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TRACI
05-14-2008, 10:02
I know this question probably has already been asked a bunch, but I did a search and could not find anything.

I am not really interested in carrying a spare air, but would like a pony bottle. What is the most efficient size (preferrebly smaller), I have been considering a 6 cu, and I am also looking a setup that is not too expensive.

I am just a novice diver, I do not do any dives that are too deep, maybe 60 - 90 or so ft.

h2odragon1
05-14-2008, 10:12
13 cuft is a good size for your 60-90 ft dives. as always;keep an eye on your guages, and stick to your dive plan.

DOWNDEEP73
05-14-2008, 10:14
I would go with a 13cf or a 19cf. They are not that much more...and you will have more spare air....and that is a good thing.

TRACI
05-14-2008, 10:43
The more I have read looks like a 13 or 19 would be better, is there any problems traveling with either?

ratown
05-14-2008, 10:46
You will probably have to empty it and take the valve off on an airplane.

Sounder
05-14-2008, 11:17
As a novice diver, I highly recommend the following reading. Understanding this will help you determine how much volume of redundant gas you need for the diving profiles you do. It'll also let you know how much gas you and your buddy need to carry incase the other one has a problem.

I seriously recommend this reading. It's no nonsense facts that will change the way you approach dive planning and which will keep you safer in the water.

Rock Bottom and Gas Management for Recreational Divers (http://www.scriptkiddie.org/diving/rockbottom.html)

No Misses
05-14-2008, 11:18
The question of pony bottle size comes up often.
Here are the questions that you need to ask yourself:
1. What is my SAC (Surface Air Consumption) Rate? My SAC rate is ~.6 cf per minute, so I will use that in the following calculations.
2. How deep to I plan to dive? Conver this to ATA for the calculations. (66 fsw = 3 ATA, 99 fsw = 4 ATA)
3. Do I want to carry enough gas for my buddy too?
4. How stressed will I be if I have an OOA situation? Your SAC rate will increase with stress.

Minutes of breathing gas = cylinder capacity in CF / (SAC*Depth in ATA)

For me that works out to...

7.9 minutes = 19 cf / (.6*4)

If I switch to my AL 19 @ 99 fsw and ascend @ <30 fpm and do a 3 minute safety stop, I should have 1.9 minutes of breathing gas left over when I surface.

Note: I did not recalculate my gas consumption as the depth decreased (All calculations at max depth of 99 fsw). So I should have a litle more that 1.9 minutes of gas remaining. YMMV

P.S. most dive computer download programs will calculate your SAC. If you do not have one, try the calculator at this site SAC Calculator (http://www.spearfishing.org/bruces_tips/java/sac.html)

Sounder
05-14-2008, 12:06
The question of pony bottle size comes up often.
Here are the questions that you need to ask yourself:
1. What is my SAC (Surface Air Consumption) Rate? My SAC rate is ~.6 cf per minute, so I will use that in the following calculations.
2. How deep to I plan to dive? Conver this to ATA for the calculations. (66 fsw = 3 ATA, 99 fsw = 4 ATA)
3. Do I want to carry enough gas for my buddy too?
4. How stressed will I be if I have an OOA situation? Your SAC rate will increase with stress.

Minutes of breathing gas = cylinder capacity in CF / (SAC*Depth in ATA)

For me that works out to...

7.9 minutes = 19 cf / (.6*4)

If I switch to my AL 19 @ 99 fsw and ascend @ <30 fpm and do a 3 minute safety stop, I should have 1.9 minutes of breathing gas left over when I surface.

Note: I did not recalculate my gas consumption as the depth decreased (All calculations at max depth of 99 fsw). So I should have a litle more that 1.9 minutes of gas remaining. YMMV

P.S. most dive computer download programs will calculate your SAC. If you do not have one, try the calculator at this site SAC Calculator (http://www.spearfishing.org/bruces_tips/java/sac.html)

Just one thing to add - a .6 is a pretty average-good RMV and newer divers will be a bit higher. Also remember that if/when you ever have to "bail out" to your pony bottle, you're most likely going to be stressed so you need to reflect that in your consumption value. Typically the stressed RMV is calculated at 1.0 but that's just an average - everyone will be different so it's best to KNOW what your RMV is instead of guessing.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-15-2008, 18:53
In my first few ocean dives I was at 0.85. A little on the air hog side. I have seen it as high as 1.0 under non-emergency but very stressful situations. Like was said above, know your breathing rates so you can make proper plans. Also, FWIW my SAC is measurably lower when I have my 40cf pony with me. I guess the 'peace of mind' more than makes up for increased drag.

