Pony bottle questions

[Uncle Pug]

I did dives 2 and 3 with my *new* set of double 72s today... what a joy... the little things are really cool and while only holding marginally more gas than a stuffed LP104 they are not much heavier, very well balanced and of course I have my redundant 1st stages plus iso manifold.

I can easily do two (or even three shallower) dives off of these. Today our first dive was 115' for 45 min. (2500 psi to 1500 psi) and the second dive was 42' for 39 min. (1500 psi to 900 psi.)

You only need one reserve held back for all the dives and that reserve gets smaller as the successive dives get shallower.

Cost to put them together was super cheap:
already had the steel 72s
SeaElite manifold $130
SS bands $64 (including shipping)

I would much rather have this little set of doubles than a single tank with pony on the side.

 

[Uncle Pug]

Razorblade once bubbled...
Is it that difficult to mount two tanks together? In other words: taking four tanks with one manifold, dive with two then surface, disassemble the tanks, link the other two together and dive again is not an option? Is it too much of a hassle?

They need to be empty install the manifold Razorblade.

 

[Razorblade]

that's too bad.
Can you mount singles on a harness after diving doubles? Or is that too hard to do at the dive site as well?

 

[Genesis]

you need a different wing and possibly a STA. Plus, your weighting will be completely different.

If you can keep track of all that and carry the gear around, its a workable solution.

 

[Razorblade]

The reason I was asking about those 4350 psi tanks is because I am thinking about starting to dive with doubles sometimes for longer shallow dives (over say 1.5 or 2 hours) and because I am very interested in getting certified in full cave and wreck diving in the future and I thought maybe it would be a good idea to have the extra gas in a smaller volume so as to not have as much trouble in tighter places.

 

[Razorblade]

STA?

Sorry, *swooosh* over my head.

 

[Rick Murchison]

MikeFerrara once bubbled...


Might want to redo the math one more time? The average depth of the ascent to 15 ft is ....

(15 + 120)/2 = 67.5

Hey, guys, if you'll re-read my post on the matter, you'll see that the ascent all the way to the surface and the safety stop are both included, but figured separately. There is no need to refigure average depth for an ascent to include the safety stop or to figure the ascent to the safety stop only... after all, you are going to continue the ascent on to the surface after the safety stop, aren't you? So you can get the accurate numbers by just calculating the safety stop as one piece of the puzzle, and the ascent as another.
By the way, if you just absolutely have to figure 'em all together, to calculate average depth you must factor in the time spent at various depths, so, for an ascent from 120' at 30 FPM with a five minute stop at 15', you must figure (((120/30)X(120/2))+(5X15))/9, or 35'. Note that 9 minutes gas consumption at 35' yields the same number as figuring a 4 minute ascent at an average of 60' plus five minutes at 15'.

35'= 2.06 ATA X 9 = 18.54 X SCR
-----------------
60'= 2.82 ATA X 4 = 11.27 X SCR
15'= 1.45 ATA X 5 = 7.27 X SCR
Total = 18.54 SCR
------------------
As far as fiddlin' 'round for a minute at depth with a catastrophic failure, I can't figure any reason to do that - I for one am gonna be ascending as I sort things out, and waste no gas making up my mind what to do. That kind of thinking is for now, on the surface, so in the event you can simply act.
Rick

 

[MikeFerrara]

Rick Murchison once bubbled...

Hey, guys, if you'll re-read my post on the matter, you'll see that the ascent all the way to the surface and the safety stop are both included, but figured separately. There is no need to refigure average depth for an ascent to include the safety stop or to figure the ascent to the safety stop only... after all, you are going to continue the ascent on to the surface after the safety stop, aren't you? So you can get the accurate numbers by just calculating the safety stop as one piece of the puzzle, and the ascent as another.
By the way, if you just absolutely have to figure 'em all together, to calculate average depth you must factor in the time spent at various depths, so, for an ascent from 120' at 30 FPM with a five minute stop at 15', you must figure (((120/30)X(120/2))+(5X15))/9, or 35'. Note that 9 minutes gas consumption at 35' yields the same number as figuring a 4 minute ascent at an average of 60' plus five minutes at 15'.

Hey Rick,

I know what you did and it looked ok to me. I was correcting the math that Genesis did when trying to correct your math.

Mike

 

[MikeS]

MikeFerrara once bubbled...


One set of doubles should get you through two dives.

Mike,

It’s not very intuitive but in thinking about it, you need less total volume with doubles than with two singles since you can “reuse” the “Rock Bottom” and any other reserve(s) from the first dive, on the second dive. The downside being that if for any reason (i.e. you or your buddy’s equipment failure), you actually consume the reserve(s) on the first dive, you may not have sufficient gas left for the second.

When I get a chance, I’m going to pull out my logs and look at the actual air consumption from some two dive off shore North Carolina trips. My logs are downloaded from a Cobra so they have the actual air consumption is PSI which I can easily convert to volume. Then by adding the “Rock Bottom” volume for the depth of the second dive to the volume of the two dives I would have the minimum volume required for a set of doubles to safely complete the actual dive profile. In this context “Rock Bottom” refers to the estimated volume of gas required to get myself and my buddy safely to the surface from the maximum depth of the second dive with an ascent rate of 30 ft/min and a 3 minute safety stop.

There are also some other tradeoffs to think about:

At the start of dive, you would tend to need more weight with doubles than with singles to compensate for the greater swing weight. Therefore, you would need to add more air to the bladder and have greater drag.

Although it can be overcome with training, the required valve manipulation associated with doubles as well as multiple SPGs contributes more to task loading than simply switching to a stage slung pony where you can clearly see the valve and SPG right in front of you.

Any comments, corrects, or suggestions are greatly appreciated.

Mike

 

[The Chairman]

The downside being that if for any reason (i.e. you or your buddy’s equipment failure), you actually consume the reserve(s) on the first dive, you may not have sufficient gas left for the second.

The UPSIDE is that you will be alive to dive another day. I don't mind losing a dive, as long as my buddy or I do not lose our lives... or sustain an injury from not having enough gas.