Recreational Limits, confusing or is it just me?

[MXGratefulDiver]

Over the past 10 years I've measured air consumption rates for probably 200 students ... a standard exercise in the AOW class ... and it's common to see their "stressed" consumption rate at least double compared to their "relaxed" consumption rate. It's not uncommon to see it triple ... and on a few occasions go even higher than that. There's a lot of variance from diver to diver, based on several factors both physical and emotional.

The other thing to consider, particularly for new divers, is that unless you've practiced making a direct ascent from depth while hooked together breathing off the same regulator it's unlikely you're going to do it "by the book". What I've experienced when doing this with students is that when left to their own devices (i.e., I'm not helping them ascend) they'll often not have the awareness to hold a constant ascent rate ... and overcompensate when they realize they're starting to accelerate to the point where they start to drop back down again. I've commonly seen several of these "correction events" during an emergency ascent drill. It's due, primarily, to a lack of practice ... most divers rarely if ever go out and practice emergency skills once class is over.

And finally ... making a direct ascent from 100 feet to the surface without any stops is rarely a good idea. Your chances of needing a chamber ride afterward will be significant if you've been down that deep for any length of time. The reason is that the typical recreational diver really doesn't understand how slow 30 fpm (or 10 mpm) really is, and will tend to come up too fast. While that beats running out of air at depth, it's still an unnecessary risk.

It's much easier to sit at a keyboard and do these mental calculations than it is to follow through with them in the water ... particularly when stressed due to an emergency, and while attempting to perform an emergency skill you probably haven't practiced since that one time you had to do it during class. And that latter is, more often than not, the reality you're going to end up being faced with.

Far better to not cut your margins that thin ... doing so is, to my concern, a significant failing in your mental approach to the dive.

... Bob (Grateful Diver)

 

[packrat12]

Hey Bob,

Not to get into a 'keyboard' match with you as I do highly respect your opinions, but....

I have had a diver on my personal boat make a free ascent from 90'. Not fun for everyone but it is done. (the SOB ran off alone after a lobster in low vis and lost us. Had he not had an emergency, I would have killed him for that alone )

As for the depth, one assumption is that everyone involved knows their limits and is skilled. You can never plan for how everyone will react otherwise we would need a tanker of air to make sure our dives are safe. As you said, you have even seen sacs at 3+X. With the 1000 PSI math, there is still a fair amount of air more that can be used. How well a buddy team can share is not relevant to this discussion as each person will be completely different and some may work well, others will refuse air and continue a free ascent without accepting shared air, as has happened to me when a diver that was not a part of my team refused my offer of air on their emergency ascent from 130'. The idea practicing an emergency ascent with buddy breathing should be stressed and practiced more in training and in the real world.

Another assumption is that this is at the worst time, e.g. at the line with 1000 PSI. Any other time in the dive will have had higher reserves.

If we have mis-calculated the amount of air required for the surface, ultimately, both divers will be closer to the surface than when the emergency started. If the consumption is 4X, they will be above 50' from the surface... etc. Remember that this is a no-ceiling dive.

At the keyboard we only have mental calculations. I know you also run them all the time. They are all you have in planning. But they are also very useful in making sure you dive in a safe envelope. Back to the tanker, we must make assumptions otherwise we really would be carrying down tons of air for simple dives.

Lastly, about the safety stop, this is an emergency. I would rather ride in a chamber than a coffin. We are both old enough to remember when a normal dive at 120' was done with a 60'/min surface rate, no safety stop. This does not mean it is smart, but we did it for years that way. As for the 'safety stop' itself, the reason it was recommended in the 90's was that Doppler had shown micro-bubbles which were thought to contribute to DCS risk. I believe all data now shows that they do not. It is my opinion that the safety stop of today is more for making a controlled ascent and building a better safety margin on NDL than the original DCS reason. I know of no computer that penalizes a diver for not doing a safety stop. The reasons for a slower ascent rate is well documented, however. It is a risk for one to go faster, maybe acceptable risk in an emergency.

