Karl_in_Calif
Banned
Thank you MD. I will just take the compliment portion of the backhanded slam and leave it at that. Dee would be soo pleased then.
Just bought a pony system, need help with answers...
- Thread starter rakjar
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Thank you MD. I will just take the compliment portion of the backhanded slam and leave it at that. Dee would be soo pleased then.
What Uncle Pug relates regarding gas management is critically important. Pony bottles ought not to figure into your gas management calculations.
Pony bottles, if carried at all, relate to unforeseeable events that occur beyond gas management calculations; and the caveat applies here that we're discussing recreational diving as opposed to technical diving. They only are worth the effort to bring along when circumstances warrant - e.g. when benefits outweigh their inherent liabilities.
Despite what Karl has to offer here, whatever that may be, if I were forced to dive a single tank to depths of 100' to 130', I would use a 30 cu. ft. pony bottle. There is little point in diving deeper on a single tank, recreationally, anyway. To illustrate calculating your requirements, lets use an RMV of 1.2 cubic feet per minute because I think Mike is right. High Anxiety does odd things to most people's respiration rates. Karl is way low with .7 for an average high-stress RMV.
Take a minute to resolve the immediate crisis. IMO, Karl is misinformed. One does not "$hit & Git" underwater (in over 99% of all potential scenarios). Plan one minute to figure out what the problem is and think. Move slowly and deliberately. Practice deployment beforehand so it isn't a scare-reflex reaction.
Ascending from 120' on your pony requires 4 minutes at 30 fpm. There is no need to 'bail out' and scream up any faster.
The 3 minute safety stop is wise. Add another minute to ascend from your safety stop. Thats nine minutes total planned for the ascent, minimum, at 1.2 cfm = 10.8 or ~11 cubic feet for the ascent minimum.
But remember, emergencies have an annoying way of not occurring like you envisioned them. You may have to deploy a lift bag. You may be dragging another diver. You may need to cut your way free after becoming entangled in a drifting gill net. Or, you may need to unclip the pony and hand it to some near-panicked diver who has an OOA going on with some other complications.
In any circumstances like that, not only will your RMV increase with exertion but the time required to resolve the issues and conduct the ascent may increase as well. Remember, every minute here is something that was outside your initial gas planning and original dive plan. How many minutes do you want to have available, at RMV >1.2 ft3 per?
IMO if you're going to carry a pony at all, a 19 cu ft tank is minimal. That might give you another five minutes at depth to resolve some issue. A 30 cu ft tank - for depths such as those you dove to with your new buddy - is prudent.
Your mileage may vary.
Doc
Pony bottles, if carried at all, relate to unforeseeable events that occur beyond gas management calculations; and the caveat applies here that we're discussing recreational diving as opposed to technical diving. They only are worth the effort to bring along when circumstances warrant - e.g. when benefits outweigh their inherent liabilities.
Despite what Karl has to offer here, whatever that may be, if I were forced to dive a single tank to depths of 100' to 130', I would use a 30 cu. ft. pony bottle. There is little point in diving deeper on a single tank, recreationally, anyway. To illustrate calculating your requirements, lets use an RMV of 1.2 cubic feet per minute because I think Mike is right. High Anxiety does odd things to most people's respiration rates. Karl is way low with .7 for an average high-stress RMV.
Take a minute to resolve the immediate crisis. IMO, Karl is misinformed. One does not "$hit & Git" underwater (in over 99% of all potential scenarios). Plan one minute to figure out what the problem is and think. Move slowly and deliberately. Practice deployment beforehand so it isn't a scare-reflex reaction.
Ascending from 120' on your pony requires 4 minutes at 30 fpm. There is no need to 'bail out' and scream up any faster.
The 3 minute safety stop is wise. Add another minute to ascend from your safety stop. Thats nine minutes total planned for the ascent, minimum, at 1.2 cfm = 10.8 or ~11 cubic feet for the ascent minimum.
But remember, emergencies have an annoying way of not occurring like you envisioned them. You may have to deploy a lift bag. You may be dragging another diver. You may need to cut your way free after becoming entangled in a drifting gill net. Or, you may need to unclip the pony and hand it to some near-panicked diver who has an OOA going on with some other complications.
In any circumstances like that, not only will your RMV increase with exertion but the time required to resolve the issues and conduct the ascent may increase as well. Remember, every minute here is something that was outside your initial gas planning and original dive plan. How many minutes do you want to have available, at RMV >1.2 ft3 per?
IMO if you're going to carry a pony at all, a 19 cu ft tank is minimal. That might give you another five minutes at depth to resolve some issue. A 30 cu ft tank - for depths such as those you dove to with your new buddy - is prudent.
Your mileage may vary.
Doc
Karl_in_Calif
Banned
So, at the very least, then you and I would hopefully agree that a "Spare Air" can is a totally inadequate piece of equipment.
Sometimes it is better to focus on the area of agreement than on the areas of disagreement.
