Planned deco on a recreational dive?

[MXGratefulDiver]

Would you give an example using one of the above two you mentioned of how you would plan that dive?

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)

 

[tursiops]

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)

This is silly, really silly, and completely non-compelling. The most thought-out and careful dive plan in the world will not help you if you are "busy taking pictures, and neglect to keep track of your tank." You insist that anything other than your method is terrible because you might get preoccupied. But your method doesn't help either if you get preoccupied.

 

[MXGratefulDiver]

This is silly, really silly, and completely non-compelling. The most thought-out and careful dive plan in the world will not help you if you are "busy taking pictures, and neglect to keep track of your tank."

Interesting observation ... but it's helped me for thousands of dives, mostly from shore, and many of them solo. Knowing how much air I use, how deep I should go, and how long I can stay frees me up to focus on the camera ... which is usually the objective.

YMMV ... and that's fine. I was simply responding to a request for an example ... feel free to continue relying on your gauge if that works for you ...

You insist that anything other than your method is terrible because you might get preoccupied. But your method doesn't help either if you get preoccupied.

... at no point in this discussion did I insist that any other method is terrible. That's just flat-out lying. Please don't go there ...

... Bob (Grateful Diver)

 

[RandomGuy1]

@NWGratefulDiver What do you do if you have a leak develop and don't notice. At that point your idea of I should have 1/2 hour at 70 feet so I don't need to check goes out the window does it now?

 

[MXGratefulDiver]

@NWGratefulDiver What do you do if you have a leak develop and don't notice. At that point your idea of I should have 1/2 hour at 70 feet so I don't need to check goes out the window does it now?

How could you not notice a leak? If it's significant enough to cut seriously into your reserves, it's going to produce noticeable noise and/or noticeable bubbles. In which case, you take appropriate action to shorten your dive.

But to your point, planning your gas in advance doesn't preclude checking your gauge ... it just means you're not relying on that check to determine the dive profile.

Now I have a question for all of you who rely on this method ... if it's so reliable, why is running out of air one of the leading root causes of diving accidents every year?

... Bob (Grateful Diver)

 

[BDSC]

Now I have a question for all of you who rely on this method ... if it's so reliable, why is running out of air one of the leading root causes of diving accidents every year?

... Bob (Grateful Diver)

Because people do this like you said:

"And what happens if you're busy taking pictures, and neglect to keep track of your tank?"

 

[MXGratefulDiver]

Because people do this like you said:

"And what happens if you're busy taking pictures, and neglect to keep track of your tank?"

... then why would you presume that relying on a method people neglect to do would be sufficient?

But I don't think that's the complete answer. I've noticed, many times, people going deeper or diving more aggressively than their air supply warrants. They do so because they aren't aware (because they haven't been tasked or trained to consider it) that they shouldn't be there with the amount of gas they're carrying.

I saw this for 12 years teaching deep diving ... people showing up with tanks insufficient for the dive. After working them through an exercise similar to the one I described above, they came to a shocking conclusion ... "I don't have enough air to do that dive".

Well ... duh ...

... Bob (Grateful Diver)

 

[BDSC]

... then why would you presume that relying on a method people neglect to do would be sufficient?

... Bob (Grateful Diver)

Bob, tell me what method will work for a person who doesn't pay attention to their gauges and gets preoccupied or distracted during a dive? You can have the greatest plan in the world and if you don't pay attention it won't matter. Will it?

 

[archer1960]

... then why would you presume that relying on a method people neglect to do would be sufficient?

But I don't think that's the complete answer. I've noticed, many times, people going deeper or diving more aggressively than their air supply warrants. They do so because they aren't aware (because they haven't been tasked or trained to consider it) that they shouldn't be there with the amount of gas they're carrying.

I saw this for 12 years teaching deep diving ... people showing up with tanks insufficient for the dive. After working them through an exercise similar to the one I described above, they came to a shocking conclusion ... "I don't have enough air to do that dive".

Well ... duh ...

... Bob (Grateful Diver)

What method do you use to prevent yourself from getting distracted and forgetting to monitor your air and time? At that level, I haven't seen anything different in your planning from anybody else's. If either of you get distracted and forget to monitor whatever data it is that you use to turn the dive, it's a problem.

 

[MXGratefulDiver]

Bob, tell me what method will work for a person who doesn't pay attention to their gauges and gets preoccupied or distracted during a dive? You can have the greatest plan in the world and if you don't pay attention it won't matter. Will it?

I'm not suggesting that they don't pay attention to their gauges ... but rather that relying on a single data point, which in this case is your SPG, is less reliable than relying on multiple data points that are based on knowledge of your gas consumption rate and how much gas you're likely to consume for a given dive plan.

People are more likely to catch those inattentive moments if they have multiple data points than they are if they have only one. Proper planning gives you more tools to work with.

... Bob (Grateful Diver)