Finally, we all can remember that assume makes an ... out of you and me

 

[rivers]

Lastly, about the safety stop, this is an emergency. I would rather ride in a chamber than a coffin. We are both old enough to remember when a normal dive at 120' was done with a 60'/min surface rate, no safety stop. This does not mean it is smart, but we did it for years that way. As for the 'safety stop' itself, the reason it was recommended in the 90's was that Doppler had shown micro-bubbles which were thought to contribute to DCS risk. I believe all data now shows that they do not. It is my opinion that the safety stop of today is more for making a controlled ascent and building a better safety margin on NDL than the original DCS reason. I know of no computer that penalizes a diver for not doing a safety stop. The reasons for a slower ascent rate is well documented, however. It is a risk for one to go faster, maybe acceptable risk in an emergency.

Finally, we all can remember that assume makes an ... out of you and me

And I'd rather have enough gas that a comfortable, slow ascent complete with stops can be made by 2 people breathing off of one cylinder than risk ending up in a chamber because the ascent was too fast. But to each their own I guess. It's not hard to watch your gas, it's not hard to dive by a minimum gas/rock bottom rule.

 

[packrat12]

Nothing in my margins says that you cannot do a comfortable ascent nor the safety stop. If you read my post, I calculated at 30'/min @ 2x. But I will also state that if necessary, I can still make the surface with contingencies beyond that, although I may give up my safety stop and if the contingencies are too great, maybe a faster ascent or even a free ascent from a shallower depth. Even with the best plans, you may find that you will also need to make one of these choices some day. A plan is only as good as the contingencies it has taken into account. Many more can arise even beyond any reserve any 'plan' can foresee.

The argument about which 'rule' to use in recreational diving is not settled... yet. Also as an aside, I look at this from clear warm water Florida diving, not the NW US or UK conditions which have much more serious considerations.

Overall, my original argument is that 200bar (2800 PSI) is not the required minimum limit for diving to 100' when diving recreationally. The dive can and is safely done with AL80's. Emergency situations can safely be handled with minimum risk. Yes all divers should understand that 500' PSI at the boat will not suffice when diving deep. They also need to determine the best minimum gas for their dive considering their conditions, divers and their level of acceptable risk (which all diving has).

 

[The Chairman]

The argument about which 'rule' to use in recreational diving is not settled... yet.

This is like a debate about which color is your favorite. There will never be a consensus among any largish pool of divers. But, like a favorite color, just pick your own for what ever reasons you feel are appropriate. Just remember that YOU and YOU alone are responsible for YOUR survival. Pick a rule and stick with it. Learn when to trust it and when it might leave you vulnerable. These limits should never be treated like a "personal best" where you're constantly trying to push them. The more you respect them, the less likely you'll incur an injury.

 

[MXGratefulDiver]

Hey Bob,

Not to get into a 'keyboard' match with you as I do highly respect your opinions, but....

I have had a diver on my personal boat make a free ascent from 90'. Not fun for everyone but it is done. (the SOB ran off alone after a lobster in low vis and lost us. Had he not had an emergency, I would have killed him for that alone )

As for the depth, one assumption is that everyone involved knows their limits and is skilled. You can never plan for how everyone will react otherwise we would need a tanker of air to make sure our dives are safe. As you said, you have even seen sacs at 3+X. With the 1000 PSI math, there is still a fair amount of air more that can be used. How well a buddy team can share is not relevant to this discussion as each person will be completely different and some may work well, others will refuse air and continue a free ascent without accepting shared air, as has happened to me when a diver that was not a part of my team refused my offer of air on their emergency ascent from 130'. The idea practicing an emergency ascent with buddy breathing should be stressed and practiced more in training and in the real world.

Another assumption is that this is at the worst time, e.g. at the line with 1000 PSI. Any other time in the dive will have had higher reserves.

If we have mis-calculated the amount of air required for the surface, ultimately, both divers will be closer to the surface than when the emergency started. If the consumption is 4X, they will be above 50' from the surface... etc. Remember that this is a no-ceiling dive.

At the keyboard we only have mental calculations. I know you also run them all the time. They are all you have in planning. But they are also very useful in making sure you dive in a safe envelope. Back to the tanker, we must make assumptions otherwise we really would be carrying down tons of air for simple dives.

Lastly, about the safety stop, this is an emergency. I would rather ride in a chamber than a coffin. We are both old enough to remember when a normal dive at 120' was done with a 60'/min surface rate, no safety stop. This does not mean it is smart, but we did it for years that way. As for the 'safety stop' itself, the reason it was recommended in the 90's was that Doppler had shown micro-bubbles which were thought to contribute to DCS risk. I believe all data now shows that they do not. It is my opinion that the safety stop of today is more for making a controlled ascent and building a better safety margin on NDL than the original DCS reason. I know of no computer that penalizes a diver for not doing a safety stop. The reasons for a slower ascent rate is well documented, however. It is a risk for one to go faster, maybe acceptable risk in an emergency.

