Shallow Decompression Dives

[eponym]

What does a recreational diver have available to them (course wise) to allow them to be trained to safely plan and execute shallow decompression dives?

Good question, nice heat-free thread.

While the current courses are geared more to the depths that make deco necessary (120 feet and beyond, give or take), I forsee a time when the major US agencies offer an "intro deco" course. SSI has added a new tech arm, in some ways similar to the SDI/TDI approach.

In the meantime I like the "hire your own instructor" advice, but as an instructor myself I would want to evaluate the prospective student's skills and use a customized liability waiver. But it does seem doable.

I would add one item to Peter Guy's excellent list of skills. When I took TDI's Deco course I was amazed, challenged, and ultimately delighted to meet an instructor who was willing to increase stress instead of defusing it, with no hesitation to wash out candidates who voiced misgivings. Given the risks involved, I would hope that anyone undertaking deco training, even for shallow dives, would have the skills and the moxie to withstand some real stressing.

-Bryan

 

[Kevrumbo]

Ratio deco is really (as I understand it, at least) just a curve fitting exercise - it's a simple set of rules that provide you with stops that approximate those that would be given by a deco model such as decoplanner. The advantage is that the rules are simple enough to allow you to calculate the stops "on the fly".

CMAS and BSAC cover deco diving in relatively entry-level courses. Echoing what has been said before, deco isn't rocket science - but for anything other than a few minutes here or there, it's worth having a bit more knowledge, irrespective of if you're diving shallower for longer or deeper.

http://www.scubaboard.com/forums/3158130-post186.html

(Recommend purchasing and taking the latest online class on Ratio Deco from UTD)

 

[Deefstes]

If you were teaching people woodworking, wouldn't you teach them how to figure out how much wood a project will require before they start cutting and sawing?

I don't think this is an appropriate analogy. If you do woodworking and you don't have enough wood you will ending up with a failure. You either have enough wood or you don't and if you don't your project is a disaster. If you do basic Open Water diving and you run low on air, you simply surface and the dive is over, but you still had an enjoyable and rewarding dive. When you start doing decompression dives, it's a different story and it becomes crucial that you know beforehand exactly how much gas you will need and how you will be using it, in this case perhaps more comparable to woodworking.

Gas planing for simple dives is relatively straightforward, especially if you focus on the most common tanks a new diver will encounter, AL80s.

I have never in my life encountered an AL80. It's an American concept and anywhere outside of the States you will be diving with cylinders in metric units. Moreover, not only are the cylinder sizes given in metric units but also as a physical size of the cylinder - as opposed to the volume of gas in a cylinder at working pressure.

The point I'm making is that the math changes when you're using different types of cylinders. Should an OW student now learn to do the math for different cylinder sizes AND types? Surely you can not expect an OW student to learn EVERYTHING? Where do you draw the line? Personally I would think that drawing the line somewhere before the actual math pertaining to gas planning is not unreasonable.

I agree with you though that gas planing for simple dives is relatively straightforward. In fact, after my second or third dive I've started calculating my air consumption and applying that to estimate my next dive, not because it was part of the OW training but because I thought it was interesting. I just think that for brand new divers there is such a lot of new information (most will never have heard of Henry's law prior to their OW course) that you have to curb it somewhere. I don't think it's necessary to have them learn imperial math when they live in a metric country, just as I don't think it's necessary to have them learn gas planning if they won't really be needing it until they do AOW anyway.

 

[Reg Braithwaite]

If you do basic Open Water diving and you run low on air, you simply surface and the dive is over, but you still had an enjoyable and rewarding dive.

Your statement is true for some definition of the terms "simply surface" and "low on air." The point of simple gas planning, the type appropriate for OW students, is to decide what it means to simply surface on a particular dive, figure out how much air is needed for that, and thus determine how much air is "low on air."

If I asked a prospective buddy what they had in mind for a dive to PADI's OW "limit" of 20m and they told me, "Dive until my computer tells me I'm out of NDL or until I'm low on air, then simply surface," I would ask them to explain what they had in mind for an ascent plan ("simply surface"), how many bar or psi would be considered "low on air," and whether the two items matched each other.

I would also be very interested in knowing whether "simply surface" takes into account the possibility that just as they run "low on air" I (their buddy) suffers a catastrophic failure and need to share their gas.

Would you consider this line of reasoning inappropriate for OW divers?

...Tthe math changes when you're using different types of cylinders. Should an OW student now learn to do the math for different cylinder sizes AND types?

If in your locale a new OW diver will encounter any different cylinders, then yes I would expect them to learn to do the math for all of the cylinders they will strap on their back. I also have access to a wide variety of cylinders, and I simplified things for myself last year by only renting one type of cylinder. I further simplified things for myself this year by buying my own cylinders.

Let's be pragmatic abut this. I am advocating that an OW student demonstrate how to do the calculations to pass their examination. And quite possibly do the gas planning as part of their OW checkout dives. Thereafter there are lots of handy tables one can carry rather than working the math out by hand. I have one laminated and tucked into my kit just in case I need to use a different cylinder.

JM2C.

