line for our ascent with 800psi. We're diving AL100s so 800psi gives us 24cf of gas. During the dive we are monitoring our air and make our way back to the grapple with 800 psi each. Just as we are getting ready to start our ascent I have a regulator failure and total gas loss. We begin to share air on our way up. The problem is that even without a safety stop we need 26cf to get to the surface and our "guideline" only gave us 24cf.
These are both common dives being executed by OW and AOW divers. I didn't contrive some off the wall scenario to prove a point. Not using basic gas planning leaves gaps. I learned how to gas plan when I was 13, without a calculator or smart phone. This isn't asking too much.

 

[admikar]

You say it's a problem that most new divers don't know how to calculate their gas consumption. I'd say it's safer for them to blindly follow typical guidelines, which are generally conservative, than to attempt to calculate gas consumption, for which they're not qualified to do anyway. Gas planning is NOT a good thing if done incorrectly.

I agree with your last sentence. With the rest, not so much. I believe every e-log software can calculate your SAC or RMV, if you care enough to write down your start and end pressures. It can be done by yourself, if you own a DC, math is simple. Whether you care to do that is a different story.

 

[boulderjohn]

I believe every e-log software can calculate your SAC or RMV,

I had a student start a tech program with me, and when we talked about SAC rates, he said he knew his already because his air integrated computer kept track of it for him. We talked about the fact that it would be different with doubles, a dry suit, etc. I was expecting something seriously great with him, because his SAC rate as a recreational diver was MUCH better than mine ever was. I was seriously impressed with it. Then we went diving. Maybe it was the doubles and the dry suit, but he never came close to matching me on a dive throughout his training. His SAC rate was not bad, but I would consider it below average for a beginning tech student. I find it very hard to believe that the number he got from that computer was close to accurate, and I am sure he was not lying about it.

 

[packrat12]

... Now were diving the U352 at 110fsw, breathing 30%. Using the NAUI tables again with an EAD of 100fsw we stay conservative and decide on a 20 minute dive time. Again the guideline tells us to be back at the up line for our ascent with 800psi. We're diving AL100s so 800psi gives us 24cf of gas. During the dive we are monitoring our air and make our way back to the grapple with 800 psi each. Just as we are getting ready to start our ascent I have a regulator failure and total gas loss. We begin to share air on our way up. The problem is that even without a safety stop we need 26cf to get to the surface and our "guideline" only gave us 24cf.
These are both common dives being executed by OW and AOW divers. I didn't contrive some off the wall scenario to prove a point. Not using basic gas planning leaves gaps. I learned how to gas plan when I was 13, without a calculator or smart phone. This isn't asking too much.

I guess I do not understand your math here. Lets look at your RMV at .7. Coming up from U-352 @ 110' is an average of 55' for simple math. RMV @ 55' is 1.86. The ascent time from 110' is just over 3 minutes. The gas used to ascend from 110' to 0' is roughly 6.2 cuft per person or 12.5 cuft for both. Add in a safety stop @ 15' adds another 3 cuft per person or a total of 6 cuft for the safety stop and <20 cuft total. Leaves you 180psi which is minimal but we can really skip the safety stop and have an additional 6 cuft available which gives us ~12 cuft and 360psi. With safety stop: <20cuft used, without safety stop: <14cuft used. Not optimal but not short from the numbers perspective.

 

[RainPilot]

I use an elevated SAC for rock bottom calculations, maybe that's where the numbers change?

 

[Basking Ridge Diver]

I had a student start a tech program with me, and when we talked about SAC rates, he said he knew his already because his air integrated computer kept track of it for him. We talked about the fact that it would be different with doubles, a dry suit, etc. I was expecting something seriously great with him, because his SAC rate as a recreational diver was MUCH better than mine ever was. I was seriously impressed with it. Then we went diving. Maybe it was the doubles and the dry suit, but he never came close to matching me on a dive throughout his training. His SAC rate was not bad, but I would consider it below average for a beginning tech student. I find it very hard to believe that the number he got from that computer was close to accurate, and I am sure he was not lying about it.

I used a VEO 2.0 and their OceanLog software for a long time. This was not AI. The key was I would before I enter the water note the PSI in each tank(s) and write it down then go dive. I had a decent SAC but nothing to write home about.

Now I use the Shearwater Perdix AI with transmitters - here when I enter the water - the tanks cool and the PSI drops I fill my drysuit I fill my wing and sure enough the AI recording does not kick in until after I go 4 feet down. So now all of a sudden my SAC looks like I run marathons and sip air... All I did was change PDC's... So when I plan I use my old numbers because that is really what I am using - not this .45 SAC that AI is spitting out...

YMMV but be careful using AI because to me it is deceptive and not showing the full picture...

 

[Diver-Drex]

I guess I do not understand your math here. Lets look at your RMV at .7. Coming up from U-352 @ 110' is an average of 55' for simple math. RMV @ 55' is 1.86. The ascent time from 110' is just over 3 minutes. The gas used to ascend from 110' to 0' is roughly 6.2 cuft per person or 12.5 cuft for both. Add in a safety stop @ 15' adds another 3 cuft per person or a total of 6 cuft for the safety stop and <20 cuft total. Leaves you 180psi which is minimal but we can really skip the safety stop and have an additional 6 cuft available which gives us ~12 cuft and 360psi. With safety stop: <20cuft used, without safety stop: <14cuft used. Not optimal but not short from the numbers perspective.

I built a spreadsheet for gas planning and used that with an RMV of 1, which is likely on the low end for an inexperienced diver in this situation. But the average depth method of calculating it gives about the same result.
Using an RMV of 1cf
1 min at 110fsw to sort it out = 8.7cf
3.5min for ascent using 55fsw as avg depth = 18.7cf
Total = 27.4cf

 

[gfaith]

@Diver-Drex add about 10cf for a safety stop and that would be the same as what my rock-bottom for 2 divers with RMV of 1.0. So the turn pressure using HP100 would be around 1400psi on either my rig or buddy's. I think it is wise to plan for the 3 minute stop knowing it is not required here.