Finally, we all can remember that assume makes an ... out of you and me

No "keyboard match" intended ... but a good exchange of perspectives is what makes a conversation worth participation.

Although we all do it to some degree, I try not to work from assumptions when it comes to gas consumption. This article is based on a module out of my AOW curriculum, and I train divers how to know how much air they'd need for a particular depth (and conditions, when those will affect air consumption), and discourage them from going deep until they KNOW how much air they'll need to begin their ascent ... based on both their own measured consumption rates and those of their dive buddy. These numbers are readily determined, and one just shouldn't consider going on a deep dive with a buddy who doesn't plan their gas needs ahead of time.

For most newer divers ... those with higher consumption rates ... going deep on a small cylinder like an AL80 is a bad idea. Sure, they do it all the time ... and mostly they get away with it, because most of the time nothing serious goes wrong. In some cases, however, divers do run OOA ... or more frequently, detrimentally LOA ... and those situations are stressful, at best. Stress isn't your friend underwater, and should be avoided ... and most of the time that's easily done by not making assumptions, but by knowing ... because you PLANNED for it ... that you have adequate air for the dive you are doing.

I do not think it's valid to assume that most divers know their limits ... many divers tend to rationalize why going beyond them will be "alright" ... and as long as they don't experience a failure or an unexpected circumstance that delays their ascent or removes part of their air supply they'll be fine. But we should not plan our dives on the assumption that we won't have a problem ... we should plan them with contingencies for what to do if the worst case happens ... that one of us would lose access to our air supply at the worst possible time. That's why we carry reserves ... and the trick is knowing how much is enough.

In AOW class I have students measure their air consumption under both normal and stressed conditions. Then I give them a dive plan for a dive to 100 feet and tell them to calculate how much air they'll need, using the rules described in the article I referenced earlier for turn pressure and rock bottom pressure. It's hardly surprising how often a student will come back after calculating their gas needs for that dive and tell me they can't do it ... "because my tank isn't big enough". Hardly surprising that they may have done the exact same dive in the past, on that tank, and were fine ... but what they hadn't done is factor in how much their air consumption goes up during a stress event ... or how much their buddy would need if they should have to share air. Then there's this to consider ... if you and your buddy have similar consumption rates, and similar size tanks, and you run out of air ... how much do you suppose your buddy's going to have available to share with you?

:shocked2:

And yes, a chamber ride is a far better alternative than a coffin ... but it's still a bad alternative. Way better would be to determine your plan based on how much air you actually have ... and if the dive profile you want to do is really that important and you need a bigger tank, then bring a bigger tank! Don't wait till you're on the dive to find out that's what you should have done.

Plan your dive and dive your plan. We all heard that mantra as part of our OW class. What they didn't tell you is that managing your air is part of your dive plan. Like anything else we do in life, being proactive usually produces better results than being reactive. If nothing else, it helps us avoid problems ... and avoiding a problem is always better than having to respond to one once it happens.

... Bob (Grateful Diver)

 

[Kevrumbo]

Why would you stop every 3m for a min from the bottom up during an emergency?!

Kevrumbo - This is for 2 Rec divers with no ceiling. Let's look at the air consumption during the emergency ascent. I will use a SAC of .8 for each diver.

100' ascent give a 50' average depth with a 3 min ascent time @ 30'/min.

50' = 2.52 ATA
consumption of 2 divers at .8: .8 * 2 * 2.52 = 4 ft3/min
3 min surface time = 12 ft3

Pressure used in an AL80: 12/(77/3000) = 467 PSI. Let us round this to 500 PSI.

So our expected air consumption is 500 PSI. Our ascent is a no-stop maybe even faster than 30'/min.

With a min of 1000 PSI in an AL80, 2 divers can easily make an ascent from 100'. They probably can do it all the way up from 800 PSI which would be the lowest PSI one should start the ascent anyway. Even if the divers both doubled air consumption (1.4 each as .8 is already very high), they would need 820 PSI. I think they could easily live.

As for dive time, with a .6 SAC, a single tank AL80 diver can expect 21 minutes at 100' with 1000 PSI left.

