Near reg failure - all ok

[Boarderguy]

My 19cf with valve and pony reg setup is right around 10. Guess if I had a 13cf it would be around 8. It's all relative -- when you have airline bag limits of 40 or 50 lbs, a 4 or 6 lb. difference is lot. Especially since all of my other gear in a checked bag is usually around 46 lbs damp when flying back (that's already putting some items in my carry-on). YMMV, but for me the result is usually to not fly with a pony.

Are those weights with the valve and full? I gave empty weights for luxor tanks, 6 & 8lbs, and then added 1 lbs for the valve. The 2nd reg set would go in your carry on so that weight is negligible.

 

[Rob9876]

Are those weights with the valve and full? I gave empty weights for luxor tanks, 6 & 8lbs, and then added 1 lbs for the valve. The 2nd reg set would go in your carry on so that weight is negligible.

With valve (off), tank empty, including reg. I've already got a heavy carry on with other gear. I'm sure we pack differently, but my point was that 4 to 6 lbs. can make a difference between bringing an alternate air source or not for some people.

 

[Thunder Struck]

Care to share the name of the charter/operator that owned the tank?

 

[johndiver999]

Care to share the name of the charter/operator that owned the tank?

yeah this is not: "one of those things" it is long term gross negligence.

 

[SlugLife]

My subjective opinion is that 13 is about the "sweet spot" between meaningful redundancy and space/weight savings, but I sadly don't have one. Because I own 1.7, 3, 6, and 19, what I actually travel with is the 6cu. My 6cu has standard 3/4 valve threading, but has bottle-regs. I'd prefer bigger, but I keep a much closer eye on my air, and figure in an emergency, I might be skipping the safety-stop.

 

[tursiops]

We bought 1.7cf ponies

Those were not full-grown ponies; those were just larvae, not even juvenile tanks. (They were probably SpareAirs.) 3 cuft is not a pony; 6 cuft is not a pony. The smallest tank of any value as a pony is 13 cuft, and 19 is much better. You can work this out by assuming a SAC of 0.5 cuft/min. PIck a depth, keep your ascent rate below 30 ft/min, assume your breathing will double or triple if you are nervous, frightened, panicked; how long will 1.7 cuft last you?

 

[Kimela]

Those were not full-grown ponies; those were just larvae, not even juvenile tanks. (They were probably SpareAirs.) 3 cuft is not a pony; 6 cuft is not a pony. The smallest tank of any value as a pony is 13 cuft, and 19 is much better. You can work this out by assuming a SAC of 0.5 cuft/min. PIck a depth, keep your ascent rate below 30 ft/min, assume your breathing will double or triple if you are nervous, frightened, panicked; how long will 1.7 cuft last you?

Yup. I agree. I suspect I'd burn through more air trying to figure out how to use the Spare Air in a panic and end up in worse shape. But that's an opinion based on just my hunch, nothing more. And the reality is that I wouldn't 'practice' using the spare air either (I'm lazy, but I'm honest!).

 

[rx7diver]

My subjective opinion is that 13 is about the "sweet spot" ...

... The smallest tank of any value as a pony is 13 cuft, and 19 is much better. You can work this out by assuming a SAC of 0.5 cuft/min. PIck a depth, keep your ascent rate below 30 ft/min, assume your breathing will double or triple if you are nervous, frightened, panicked; how long will 1.7 cuft last you?

Another approach: Assume a depth of 130 fsw (say), and that you've reached NDL. Calculate how much gas you require to surface. Do this calculation two ways. First time, assume a direct ascent to the surface. Second time, assume you must surface by retracing the route you took to get to where you are.

Assume your pony will contain the exact same gas (Air? EAN28? EAN32? Something else?) that's in your back cylinder, and that your vertical ascent rate will be 30 fpm.

Since, you're right at NDL, any delay will put you into mandatory decompression. So, your calculations should take this into account, too.

Use your swimming/finning RMV (SAC rate), rather than your resting RMV, in your calculations, maybe.

Your calculations will suggest the minimum capacity of the pony you "should" (perhaps) be diving with.

Okay. When you're deciding exactly what capacity pony to purchase and use, weighing all the pros and cons, then keep in mind the baseline minimum capacities you have just calculated.

rx7diver

 

[tursiops]

Another approach: Assume a depth of 130 fsw (say), and that you've reached NDL. Calculate how much gas you require to surface. Do this calculation two ways. First time, assume a direct ascent to the surface. Second time, assume you must surface by retracing the route you took to get to where you are.

Assume your pony will contain the exact same gas (Air? EAN28? EAN32? Something else?) that's in your back cylinder, and that your vertical ascent rate will be 30 fpm.

Since, you're right at NDL, any delay will put you into mandatory decompression. So, your calculations should take this into account, too.

Use your swimming/finning RMV (SAC rate), rather than your resting RMV, in your calculations, maybe.

Your calculations will suggest the minimum capacity of the pony you "should" (perhaps) be diving with.

Okay. When you're deciding exactly what capacity pony to purchase and use, weighing all the pros and cons, then keep in mind the baseline minimum capacities you have just calculated.

rx7diver

Here is a calculation that illustrates the problem. It assumes 1 cuft/min surface consumption rate, and a 30 ft/min ascent rate. NO time spent on the bottom, NO searching for an ascent line, NO deco needed, NO panic. THIS is why you want a 19 cuft pony.

Added later: see posts below to show how this spreadsheet is not quite right.

 

[L13]

Here is a calculation that illustrates the problem. i tassumes 1 cuft/min surface consumption rate, and a 30 ft/min ascent rate. NO time spent on the bottom, NO searching for an ascent line, NO deco needed, NO panic. THIS is why you want a 19 cuft pony.
View attachment 749971

Your "V consumed during ascent" is 0.5Cf/min off. The average ATA from depth is:
(ATA + 1)/2 = ATA/2 + 1/2,
not:
ATA/2

4th column should start with 0.61, and end with 13.20. The last column needs correction also.