Planned deco on a recreational dive?

[BRT]

Sure, I'm a photographer, so I do it all the time knowing that I'm going to want to focus primarily on my camera.

I'm doing a shore dive, getting in at Point A, getting out at Point B. In order to go from Point A to Point B I have to go around a pier, and avoid the pier by a minimum of 150 feet ... which means I'm going to spend a fair bit of the dive in the 60-70 foot range in order to keep this distance. For the sake of planning, I'll figure I'm going to average roughly 65 feet, or 3 ATA's for most of the dive. That means I'll be using my air at 3 times my SAC rate. Let's say that I know my SAC rate to be 0.6 cubic feet per minute. That means I'll be using roughly 1.8 CFM on average for this dive. But it's a shore dive, so I will be spending the early and later parts shallower ... which means I can safely round down to 1.5 CFM to account for swimming down to depth, swimming up from depth, and spending time at safety stop. I want to plan for a 60-minute dive. That means I'll be needing 1.5 x 60 cubic feet of gas to do this dive (I use EAN32, so NDL's won't be a factor ... therefore I'm only going to look at my gas needs). 60 x 1.5 is 90 cubic feet of gas. That means if I want to do this dive I should be carrying a tank no smaller than 100 cubic feet, to have sufficient reserves to account for anything that doesn't go according to plan. If I don't have a tank that big, I should shorten the dive or shallow it up ... which in this case would mean choose a different profile. I make that decision before getting in the water. Now, this also gives me some other information to work with during the dive. Suppose I see something interesting, and want to extend my dive? That tells me without even much thought that I'll need to get shallower sooner than planned. Or suppose I want to go deeper for some reason ... say I see something below me that I want to take pictures of. That tells me that I'll need to shorten the duration of the time spent at depth. These are decisions I can now make based on real information, rather than feedback from watching my gauge.

Compare that to watching your gauge. Without giving your gas consumption any forethought, you went on that dive using a smaller tank. Now you're three-quarters of the way around that pier and realize you're approaching your 700 psi (or whatever you chose). You're at a depth of 70 feet, and a direct ascent puts you in front of the pier ... and into potential boat traffic. What do you do?

Sure, you'll probably be OK. But why put yourself into that situation when, with a little bit of planning you could have avoided it?

And what happens if you're busy taking pictures, and neglect to keep track of your tank? Suddenly, at 70 feet, you breathe your tank dry. OK, you signal your buddy, get on their air (and if you just ran out, how much do you suppose your buddy might have to spare?). Maybe you'll be OK ... but why put yourself in that situation?

With a little bit of forethought, you'd have known ... because you planned for it ... that you had sufficient gas to do the dive you planned for. Or you'd have known ahead of time that the tank you planned to use wasn't adequate for that dive, and changed your plan accordingly.

It's not rocket surgery ... it's elementary school arithmetic. Is it necessary? No ... not on every dive. But a little bit of forethought can help you avoid the most common mistakes a lot of divers make fairly regularly when it comes to managing their air supply.

Monitoring your gauge is good. But knowing before the dive that you have an adequate air supply for the dive plan is way better.

... Bob (Grateful Diver)

But with boat traffic overhead you are actually in an overhead environment. This is not rec diving.

 

[MXGratefulDiver]

Bob, what did you rely besides turn pressure? I'm assuming that you are going to say that when you noticed that the current was going to be against you coming back you turned sooner? That is not preplannable but it seems obvious. We always go against current out if possible and if I found myself going with current on the way out and had to get back to the entry point I would cut the swim short and stay close to the exit.

It seems obvious, and it is. The question is "by how much" ... and understanding and applying your consumption rate helps you better determine what that should be. Again, it's not a question of methods, so much as simply having more information to work with when making those decisions.

It's not an either/or ... it's not a good/terrible (as has been suggested) ... it's a simple matter of understanding the relationship between conditions, depth, and breathing rate to help you make better decisions. If all you do is watch your gauge, it's unlikely you're going to develop that understanding. And while you will usually make the right decisions, it will be based more on intuition or prior experience than on knowledge. That'll work quite OK for some people ... less so for others.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

But with boat traffic overhead you are actually in an overhead environment. This is not rec diving.

Of course it's rec diving. Have you never dived in an area where boat traffic is likely before? You can still ascend directly to the surface ... you just have to take your chances that someone might come by in a boat and not see you. The question I posed is why put yourself in that situation if you don't have to?

... Bob (Grateful Diver)

 

[MXGratefulDiver]

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.

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)

 

[BRT]

Of course it's rec diving. Have you never dived in an area where boat traffic is likely before? You can still ascend directly to the surface ... you just have to take your chances that someone might come by in a boat and not see you. The question I posed is why put yourself in that situation if you don't have to?

... Bob (Grateful Diver)

I am happy diving with boats going overhead, in spite of the fact that I claim to be a strictly recreational diver. I am not going to put myself into a situation where I will have to do a long swim against current to get free from that boat area where they are not expecting divers. On Coz we shore dive with boats flying overhead but a short swim will put us close enough to shore that they will not be overhead. The backup plan to the plan of heading for shore is of course to shoot a marker. As you know anytime you are diving off a boat in Cozumel you have boats overhead and it makes no difference where you surface unless you can figure out which boat is yours and get close to it.

 

[MXGratefulDiver]

So it someone is out lobstering and not watching their gauges what plan would save them?

... 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)

 

[MXGratefulDiver]

I am happy diving with boats going overhead, in spite of the fact that I claim to be a strictly recreational diver. I am not going to put myself into a situation where I will have to do a long swim against current to get free from that boat area where they are not expecting divers. On Coz we shore dive with boats flying overhead but a short swim will put us close enough to shore that they will not be overhead. The backup plan to the plan of heading for shore is of course to shoot a marker. As you know anytime you are diving off a boat in Cozumel you have boats overhead and it makes no difference where you surface unless you can figure out which boat is yours and get close to it.

In Coz the boats are used to divers, and expect them to be there. It's a bit different than diving in someplace like the Straits of Juan de Fuca, or some of the wall dives along the Gulf Islands off the coast of Vancouver, where you're dealing with tugs towing barges or pulling rafts of logs ... you may start the dive in open water conditions that turn into an overhead while you're down there, in which case you really want the ability to swim close to shore, or stay down until the boat, barge, or log raft move on by.

Carrying and deploying an SMB is an awesome idea, and I don't dive in the above mentioned areas without one. But never assume that the crew of a working boat either knows or cares what it means ...

... Bob (Grateful Diver)

 

[gfaith]

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..

 

[tursiops]

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..

?? By the time you get to 15 ft for the SS, the NDL will be huge, whether you spent 12 or 16 mins at 110 ft. So I don't understand.

 

[gfaith]

The point is that even if you spend the entire allowed time of 16 minutes the divers in the example have more than 700psi. I agree the NDL would be reset by the time we reach the SS. The main reason we would cut it back to 12 minutes would be to have enough air to share if needed on ascent. By the way, we most likely would not make this dive on single AL80s.