Depends on application....if you are at your desk dusting off a keyboard...or using it as a compressed air can for a paintball gun then fine.....
as for diving the thing is a death trap
Mike,
I agree with every thing you except maybe this part....
In my opinon if you still have to consider if a saftey stop is warranted, you need to make the stop and stay unitl the stop is done even if the SPG is bouncing on the peg. Most SPG's are not horribly accurate at the low end of the scale and tend to be a little conservative. A couple hundred psi in the tank at the surface after the dive that the SPG did not think you had, does you no good at all. You are far better off using it at 15 ft. Virtually without exception any reg wil start to breathe harder whenthe tank pressure falls below IP.
On a large tank this allows several breaths and will still take a couple minutes to breath down at 15 ft if you are not hoovering the air. With a reasonably sized pony (19-30 cu ft) you still have a breath or two and enough air for the ascent from 15 ft. So in the situation you describe at a saftey stop and even more so at a deco stop, I'd stay until the reg said "go" regardless of the SPG reading.
An SPG on the pony is a very good idea though just for peace of mind and also to clue you into when the pony is low and when you may want to start paying attention to the increased work of breathing.
In other news...
I agree with whoever said that a 19 cu ft pony should be considered the minimum for deep diving. Under normal circumstances and with a SAC rate of .5 cu ft/min, I can ascend from 130 feet on 10 cu ft of air with a 1 minute deep stop at 65 ft, a 1 min stop at 30 ft, a 3min stop at 20 ft and a 1 min stop at 10 ft.
In this case a 19 cu ft pony would give me just enough air to do a normal ascent under less than normal conditions where my air consumption is significantly higher or where I may be on the bottom a minute or so longer before startgn the ascent. A 30 cu ft pony is even better as it allows more margin of error and is preferable if your normal sac rate is substantially higher.
In any case it is a really good idea to get away from the math and best guestimate excercise and actually try some normal ascents with your pony to see what it can do really do. It builds confidence and also builds the skills you need to actually do it when things go south on you.
I agree with every thing you except maybe this part....
In my opinon if you still have to consider if a saftey stop is warranted, you need to make the stop and stay unitl the stop is done even if the SPG is bouncing on the peg. Most SPG's are not horribly accurate at the low end of the scale and tend to be a little conservative. A couple hundred psi in the tank at the surface after the dive that the SPG did not think you had, does you no good at all. You are far better off using it at 15 ft. Virtually without exception any reg wil start to breathe harder whenthe tank pressure falls below IP.
On a large tank this allows several breaths and will still take a couple minutes to breath down at 15 ft if you are not hoovering the air. With a reasonably sized pony (19-30 cu ft) you still have a breath or two and enough air for the ascent from 15 ft. So in the situation you describe at a saftey stop and even more so at a deco stop, I'd stay until the reg said "go" regardless of the SPG reading.
An SPG on the pony is a very good idea though just for peace of mind and also to clue you into when the pony is low and when you may want to start paying attention to the increased work of breathing.
In other news...
I agree with whoever said that a 19 cu ft pony should be considered the minimum for deep diving. Under normal circumstances and with a SAC rate of .5 cu ft/min, I can ascend from 130 feet on 10 cu ft of air with a 1 minute deep stop at 65 ft, a 1 min stop at 30 ft, a 3min stop at 20 ft and a 1 min stop at 10 ft.
In this case a 19 cu ft pony would give me just enough air to do a normal ascent under less than normal conditions where my air consumption is significantly higher or where I may be on the bottom a minute or so longer before startgn the ascent. A 30 cu ft pony is even better as it allows more margin of error and is preferable if your normal sac rate is substantially higher.
In any case it is a really good idea to get away from the math and best guestimate excercise and actually try some normal ascents with your pony to see what it can do really do. It builds confidence and also builds the skills you need to actually do it when things go south on you.
Karl_in_Calif
Banned
Then you might as well use a 40 and fill it with EAN36 so that you could egress from your first dive with 1400 psi from it, and have the remaining 1600 psi for your second dive's egression, and simply leave 1000 psi in your main tank under the rule of thirds.
I find there is little use to owning either a 20 or a 30 cu ft stage tank for use as a pony, for any reason.
cornfed
Mindless lemming
Karl, why are you advocating diving EAN36 at these depths!?
Let's also ask why he is advocating use of a pony as part of his gas management plan instead of as an emergency backup in the event of a catastrophic system failure.
Karl_in_Calif
Banned
Cornfed, let me help you with this.
PO2 = FO2 x P(total)
P(total) = PO2 / FO2 = 1.4 / 0.36 for backgas or
P(total) = PO2 / FO2 = 1.5 / 0.36 for deco
1.5 / 0.36 = 4.167 ATAs = 105 fsw or 107 ffw
It would be a bad idea to dive deeper than 105 fsw or 107 ffw with EAN36 in your 40 cu ft stage tank, if that was your sole bail out gas, AND you did not have a buddy. However 95 ft would probably be a much safer limit. Hope that helps.
I know you can do these calculations yourself. You would not have gotten your government job if you were not a really good mathematician and a really bright person.
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