Good! That's how you begin realizing your vital limitations --Start doing the gas planning math with your own conservatism estimates; mine was for accounting for plenty of margin with a controlled sane & slow ascent in most conditions and physical exertion states in an easy quick calculable method.

Example Gas Planning for the Basic OW 18m/60' Limit
. . .
Here's another slightly more elaborate conservative Emergency gas planning example & exercise for the OP, for a dive to the Basic Open Water Limit of 18m/60':

Emergency "Stressed" 28 litres/min SCR (Surface Consumption Rate, also referred to as SAC), with 18 meters (2.8 ATA) depth NDL, and with one minute stops every 3 meters to surface:

2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28

Sum Total: 372.4 litres gas needed to ascend to surface for an emergency contingency.

Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank again which has a metric rating of 11 litres/bar. So 372/11 = approx 35 bar.

That's 35 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 35 x 2 = 70 bar. Therefore your Rock Bottom Reserve is 70 bar showing on your SPG --if there is no emergency air sharing contingency at that instant, just continue the dive but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5m safety stop with your buddy with no less than 50 bar showing on the SPG.

Your usable gas for the dive is your starting pressure subtracted by the Rock Bottom Reserve --so 200 bar minus 70 bar equals 130 bar usable. Let's use 20 bar of this 130 to get squared away in good trim & buoyancy on the descent to 18m which leaves 110 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 110 bar or 55 bar consumed.

So for a nominal dive with an SCR of 22 litres/min on a 11L (AL80) tank, your resulting pressure SCR will be 2 bar/min [22 divided-by 11 equals 2bar/min]. Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 5.6 bar/min. [2.8 times 2bar/min equals 5.6 bar/min]. So 10 minutes at depth 18m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 56bar of gas (10min times 5.6 bar/min equals 56bar) and your SPG reading to show a delta down of 56bar. . .

Okay, you splash in to start the dive with 200bar and a 70bar rock bottom. You use up 20bar on descent --initial exertion, inflating your wing etc. -so you now have 180bar with 70bar rock bottom at present depth 18m, a net usable of 110bar (180 minus 70 is 110). Finning out for 10 minutes, an easy swim looking at all the cool marine life around -- you already know by the end of this 10 minute interval that you've used up 56bar (see previous paragraph above), so you decide to turn the dive. You should now know you have roughly 50bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent within the next 10 minute interval. Alright, so while turning back, you see a turtle and decide to chase it for a few minutes, huffing & puffing on your reg, until it dives down below your operational depth of 18m; you immediately check your SPG and it reads 70bar --bingo! Rock bottom has arrived so start ascending to the shallower depths . . .or if your Buddy just happens to blow his tank neck O-ring at that instant --you know you've got plenty of breathing gas margin for a controlled air-sharing Emergency ascent profile as described above. . .

Be aware that depending on environmental conditions and physical exertion (cold water, stiff current, long surface swim, heavy workload at depth etc), you may have to reserve a greater Rock Bottom Reserve --perhaps as high or even over 100 bar on the 11L/AL80 tank to be conservative. (Even better just abort the dive, wait another day or find another easier site to dive!)
____
Notes for American Divers using US Imperial Units:
Stressed SCR of 28 litres/min is equivalent to 1 cuft/min;
Nominal SCR of 22 litres/min is approx 0.75 cuft/min.
2 bar/min is same as 29psi/min;
5.6 bar/min is 81psi/min

Easy imperial US/metric conversions for depth & pressure, that you can do in your head:

Depth in meters multiplied by 10/3 gives depth in feet;
Feet multiplied by 3/10 gives meters.
Example: 18m(10/3) = 60' ; 60'(3/10) = 18m

Pressure bar multiplied by 3/2, and multiplied again by 10 gives pressure psi;
Pressure psi multiplied by 2/3, and divided by 10 gives pressure bar.
Ex): 200bar(3/2)(10) = 3000psi ; 3000psi(2/3)/10 = 200bar.
___
Your common counting numbers, or reference cardinal numbers, for depth in most scuba tables are:

Imperial US (feet) by 10's:
Ex): 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 etc

Metric system goes by 3's:
Ex): 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33 etc

Hence the 3/10 or 10/3 conversion factors between imperial to metric and vice versa. . .

