Thanks. Sounds like slinging may be the best route here.
Now I'm trying to figure out the size. I know that a dive almost at the surface I can last about 60 minutes, and 130 feet is 1/5 the volume, so it equals 12 minutes at 130 feet. I'd like 3 minutes to safely ascend, so being very conservative (ignoring the fact I'll use progressively less air while I ascend), I'll need a tank 1/4 the size of my main tank. If my main tank was 120cf, then I should probably get a 30. If my main tank was 80cf, then 20 may suffice.
Does anyone know what a standard tank size is that you'd be likely to get for open water training?
CC,
There really is no 'standard tank' for bailout bottles - as each diver needs to calculate their own needs based on their own individual circumstances (anticipated depths, purposes, potential for entanglement, etc.)
You are likely looking for a Luxfer 19 or 30.
Regardless, the above quote is close-but-no-cigar...you've got the idea, but you've left out a few steps in the calculations.
First you're looking for your Surface Air Consumption (SAC) rate. (Keep in mind, you're not measuring it in an emergency...)
SAC = Total Gas Consumed (psig) -:- [Depth (ATA) x Time (minutes)]
You next convert SAC to Respiratory Minute Volume (RMV) to quantify a "cubic feet of gas consumed per minute at X depth" measurement. You're looking for your personal rate of gas consumption. (Obviously the calc includes depth; not so obviously it also includes which tanks you're using)
RMV = SAC -:- [rated cylinder working pressure -:- rated cylinder volume]
To find RMV:
1. Divide the rated cylinder working pressure by the rated volume;
2. Divide your SAC by this value (from step 1) to find your RMV.
Those are the formal steps, however, none of it accounts for the extremely elevated respiratory rates that are experienced by divers in a state of high anxiety. As this is difficult to quantify, most guys estimate.
An estimate of approximately 1.3 cfm is a conservative figure, but OTOH, those of us who have screwed the pooch a few times can vouch for the fact that divers can go through gas real quick under certain circumstances.....
So, using an estimate of 1.3 cfm at say ~99' (4 ATA), you then count the estimated number of minutes.
You may want to allow, say, 4 minutes to deal with whatever issues you may have (entanglement, panicked OOA diver, clusterpuckery, whatever) at depth. You may be assisting someone with a large camera, a heart attack, a lost mask, its tough to predict.
At a relatively composed rate of ascent of 30 fpm you have 3 min from 4 ATA to 10', then a 3 minute safety stop, say 6 minutes of total ascent. That's 10 minutes total.
At an average consumption of approximately 1.3 cubic feet per minute, over a 10 minute period, you'll be needing a minimum of 13 cubic feet of gas.
But you could easily add environmental or situational parameters such as you are inside a wreck, and must swim horizontally out of the wreck (back to the upline, against potential current, with an OOA diver or large camera, etc. - whatever your personal potential characteristics might entail). If you are going to traverse any distance underwater at depth before ascending, then add the approximate number of minutes it might require you to traverse a distance horizontally at depth.
If you add another 5 minutes for dealing with environmental or situational parameters, whatever these may be in your case, extending your time on bailout to 15 minutes total, then your max cubic feet of gas needs may approach 19.5 cubic feet (at 1.3 cfm).
Add other issues? Your max gas needs may increase accordingly...
(Depth is huge. At 5 ATA (~130') and 1.3 cfm, a 40 will go pretty fast given any sort of issues. Give it a try, like I did, and you'll notice this even without a elevated rate of respiration.....)
Hopefully you can see that "what you need" is not a standard answer. "What you need" depends on you, what you're doing at depth (wrecks?), where you're diving - under what conditions (currents?), with whom (solo or around other divers), etc.
No one can answer these questions but you. So you sort of need to sit down and from a conservative perspective describe what you think you need a bail-out FOR, under what conceivable conditions would you deploy one? Then, based on that personal review, you can calculate out the conditions under which you'd use it, and how much gas you'd potentially need.
There are many threads on this topic, some of which you might find useful. You may want to do a search. Here is an older thread, one example among many in the archives:
http://www.scubaboard.com/forums/ne...g-diving/216419-pony-bottle-vs-spare-air.html
Best,
Doc