Preparing for Devil's Throat

[John Larsen]

I would call devils throat more of a glorified swim through than a Overhead environment...Cenotes have much longer swimthroughs...and My depth gauge always say 122-128 feet when I exit...Still within Rec Limits....If you don't want to dive it or feel unsafe diving it then by all means don't do it...but to call it a Dangerous/tech dive is a bit of a stretch.

Since when are overhead penetrations and depths exceeding 130' considered recreational?

 

[TSandM]

I think committing yourself to a dive where you do not have enough gas to get you and your buddy to the surface in the event of a freeflow or a dip tube blockage is at best unwise. Doing so without even having thought through the potential issues is worse. Doing so without the tools to calculate whether you even have enough gas in the tank you are using to get THROUGH the dive is just plain foolish. We lose divers every year in Puget Sound, when they finish their AOW and go do deep dives for which they think they are prepared, but they are not.

All diving is risk tolerance -- some people won't tolerate the risk of going underwater at all. Others are very conservative, and some do 200 foot bounce dives on single tanks. What I think is dangerous may not be what you do -- but I have some rational reasons for describing why I think some dives should be regarded very carefully before they are undertaken. Saying that you have done a dive multiple times without an issue does not mean that what you are doing is either reasonable or safe. It may mean you have been playing Russian Roulette and haven't hit the full cylinder yet.

 

[Ms. Ann Thrope]

I don't think it's generally a big-deal danger dive either, and it's kind of pretty and serene inside the throat/swim-through (assuming you're a relaxed, reasonably skilled diver), if no one has kicked it the heck up. And it's got a great name. I think that's 80% of why people want to dive it. If they called it "Deep Sand Chute," I don't think it would be on so many people's wish lists. I'm glad I've seen it, but I like nearby Cathedral better, along with any number of other dives. But I don't think any of that will matter to you. If you're diving Devil's Throat with a conservative computer, it may be an especially short dive--my first time there, with my UWATEC set on air, I was out of no-deco time pretty much the minute I popped out over the wall. Disappointing. The part that gives me pause is that you described yourself as an air hog. I don't know if that means on an average dive with an 80 sf tank your air lasts 40 minutes or 60, or what--but that's a discussion worth having with yourself. We did it on 80s and I don't have any qualms about that, but some might. How much tank are people generally using on an average DT dive? I have no idea. I breathe light and my dive was short--I still had lots and lots of air on that first DT dive, but that might not be the case in every situation.

I guess my only advice, beyond deciding if you think you should and want to do it (and your operator might not even give you the chance, depending on the skills and desires of the boat as a whole), is to know your computer should you go into deco, and be weighted properly so that if you do find yourself with the combination of a light, near-empty tank and a bit of deco obligation left, that you're equipped to deal with it. I say this having once been on a poorly led Barracuda dive where a couple of people didn't know how to read their computer deco stop instructions and blew through their stops because they just couldn't stay down any longer--and then had to decide whether they should worry a little or a lot about how their computers had locked them out with their flight being the next day. I think most people manage to do the Throat without going significantly into deco, but it's just good to know what to do in case you do. Oh, and the other part is, keep your distance from other divers lest you get kicked, crawled over or otherwise affected by someone who suddenly realizes they are very, very anxious down there.

 

[TSandM]

I sat down and did the rock bottom calculations (and please don't niggle about a half cubic foot here and there -- there are some rounding errors).

If you assume an ascent from 130 feet, AND that both divers are calm enough to manage a 1 cf/min consumption rate (which is not that much higher than the .7cfm that is the assumed average for an adult male), AND you omit the safety stop, you're looking at 35 cubic feet of gas to do that ascent, which is roughly 1400 psi in an Al80. That's half the tank, assuming you are willing to hit the surface with the tank dead empty. Note that every minute at depth is costing you 3.5 cubic feet of gas with normal consumption, or a minimum of 7 if the two of you are sharing gas. It's 10 cubic feet per minute with the usual 1 cfm stressed consumption rate -- but that may be WAY too conservative an estimate if the divers don't have any stress training.

People don't DO these calculations. They become very sobering.

 

[Mossman]

I would call devils throat more of a glorified swim through than a Overhead environment...Cenotes have much longer swimthroughs...and My depth gauge always say 122-128 feet when I exit...Still within Rec Limits....If you don't want to dive it or feel unsafe diving it then by all means don't do it...but to call it a Dangerous/tech dive is a bit of a stretch.

Any time you can't immediately ascend to the surface, you are exceeding recreational standards. Call it a cavern dive, maybe, with narrow restrictions and the potential for silt outs and deco obligations, but don't call it a recreational dive if one adheres to the standard definition of a "recreational" dive.

Just because people routinely do such dives recreationally, i.e. without redundancy and proper training, doesn't mean they're recreational dives.

---------- Post added October 22nd, 2013 at 12:47 PM ----------

I sat down and did the rock bottom calculations (and please don't niggle about a half cubic foot here and there -- there are some rounding errors).