 

[Edward3c]

...
The other thing to consider, particularly for new divers, is that unless you've practiced making a direct ascent from depth while hooked together breathing off the same regulator it's unlikely you're going to do it "by the book". What I've experienced when doing this with students is that when left to their own devices (i.e., I'm not helping them ascend) they'll often not have the awareness to hold a constant ascent rate ... and overcompensate when they realize they're starting to accelerate to the point where they start to drop back down again. I've commonly seen several of these "correction events" during an emergency ascent drill. It's due, primarily, to a lack of practice ... most divers rarely if ever go out and practice emergency skills once class is over.
... Bob (Grateful Diver)

Last time I assessed some BSAC Advanced Divers on the 20m Assisted ascent, I wouldn't sign them off as they dropped 0.5m at one point on the way up.

 

[beanojones]

Over the past 10 years I've measured air consumption rates for probably 200 students .. it's common to see their "stressed" consumption rate at least double...triple ... and on a few occasions go even higher than that.

The other thing to consider, particularly for new divers, is that unless you've practiced making a direct ascent from depth while hooked together breathing off the same regulator it's unlikely you're going to do it "by the book". What I've experienced when doing this with students is that when left to their own devices (i.e., I'm not helping them ascend) they'll often not have the awareness to hold a constant ascent rate

Excellent on Topic post, that I am going to cannibalize for some other points, that are largely off topic in a sense and largely on topic in a sense.

First gas planning (as usually advocated on SB) depends on a stable SAC. Since I get have seen similar huge jumps in air consumption that are not based on the state of emergency but rather on environmental conditions, I simply cannot ever say that gas planning (as advocated at SB) is ever of real use in the vast majority of real world diving. We don't know two of the three variables (SAC rate, Max Depth) until we are in the water, which makes the pre dive machinations pointless.

In real OOA situations I have been a part of, there is no correction of ascent rates, because the only thought of an OOA diver is Surface! Now!, including running away from an usable air source when the donor would not move to the surface fast enough. But then again, I disagree wholeheartedly with the design of most OOA exercises in OW classes (and in tech classes as I have seen them).

And finally ... making a direct ascent from 100 feet to the surface without any stops is rarely a good idea. Your chances of needing a chamber ride afterward will be significant if you've been down that deep for any length of time. The reason is that the typical recreational diver really doesn't understand how slow 30 fpm (or 10 mpm) really is, and will tend to come up too fast. While that beats running out of air at depth, it's still an unnecessary risk.

I disagree with the idea that a fast ascent from a recreational depth within NDL limits really does much to increase chamber risk. Fast ascents are not a good idea, but the idea that anything other than surfacing quickly should be on the mind of someone helping an OOA diver has caused the problems I have seen first hand in actual OOA situations. One of which is, as mentioned, OOA divers abandoning working octos and buddies because they wanted to get to the surface, now, and the erstwhile donor was thinking about everything but that.

Real OOA situations often look more like wrestling matches than anything else, with the supposed helper diver getting in the way of Surface! Now! from the OOA divers point of view. The thing that calms the OOA diver down is the donor starting up with them at a good pace, and not doing some ridiculous hand motions or trying to slow the OOA divers ascent rate. Worrying about ascent rates during practice means less success in the real world, if success is defined as successfully taking an OOA diver to the surface.

What these points add up to, to me, is the idea that we stress limits based on the divers abilities. I get laughed out of the gym here at SB, every time I suggest that people should be able to do CESAs from any depth they are diving to in non-redundant gear, and that is a good way to get comfortable doing deeper dives. And yet, if we forced people to live up to that, there would be a completely non-arbitrary reason for 18m/60' limits on new divers.

And we might just get people realizing that fully redundant (ie not a tiny pony bottle) is not just for tech anymore.

(The real value for sidemount is that we have finally have a real way for traveling recreational divers to carry actual amounts of redundant gas, instead of a 13cf bottle that will not solve real OOA problems in the real world. Even if they backmount one bottle, and sidemount another.)

 

[AfterDark]

it is confusing since the official rules are vague: "within training and experience". what kind of training? how much experience?

but this doesn't matter since it really boils down to the dive op rules.

This is true only if you dive with ops. Those of us that have access to deep water from shore like here in RI, we can set our own limits. I dove for decades beyond my certification. Thirty years ago there weren't any dive ops that I remember in RI, we'd charter a fishing boat that wasn't fishing that day to take us to wreck XXX, or somebody had access to a private boat, like I did for a time with my fathers boat. The fishermen didn't care if we were certified and the bottom was the limit.

These limits are to try to make up for the lack of training and training time given to dive training these days.