Scuba
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Ahhh, maybe "Right" is going out of style, and the new in word is "True".
Please explain why dual 80's are "true" and a 120 and a 40 "lie"?
Should I get a Spare Air?
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Ahhh, maybe "Right" is going out of style, and the new in word is "True".
Please explain why dual 80's are "true" and a 120 and a 40 "lie"?
Jeff,
Please explain true redundancy. My understanding is there are three options for doubles, independent, manifold, and isolation manifold. The only option that provides total redundancy is independent doubles, but only at the cost of the complication of having to switch back and forth between tanks to maintain trim. In a manifold system the air supply is not redundant only larger. An isolation manifold provides redundant air supplies only based on manual manipulation of the isolation valve, and the isolation valve itself is a single point of failure.
How do any of those options provide truer redundancy than a pony with its own regulator?
As to the use of doubles, a good set of doubles will run about $1,000, not including regulators. I purchased a 30 cf pony, an Apex DS4/ATX40, and a SPG for less than $300. When I dive where I can use my own tanks I dive a PS HP120 and typically either hit the NDL, get cold, or just get board before the dive plan calls for surfacing due to gas supply, even diving Nitrox. The only reason that I can see for going to doubles is if at some point I decide to get the proper training and do staged decompression or penetration diving. Then to make two dive trips practical, I would need to get two sets of doubles; there goes $2K. And even then it would not be practical to take the doubles anywhere that required air travel.
Ive heard, and will try it on the way to the Florida Keys this month, that if you remove the valve and put in a plastic plug, there is no problem taking a pony in checked baggage.
As to the aliens, Im hoping that they metabolize nitrogen, producing nitrous oxide, in which case we should get along just fine.
Mike
JeffG
Contributor
No, "Right" is not going out of Style, but I am a stroke so it didn't fit to use that term. But used the word true to appease the "True" god
Actually, When I read his posting I was thinking it would be just easier to rent another 80 and add it to his double system (if he had one) than rent a pony bottle and/or fight the airlines with a compressed air bottle.
But IMO I think the pony bottle is a solution to a non existent problem. (i.e the alien attack possibility) but its not my place to say yea or nea.
Jeff
JeffG
Contributor
Well, the independent doubles idea was kinda what I was thinking, But I was thinking more about if he used a double setup because it seem to me that it would be easier to rent 80's instead of hauling around ponies. If you can use a plastic plug, great, I can learn something new everyday
You don't have to beat me over the head tooooo many times:bonk: 
Wreck/Tec
Contributor
MIKES
I believe if you take tanks on a plane they have to be empty, and you will pay more reguardless, by virtue of weight and purpose. Ask!
Wreck/Tec
I believe if you take tanks on a plane they have to be empty, and you will pay more reguardless, by virtue of weight and purpose. Ask!
Wreck/Tec
Ive heard that not only do they need to be empty but also the valve needs to be removed. Thats why I was planning to remove the valve and put in a plastic plug. I know that American Airlines allows tanks because they are listed under Sports Items for which an excess charge applies.
http://www.americanairlines.com/content/customerService/baggage/baggageAllowance.jhtml
I think that I will play dumb put the pony in a check bag and see what happens. I think maybe the charge only applies to full size tanks because of the special handling required.
As to paying more based on weight, I think that that depends on your situation. Per American Airlines each passenger is allowed two 50 checked bags and a 40 pound carry on. If I cannot get a week of clothes for warm weather and all my SCUBA gear including the pony (less weights and tanks) under 140 pounds Im carrying to much. Worst case is Ill have to pay $80 for an extra bag.
Mike
I've always seen posts complaining about the amount of time a 3 cuft spare air gives you and how it is utterly useless but I've never seen anyone do the math.
Here's the equation I came up with to determine the amount of volume of air needed to ascend from depth:
Volume = (breathing-rate-at-surface * depth / ascent-rate) * (depth / 30ft + 1)) / 2
Volume is in cu.ft.
Depth is in feet
ascent-rate is in ft/min
breathing-rate is in cu. ft. / min
The 30ft is in there to express the depth for 1 atm pressure.
Assuming a very stressed out diver with an at-surface breathing rate of 1 cu. ft./min (always estimate on the conservative side), a 3 cu. ft. spare air will bring you to the surface from 60' using a 30'/min ascent rate.
If you were at 130 ft, the spare air will only get you to 110 ft using the same breathing rate.
For realtively shallow dives ( < 70 ft), spare air looks advantagous for getting you within a comfortable depth for a CESA.
