Planned deco on a recreational dive?

[admikar]

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 will give your student benefit of doubt and say he said truth about his SAC. But, was it his average SAC or "best" SAC. For some reason, my best SAC is on night dives and there is a difference of 10 lit/min of my best and worst. My average is 17 lit/min, but I always plan on 20. As for accuracy, I do not know when his computer starts tracking pressure, my numbers are before I splash and when I exit water, I don't care if there is a cooling phase on my thank, I care how much air I have available.

 

[Diver-Drex]

@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.

Agreed. I wasn't trying to give a recommendation for rock bottom calcs, just illustrating that the "general guidelines" in the SCUBA community aren't adequate in many cases.

 

[gfaith]

Agreed. I wasn't trying to give a recommendation for rock bottom calcs, just illustrating that the "general guidelines" in the SCUBA community aren't adequate in many cases.

And I agree with you "general guidelines" are not adequate.

 

[EricTheDood]

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.

In your first example, divers would turn when they hit "X" psi or 25 min, whichever comes first. Your calculations are valid, and thus in many cases the divers would turn before reaching 25 min. The SPG overrules the clock.

In your second example, you've described a near-worst-case emergency situation. Again, your numbers are valid, but they factor in a non-emergency ascent rate. Most divers would ascend faster, e.g. 60fpm, before reaching shallower water and slowing down.

 

[MXGratefulDiver]

The line between no stop and deco diving is very clear.

With no stop diving, follow your computer and monitor your air. Gas planning is an afterthought.

With deco diving, gas planning is necessary.

It's not about danger, it's about planning.

Is this a joke? If not, I suggest you should go talk to your OW instructor and ask for a refund ... he apparently neglected to teach you some fairly fundamental concepts about decompression and gas management.

Breathing is not an afterthought ...

... Bob (Grateful Diver)

 

[boulderjohn]

While I do teach gas planning to OW divers and believe it is important, I have to admit that for a lot of dives,....well, let's compare two dive scenarios and see if you can catch my drift. The first is a well-planned dive done by fully trained and experienced cave divers, with good gas planning. The second involves two OW divers who do what most people would consider to be no gas planning whatsoever.

1. The two divers are planning to dive a cave with no noticeable current. They are using identical tanks filled to the same amount. They will follow the contour of the cave as they dive. They agree that they will turn the dive and return to the entrance when one of them reaches one-third of their pressure.
2. The two divers are planning to dive a shallow wreck with no noticeable current. They are using identical tanks filled to the same amount. They will follow the contour of the wreck as they dive. They agree that they will ascend when one of them reaches 700 PSI.

See the difference?

 

[tursiops]

See the difference?

No. I guess I am missing your point. Sorry.

 

[boulderjohn]

No. I guess I am missing your point. Sorry.

It's called irony.

I don't see the difference either.

 

[oldschoolto]

While I do teach gas planning to OW divers and believe it is important, I have to admit that for a lot of dives,....well, let's compare two dive scenarios and see if you can catch my drift. The first is a well-planned dive done by fully trained and experienced cave divers, with good gas planning. The second involves two OW divers who do what most people would consider to be no gas planning whatsoever.

1. The two divers are planning to dive a cave with no noticeable current. They are using identical tanks filled to the same amount. They will follow the contour of the cave as they dive. They agree that they will turn the dive and return to the entrance when one of them reaches one-third of their pressure.
2. The two divers are planning to dive a shallow wreck with no noticeable current. They are using identical tanks filled to the same amount. They will follow the contour of the wreck as they dive. They agree that they will ascend when one of them reaches 700 PSI.

See the difference?

There is no difference, Because both are doing gas planning... That is not the same as just blindly getting in the water without talking to any buddies you are diving with.. It doesn't have to be a big deal.. But having a talk about air and what "WE" are going to do is important...

Jim..

 

[boulderjohn]

There is no difference, Because both are doing gas planning... That is not the same as just blindly getting in the water without talking to any buddies you are diving with.. It doesn't have to be a big deal.. But having a talk about air and what "WE" are going to do is important...

Jim..

It's still not planning the way some people would demand it. There is no talk about SAC rates. There is no pre-planned time at any specific depth.