13 Cu. Pony Tank

[markfm]

As I mentioned before, perhaps try it with an AL80.
AL80 = 77.5 ft3 13/77.5 = 0.1677 0.1677*3000 = 503

A full 13 has 13CF of gas. An AL80 holds 77.5 ft3 (varies a bit from manufacturer to manufacturer). The 13CF is, therefore, equivalent to about 503 psi of the AL80. I call it 500 -- am not interested down at the <1% error level.

If you have an AL80, see how many PSI you burn during the 20 minutes at 20'.

Similar for the 19CF. 19/77.5 = 0.24516 .24516*3000 = 735. A full 19 is equivalent to 735 psi on an AL80. I treat it as 750, a good approximation (2%). If I can do something with 750 psi on an AL80, I'm personally comfortable that I can do the same on a 19.

These numbers are what I used in deciding what I wanted to buy and use. I watched my gas use on an 80. Then actually practice using it, plan, and do, a pony based ascent (where I still have plenty of back gas, and am with my buddy who knows up front that on this dive I will use my pony for the ascent).

This is just for me, what I chose to do.

 

[dkktsunami]

As I mentioned before, perhaps try it with an AL80.
AL80 = 77.5 ft3 13/77.5 = 0.1677 0.1677*3000 = 503

A full 13 has 13CF of gas. An AL80 holds 77.5 ft3 (varies a bit from manufacturer to manufacturer). The 13CF is, therefore, equivalent to about 503 psi of the AL80. I call it 500 -- am not interested down at the <1% error level.

If you have an AL80, see how many PSI you burn during the 20 minutes at 20'.

Similar for the 19CF. 19/77.5 = 0.24516 .24516*3000 = 735. A full 19 is equivalent to 735 psi on an AL80. I treat it as 750, a good approximation (2%). If I can do something with 750 psi on an AL80, I'm personally comfortable that I can do the same on a 19.

These numbers are what I used in deciding what I wanted to buy and use. I watched my gas use on an 80. Then actually practice using it, plan, and do, a pony based ascent (where I still have plenty of back gas, and am with my buddy who knows up front that on this dive I will use my pony for the ascent).

This is just for me, what I chose to do.

Great way to practice.

 

[markfm]

Only way to do it. Familiarity with a skill, the steps, is what is needed to minimize the air-sucking modes that some folks, accurately, refer to.
I check my pony pressure at the start and end of each dive, am confident in the gas that it has. Actually doing some ascents from depth is what it takes to make it a no-big-deal, minimal stress, knowing that yes, indeed, it has enough gas to get me from a given depth, with the safety stops I use (one minute at half depth, then the standard stuff).
If Mr Ed goes dead during practice, I've pushed it to a new depth attempt that goes outside my margins, no harm -- I'm practicing with back gas still there, plus my buddy.
(Never using it to extend my dive time -- if I would normally start an ascent from depth at 1250 psi, I would do the practice ascent on pony when my back gas has that 1250.)

 

[lamont]

With an Al80 I plan around 300 psi / 10 mins / ata (0.75 SAC). At 20 fsw, call it 1.5 ata and 450 psi / 10 min. For an Al19, that'd be about 4x the rate, so 1800 psi / 10 min, which would be around 16 mins.

Or, assuming a 0.60 cu ft / min SAC rate, do it exactly:

19 cu ft / (0.60 cu ft / min * (20 fsw / 33 + 1) ) ~= 20 mins

I might be able to get my SAC rate below that on deco not moving around much, but for planning purposes I would never assume below 0.60 for myself. YMMV.

 

[Web Monkey]

Because you're freezing your butt off in New Jersey and the low numbers are in warmer water?

I usually run about .65 in cold water in a drysuit and a little less than .5 in warm water in a wetsuit.

Also it gets lower with more diving.


Terry

For comparison, sitting on the sand at 30 feet for 5 minutes, I computed my SAC rate to be about 0.5/min. When I'm swimming relaxed with no stress, I seem to be between 0.6 and 0.8. With any expenditure of effort, I'm around 1!

Any thoughts as to why my numbers are so high? Big lungs, maybe? Or why yours are so low?

 

[Hoosier]

It has been covered many times, but the answer is always same, at least 19cf tank.

6 and 13 cf are more likely argon bottle, not bail out bottle. Of course, if you say above 60 fsw, 13 cf can be used as a bail out bottle.


Scuba rule 101
More gas never hurt you at all..

 

[BigTuna]

I dont really know some of the calculations being done here, so here is another question on the same topic. How long will a 13 cu. last me at 20ft breathing calmy at say 25-30 breathes a minute. Will is suffice to do a 20 minute deco at 10 at 25 breathes per minute? Please dont ask why a deco like that would be necessary this is just a more practical analysis than going out of air at 100ft.

The rock bottom spreadsheet says you need 32 cf to stay 20 min at 20 ft, assuming a SAC rate of 1 cf/min, which is higher than the average. For lamont's SAC rate of 0.6, it says you need a little over 19 cf. A 13 cf bottle wouldn't cut it.

Since you're interested in a 13 cf bottle, how long COULD you stay at 20 ft with it? Assuming it starts at 3000 psi, the rock bottom spreadsheet says 8 min for the high-side SAC rate of 1 cf/min, or 13 min for lamont's SAC rate of 0.6.

How long at 10 ft? Answer: 10 min for the 1 cf/min SAC rate, or 16 min for lamont's 0.6 SAC rate.

The replies you've gotten are the best we can do for you without knowing how much air is in a "breath."

 

[cebudiver]

On a AL 80cu. @ 500PSI I personally can do 20 minutes. Enough time to complete most deco's under normal conditions. "What is Normal?" Poses another question. Lets just say in 90% of the conditions which I dive in. I have decided to go with the small 13cu. Thanks for all your inputs.

 

[cebudiver]

Correction 80cu. @ 500PSI @ 10ft. 20 minutes.

 

[lamont]

I dont really know some of the calculations being done here, so here is another question on the same topic. How long will a 13 cu. last me at 20ft breathing calmy at say 25-30 breathes a minute. Will is suffice to do a 20 minute deco at 10 at 25 breathes per minute? Please dont ask why a deco like that would be necessary this is just a more practical analysis than going out of air at 100ft.

I'm honestly still really confused about why you think that this is a more practical analysis? If you're just looking for a safety net if you happen to be on deco without enough air to complete it, then hang a bottle with a high O2 content. If you're looking for bailout, then you want to be doing the analysis of what the pony bottle will get you if you go OOA at 100 fsw.