For math buffs, here's how I came up with this equation (it's been years since college so please correct me if I made a mistake):
(1) t(d) = d / r, time to ascend, d being depth in feet, r being ascent rate in ft/min
(2) b(d) = b0 * (d / 30ft + 1), breathing rate in ft^3/min at a depth, b0 being the breathing rate at the surface in ft^3/min
(3) V(d,t) = b(d) * t, V being the volume of air used at a depth d for time t, t in minutes
Combining 1 and 3 and integrating over time, you get
Vasc(d) = Integral of (b0 * (d / 30ft + 1)) with respect to time (dt) from t = 0 to t = d / r
Solving, you get
Vasc(d) = b0 * (d/r) * (d / 30 + 1) / 2
Did I get this right?
Here's the equation I came up with to determine the amount of volume of air needed to ascend from depth:
Volume = (breathing-rate-at-surface * depth / ascent-rate) * (depth / 30ft + 1)) / 2
Volume is in cu.ft.
Depth is in feet
ascent-rate is in ft/min
breathing-rate is in cu. ft. / min
The 30ft is in there to express the depth for 1 atm pressure.
Assuming a very stressed out diver with an at-surface breathing rate of 1 cu. ft./min (always estimate on the conservative side), a 3 cu. ft. spare air will bring you to the surface from 60' using a 30'/min ascent rate.
If you were at 130 ft, the spare air will only get you to 110 ft using the same breathing rate.
For realtively shallow dives ( < 70 ft), spare air looks advantagous for getting you within a comfortable depth for a CESA.
For math buffs, here's how I came up with this equation (it's been years since college so please correct me if I made a mistake):
(1) t(d) = d / r, time to ascend, d being depth in feet, r being ascent rate in ft/min
(2) b(d) = b0 * (d / 30ft + 1), breathing rate in ft^3/min at a depth, b0 being the breathing rate at the surface in ft^3/min
(3) V(d,t) = b(d) * t, V being the volume of air used at a depth d for time t, t in minutes
Combining 1 and 3 and integrating over time, you get
Vasc(d) = Integral of (b0 * (d / 30ft + 1)) with respect to time (dt) from t = 0 to t = d / r
Solving, you get
Vasc(d) = b0 * (d/r) * (d / 30 + 1) / 2
Did I get this right?
LOL. I'm not going to go through the calc right now but I would just use the average depth during the ascent. True that won't give you the gas used at any intermediate time but...
Charlie99
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First, 1atm is 33', not 30.
For a constant rate ascent you will use the same amount of gas as if you spent all of the ascent time at the average depth.
If you have an emergency at 100' and are bailing out on a 3 cu ft tank you will NOT be poking along at 30fpm.
1 cfm SAC is NOT a conservative number for a panicked diver. 2 or 3 is not unreasonable.
Even your equations wouldn't have the diver running out after 10 seconds (ascent time from 100' to 95').
After noting the above, I decided it wasn't worth my time figuring out if your equation was right.
How about this calculation:
100' to surface at 60fpm is 1.67 minutes.
50' avg = (50+33)/33 = 2.52 ata.
1.67 min * 2.52 ata = 4.2 equivalent surface minutes.
This means 4.2 cu ft if you assume 1 cu ft / min breathing rate,
8.4 if you assume 2cfm.
Here's another interesting calculation:
99'=4ata.
A calm diver will breath something around 0.6 cfm on surface, or 2.4cubic feet per minute.
3 cubic feet of spare gas gives you about a minute to hunt down your buddy (he IS close by isn't he?) and get hold of an adequate air supply (your agreed upon turn point DID allow for 2 agitated divers ascending on one tank, didn't it?)
For a constant rate ascent you will use the same amount of gas as if you spent all of the ascent time at the average depth.
If you have an emergency at 100' and are bailing out on a 3 cu ft tank you will NOT be poking along at 30fpm.
1 cfm SAC is NOT a conservative number for a panicked diver. 2 or 3 is not unreasonable.
Even your equations wouldn't have the diver running out after 10 seconds (ascent time from 100' to 95').
After noting the above, I decided it wasn't worth my time figuring out if your equation was right.
How about this calculation:
100' to surface at 60fpm is 1.67 minutes.
50' avg = (50+33)/33 = 2.52 ata.
1.67 min * 2.52 ata = 4.2 equivalent surface minutes.
This means 4.2 cu ft if you assume 1 cu ft / min breathing rate,
8.4 if you assume 2cfm.
Here's another interesting calculation:
99'=4ata.
A calm diver will breath something around 0.6 cfm on surface, or 2.4cubic feet per minute.
3 cubic feet of spare gas gives you about a minute to hunt down your buddy (he IS close by isn't he?) and get hold of an adequate air supply (your agreed upon turn point DID allow for 2 agitated divers ascending on one tank, didn't it?)
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