How to play no-deco time vs. air time?

[BigTuna]

Usually when I dive, when to turn or ascend is governed by either the remaining air time (i.e., turn pressure or rock bottom pressure) or remaining no-decompression time. I'm wondering if there is some good advice for playing the two factors together.

For example, consider an out and back "tour" on a simple, square-profile dive, with a varying rate of air consumption. What's the best way to play tank pressure and no-deco time numbers to figure out when to turn?

In planning the dive, I can use tables or my computer to estimate the total no-deco time and plan to turn back at half that time. I can also use my personal rate of air consumption to estimate what my turn pressure should be, and therefore what the air-determined time should be for turning around. That's pretty elaborate planning for a simple recreational dive. Does anybody do it? If so, how do you use the planning numbers during the actual dive?

 

[PerroneFord]

Usually when I dive, when to turn or ascend is governed by either the remaining air time (i.e., turn pressure or rock bottom pressure) or remaining no-decompression time. I'm wondering if there is some good advice for playing the two factors together.

Yes, Figure out before the dive whether NDL or Rockbottom will be the limiting factor on the dive. Then dive it. I would say that in most recreational scenarios, dive time will be limited by gas, rather than by NDL.

For example, consider an out and back "tour" on a simple, square-profile dive, with a varying rate of air consumption. What's the best way to play tank pressure and no-deco time numbers to figure out when to turn?

Figure out average depth, and your emergency SAC (use 1.0 as your SAC), and figure out how much gas will be necessary to get you AND your buddy safely to the surface, including the safety stop. Then calculate your turn pressure. Likely this will be WELL before NDL.

In planning the dive, I can use tables or my computer to estimate the total no-deco time and plan to turn back at half that time. I can also use my personal rate of air consumption to estimate what my turn pressure should be, and therefore what the air-determined time should be for turning around. That's pretty elaborate planning for a simple recreational dive. Does anybody do it? If so, how do you use the planning numbers during the actual dive?

Yes we do it. I turn the dive after making a determination of how much gas I will need to get to the surface with my buddy and I sharing. Simple.

 

[BigTuna]

Perrone,

What you describe is what I do. But my breathing is improving, and more and more I'm finding NDL is limiting, not gas. I want to turn the dive before it becomes a mandatory deco dive, regardless of remaining gas. The thrust of my question is how best to use BOTH numbers, in real time, and for an arbitrary profile, to determine the turn point.

BT

 

[PerroneFord]

Oh I see. I typically carry enough gas so that it's not an issue. Or I dive thirds. The ambiguity of not knowing what will limit the dive is not something I want to deal with.

Best of luck finding your answer.

 

[do it easy]

Perrone,

What you describe is what I do. But my breathing is improving, and more and more I'm finding NDL is limiting, not gas. I want to turn the dive before it becomes a mandatory deco dive, regardless of remaining gas. The thrust of my question is how best to use BOTH numbers, in real time, and for an arbitrary profile, to determine the turn point.

BT

If I understand your question correctly:

At any point, 1) you should have enough gas to get back to the surface, whether that is straight up or swimming underwater and 2) enough NDL to return to the surface, again, straight up or swimming underwater.

Think of it as two dives, one limited by gas and the other limited by NDL, and turn before you hit either limit.

 

[Sideband]

Figure them both and the shortest one controls the dive.

 

[ClayJar]

For example, consider an out and back "tour" on a simple, square-profile dive, with a varying rate of air consumption. What's the best way to play tank pressure and no-deco time numbers to figure out when to turn?

For a simple square profile, the "best" answer is trivial. As has been said, you figure out what your turn pressure will be, and take half your available time at depth. If you reach either, you turn. If that seems elaborate for a simple recreational dive, it is only because many people do recreational dives without thinking... at least until they get to the "Oh, nuts! I'm (out of air / into deco)!" moment. For simple profiles, the planning is indeed simple... once you get the hang of actually doing it.

Playing NDLs against air time remaining becomes somewhat less trivial, however, for distinctly non-square profiles. If you spend a little less time at the deeper depths, it may give you much more NDL time at the medium depths, allowing you a longer dive and using more of your gas. If you spend all your NDL at the sand, you have nothing left for touring the top of the wreck, so you hit 15 feet with half a tank left. (Note: This implies some way of calculating multi-level dives -- most commonly a dive computer, at least for recreational divers I've known.)

(The general rule for recreational NDL diving is that the shallower dives are gas-limited, while deeper dives are NDL limited. The breakpoint between the two limits is based on your air consumption rate. For me, an 80-foot dive on air is always NDL-limited; for harder breathers, it may be that they have to get deeper before the more-rapidly-decreasing NDLs overtake their air times.)

 

[Teamcasa]

Deep air dives will most certainly be governed by NDL provided you have a sufficient air reserve and a decent SAC rate.

Dave

 

[BigTuna]

What do you think about this possible algorithm? (It doesn't require a square profile.)

1- Convert Rock Bottom for my current depth to time remaining at this depth, using a SAC rate of 1 cuft/min, and call this my "minimum gas time."

2- Now calculate the "remaining gas time." This would be the total time available from all the gas currently in the tank, minus the min gas time from step 1. [I could probably construct a table that gives remaining gas time as a function of depth and tank pressure and carry it along on a slate.]

3- In real time, compare the remaining gas time directly against my computer's report of remaining NDL time. Turn back no later than 1/2 times the smaller of the two times.

 

[rockjock3]

Whichever you reach first is your turnaround point.