 

[Peter Guy]

re the math and figuring stuff out at the OW level:

At least where I am, at the end of each OW experience dive, the Instructor has the OW student fill out her logbook and requires the following information:

Max depth
Dive time
Starting PSI -- ending PSI
Beginning Table Pressure Group -- Ending Table Pressure Group
Surface Interval

From that information, it is a very small step to estimate "average depth" (in fact, the computer also has that information). Of course, once they have the average depth of the dive, they have all the info they need to then compute:

Volume of air used
Volume of air used per minute and thus
A "normalized" volume of air per minute -- aka RMV or SAC -- something that would take maybe another minute. IF they did this, over the four dives they'd see how usage changed and maybe, perhaps, possibly, begin to get the idea about guestimating volume needed for a dive. If only.....

 

[Deefstes]

If I asked a prospective buddy what they had in mind for a dive to PADI's OW "limit" of 20m and they told me, "Dive until my computer tells me I'm out of NDL or until I'm low on air, then simply surface," I would ask them to explain what they had in mind for an ascent plan ("simply surface"), how many bar or psi would be considered "low on air," and whether the two items matched each other.

Yeah, I guess that's not unreasonable but but don't be surprised (or disappointed) if the response is a confused look and an answer of "well my ascent plan is to swim up slowly and do a safety stop at 5m for 3 minutes and 50bar is low on air. Do the two items match? Yes. Next question."

So that's not as precise as some of the more experienced divers around here go about their business but then this is not an experienced diver. Surely such a dive plan is not problematic?

I would also be very interested in knowing whether "simply surface" takes into account the possibility that just as they run "low on air" I (their buddy) suffers a catastrophic failure and need to share their gas.

Two divers ascending slowly (10m/min) from 20m (PADI OW limit) with a 5 min safety stop at 3m and each an RMV of 25 l/min will only use 425l between them, that's only 35 Bar in a 12l cylinder. That's the math proving that your buddy plus you with your catastrophic failure would comfortably reach the surface on 50 Bar. I'm suggesting that the beginner diver doesn't have to calculate this prior to each dive, but simply know that 50 Bar means "low on air" which means that it's time to "simply surface" because someone else calculated it already.

Let's be pragmatic abut this. I am advocating that an OW student demonstrate how to do the calculations to pass their examination. And quite possibly do the gas planning as part of their OW checkout dives. Thereafter there are lots of handy tables one can carry rather than working the math out by hand. I have one laminated and tucked into my kit just in case I need to use a different cylinder.

Yep, nothing wrong with your argument. Personally, I'm not entirely convinced it's necessary though and if I were required to put together an OW curriculum (don't worry, I'm not and I won't) I would not have included that part. It would seem to me that most agencies agree. If an agency felt that it should be part of the curriculum, I would not consider them outrageous. On the contrary, I would consider them more thorough and hence better but I know for a fact that my friend's girlfriend would not have been able to get her cert through that agency as this sort of math, simple as it may be for you and me, is well beyond her.

 

[Reg Braithwaite]

I'm not entirely convinced it's necessary though and if I were required to put together an OW curriculum (don't worry, I'm not and I won't) I would not have included that part. It would seem to me that most agencies agree. If an agency felt that it should be part of the curriculum, I would not consider them outrageous. On the contrary, I would consider them more thorough and hence better but I know for a fact that my friend's girlfriend would not have been able to get her cert through that agency as this sort of math, simple as it may be for you and me, is well beyond her.

I shall dub this "Deefstes' LOAF (Lady of a Friend) Rule." In short, if the math is too hard for a friend's lady friend, it will not be part of the OW curriculum. Because, no matter which way you slice it, to the big agencies it's "all about the bread."

(running and ducking!)

 

[Monkey_Gland]

BSAC also has a darn aggressive set of deco tables and I am not sure they are a good resource.

Go say that on Yorkshire Divers....they'll try and skin you!

 

[Blackwood]

The point I'm making is that the math changes when you're using different types of cylinders.

Not really.

You track how much pressure you use and relate that back to volume with the end goal of understanding how much gas you consume per unit time * depth.

Sure, you may be computing Liters/Bar as opposed to CF/PSI, but that's not a fundamental difference.

Right?

 

[waterpirate]

I started diving in 1981. Naui old skool, we learned the navy tables. Deco diving was something we did. It was on the tables, follow them and life was good.
Fast forward to now. I am scheduled to take adv.nitrox/intro to deco in 2 weeks. A lot has changed. Diving philosophy, mixed gas, deco gas, redundant gas supply, all for the better. At my age I do not plan on any "deep dives" just a really long shallow dive.
With the choice of gasses available today and the software to plan your dive I believe that all of this should be introduced to the advanced open water student. It would make everyone safer and bridge the knowledge gap that is so absent in intro classes today.
The introduction of nitrox was a huge benefit, and I see a more widely accepted view of helitrox in the very near future. Helitrox will revolution the rec diver much like nitrox did then.
With a mix of 32/15 I can do a dive to 80fsw for 80 min and surface with only a 3 min obligation on back gas, I would hardly call that deco diving, but I would call it extended bottom time with a very limited exposure risk. With the right training, and gas management all of this is safe at shallow depths.
Why hold back info from new divers. If they seek advanced training then let us provide it, not spoon feed them adventure dives and silly certs for the sake of card collecting.
Just my .02psi
Eric