If you assume an ascent from 130 feet, AND that both divers are calm enough to manage a 1 cf/min consumption rate (which is not that much higher than the .7cfm that is the assumed average for an adult male), AND you omit the safety stop, you're looking at 35 cubic feet of gas to do that ascent, which is roughly 1400 psi in an Al80. That's half the tank, assuming you are willing to hit the surface with the tank dead empty. Note that every minute at depth is costing you 3.5 cubic feet of gas with normal consumption, or a minimum of 7 if the two of you are sharing gas. It's 10 cubic feet per minute with the usual 1 cfm stressed consumption rate -- but that may be WAY too conservative an estimate if the divers don't have any stress training.

People don't DO these calculations. They become very sobering.

If every minute costs you 3.5 cf of gas, 35 cf would mean a 10 min ascent. PADI advises a 60 fpm ascent rate, which means the ascent from 130' would take 2:20 without safety stop or deep stop, not 10 min.

 

[nimoh]

another thing to consider on a dive such as this is even if you are confident in your own skills to manage a problem, inexperienced divers are commonly taken in groups and something as simple as a broken mask strap can put some people into a full panic. If these people happen to be in front of you, blocking your exit, you can end up going through a lot of gas waiting for them to resolve the problem, or you can go through a lot of gas helping them resolve the problem.

 

[Bob DBF]

I sat down and did the rock bottom calculations (and please don't niggle about a half cubic foot here and there -- there are some rounding errors).

If you assume an ascent from 130 feet, AND that both divers are calm enough to manage a 1 cf/min consumption rate (which is not that much higher than the .7cfm that is the assumed average for an adult male), AND you omit the safety stop, you're looking at 35 cubic feet of gas to do that ascent, which is roughly 1400 psi in an Al80. That's half the tank, assuming you are willing to hit the surface with the tank dead empty. Note that every minute at depth is costing you 3.5 cubic feet of gas with normal consumption, or a minimum of 7 if the two of you are sharing gas. It's 10 cubic feet per minute with the usual 1 cfm stressed consumption rate -- but that may be WAY too conservative an estimate if the divers don't have any stress training.

People don't DO these calculations. They become very sobering.

I'd be willing to hit the surface with the tank dead empty, rather than start that way on the bottom.

Even though we use larger tanks (and ponys), this is the reason we head up to 60'-80' and sort out issues if anything in our dive plan at 130' is not going perfectly. It is so easy to burn air and go into deco, which needs even more air, dealing with anything on the bottom. We might not accomplish anything, but we get to go back later.

That's what calculations are for, to scare the crap out of you, or we could just bring in the unicorns and rainbows.




Bob
--------------------------
A man's got to know his limitations.
Harry Callahan

 

[Mike]

I sat down and did the rock bottom calculations (and please don't niggle about a half cubic foot here and there -- there are some rounding errors).

If you assume an ascent from 130 feet, AND that both divers are calm enough to manage a 1 cf/min consumption rate (which is not that much higher than the .7cfm that is the assumed average for an adult male), AND you omit the safety stop, you're looking at 35 cubic feet of gas to do that ascent, which is roughly 1400 psi in an Al80. That's half the tank, assuming you are willing to hit the surface with the tank dead empty. Note that every minute at depth is costing you 3.5 cubic feet of gas with normal consumption, or a minimum of 7 if the two of you are sharing gas. It's 10 cubic feet per minute with the usual 1 cfm stressed consumption rate -- but that may be WAY too conservative an estimate if the divers don't have any stress training.

People don't DO these calculations. They become very sobering.

Must be a ton of conservatism built into your calculations or I'm an underwater God. No way in the world have I used 1400 PSI coming up from Devil's throat. Last time I did it, we spent an awful long time swimming to it, (I think the boat drop was a bit off), we went through, hung out on the wall, then turned around and went back through the opposite way, finned all over the reef, watching the bubbles come up through the top of the reef from all the expelled air in the throat from all the divers, went over the reef and then did a 5 minute safety stop. 1400 psi just seems like a huge amount of consumption.

Big thing I don't like about the throat is the fear of getting stuck behind somebody in the middle of it who freaks out and goes bat sh*t and having to defend yourself in that tight environment while others start piling into you from behind. As long as everybody goes on merrily through one after another it's never a problem.

 

[TSandM]

If every minute costs you 3.5 cf of gas, 35 cf would mean a 10 min ascent. PADI advises a 60 fpm ascent rate, which means the ascent from 130' would take 2:20 without safety stop or deep stop, not 10 min.

Note that the calculation I did is for TWO divers, not one, and I used the currently recommended ascent rate of 30 fpm. You can certain exceed that, but then you have to ask the question whether planning to compound the problem of being out of gas with a rapid ascent is a reasonable strategy.

I should add that, with the ascent strategy that I was trained to use and WOULD use in such a situation, the rock bottom reserve is much larger, which is one of the reasons I don't do dives like this on a single tank, no matter what size.

 

[Mike]

Note that the calculation I did is for TWO divers, not one,

Ah... I see. Well there goes my dive God theory