#850
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From Aikidiver: Here are the limits for the dives during this certification: Wreck penetration limits. The overhead environment poses three limits beyond the normal open water constraints of depth, air supply and no-decompression limits: 1. Edge of light zone — You should never penetrate a wreck past the point where you can see the natural light of the entrance. For this reason, you do not make penetration dives at night or in water so deep and murky that there is little or no natural light visible from inside ...
The pony bottle is not used for breathing. (AS GAS PLANNING GOES) It is a replaement for a failed primary gas supply.
You refer to rule of thirds. So worst case will be you are 1/3 in ,at the turn point, and you have a failure of your porimary gas supply, You now have no supply to breath. You switch to your pony and exit the wreck. Your pony will now have to have a min of 1/3 the supply of the original gas supply. If you are using a al s80 then you have now lost 2/3 of 77 cuft. so you need a min of 1/3 of 77 or 23 cuft. Now that leaves you with a min pony size of a 30. Your ascent TO THE SURFACE can not include the safety stop when thinking of a pony with 23 cuft. Remember a safety stop is a deco stop that is not a required one. This is why the no deco limit was placed.
Other things to consider.
23 cu ft is technally 1/3 of your back gas but if you have switched to it you are most probably stressed and your body will respond as such. Respiration will increase to 1.5 or 2 times the consumption untill you FULLY calm down. 1/3 in may be 23 cuft of gas but out on a pony may use 30 or more because of stress related issues. This makes the al40 the appropriate size of a pony.
The degree of stress incurred may cause more problems like breaking your line and not knowing your way out. Loss of visibility (RULE #1) from silting is a common one to deal with. Silted water blocks natural sunlight from penitrating your location and increases the stress level. It is easy to vilote rule #1 and not know it. No matter how good you are at not silting, the silting is the result of the entire group not just you. Lastly a sudden cloud cover can reduce the light level to near nil and it only takes one of the group to silt things up in responce to the sudden low light.
There is one rule to remember. The only emergency underwater is an out of air. All other issues are irritating problems allowing you to stop, think, plan, and act. So a sudden loss of visibility is not an emercency. stop and lay stationary for a few minutes let the silt settle and exit when vis improves via the nearest exit. If those few minutes require 10 cuft then once again the 40 is the way to go.
Anything over the al 40 is normally not needed as everyone in the group will be toting a al40 as a pony. A 40 fits very nicely snap bolted to your rig as opposed to bottles less than 30. been there done that.
General rules for pony’s are that they be al tanks for the bouyancy charactoristics. neutral at half filled. Tank valve is shut till needed to guarentee a supply is there if needed. Possibly opened upon entrance to wreck.
Things that can cause a primary gas failure: in an overhead environment these examples are not that uncommon.
regulator failure. primary secondary or cut hose
blown tank o-ring
shered burst disk
tank valve rolled off.
Arguments for using less than a al40 for a pony:
Rule#2 you hit turn point long before you hit the 1/3 rule.
Rule #3 may be based on the primary air supply available at wreck entry point
More reasons to use a al40.
You can use this size as a deco bottle in the future
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