I think the 19 is the sweet spot for the typical diver at these depths in good conditions. For me and my dives it would be pushing the envelope a little too close.

ChrisA
05-15-2008, 19:15
I know this question probably has already been asked a bunch, but I did a search and could not find anything.

I am not really interested in carrying a spare air, but would like a pony bottle. What is the most efficient size (preferrebly smaller), I have been considering a 6 cu, and I am also looking a setup that is not too expensive.

I am just a novice diver, I do not do any dives that are too deep, maybe 60 - 90 or so ft.

You have to calculate it. the first hing you should do is figure out how much air you breate in a minute. Then how many minutes it might take you to reach the surface allowing for a 3 minute saftey stop and a small reserve. Nex you have to account for the pressure, you will use more air at depth then near the surface. Sounds complex but it's not.

First step is to measure. Next dive, bring a slte with you. Stay at some constant depth and just swimm around for 10 minutes. Record the tank pressure in PSI at the beginning and end of a 10 minute period. Record the depth. Record what kind of tank you were using too. Then with this data come back here and ask someone to help computer your SAC

You are a new diver. Lets assume your rate is 1 cu ft per minute.

If you do a one foot per second accent from 120 feet it will take you 120 seconds to reach the surface. Durring your acentyou average depth will be 120/2 or 60 feet. At 60ft the pressure is 3 atm. So you wil breathe 3 cu ft per minute on average for 2 minutes. In theory you could just use a 6 cu ft tank but yo did not alow for a 3 minute saftey stop or allow a reserve to account for


The pony tank may not be 100% full when you start yuor dive
You can not breathe all the air from a tank the reg stops working at about 200psi
when you are excited you breathe faster, maybe even dounble the normal rateRelitically you will need to double that 6 cu ft tank to the next avilable size which is 13 cu ft. The cost is only a few bucks more.

Howeverr if you want a 6 cu ft tank even after working out the math. I sell you one at a reasonable price. I bought it when I was new before I figured out how to compute air consumption. It's yellow and looks like new.

ChrisA
05-15-2008, 19:32
Note: I did not recalculate my gas consumption as the depth decreased (All calculations at max depth of 99 fsw). So I should have a litle more that 1.9 minutes of gas remaining. YMMV

The effect of assuming you will do your entire "accent" horizontally along the bottom is very, very conservative. It almost doubes the amount of gas you think you will need. Some would even argue that a calculation that is "off" by nearly a factor of two is "wrong".

It's easy to get this right. Then after you have the exact answer add in a suitable reserve.

First I tried to treat this as a basic Calculus problem and integrated the air consumed over time but then when I saw the answer I knew I had over killed a simple problem. Here is the easy way: If you start at 100 feet and end at zero and spend 100 seconds doing the asent then your average depth is 50 feet. You can compute the gas used as if it you remained at a constant 50 feet for 100 seconds

If you don't like 60 ft/sec or 100 feet then change the numbers. Either way if the asent rate is constant then use half the max depth as the average depth.

A good scenario for using a pony is to add up the gas usage from each of the assent's component parts:
One minute on the bottom to get oriented, calm down and start the assent.
A one foot per second assent to 30 feet
A one foot per two second assent from 30 feet
A three min stop at 20 feet.
A 500 psi reserve in the pony tank at the surfaceIf you do this the answer comes out to either 13 or 19 cu ft. unless you do only shalow dives then 6 cu works. But "why bother" if you are only doing a 30 foot dive and run out of air, just swimm up.

No Misses
05-16-2008, 07:44
Hmm. You removed my conservatism and replaced it with your own. It still came out to being a 19 cf bottle :-)

Grin
05-16-2008, 08:38
simple math tells you a 13 is equal to a AL 80 with 490 psi left in it.
simple math also says a 19 is equal to a AL 80 with 712 psi left in it.
" a 6 225 psi " Yikes!
how about a spare airs 3 112 psi :smiley11:

Most people, know 500lbs in a AL 80 is a easy accent from less than 100ft. When I used to dive AL 80s, every dives goal was to leave the bottom at 500lbs. Many times I screwed up and left at 300-400 and did accents that were way better than 30fpm averages.

TRACI
05-16-2008, 08:45
I would consider a 13 cf, I just need to see how much difference in size between the 6 and the 13.

ScaredSilly
05-16-2008, 11:52
The difference in size is a bit. 6cuft bottles tuck out of the way very easily. 13 are bit harder but still reasonable and there is not much difference between a 13 and 19 only length.

TRACI
05-16-2008, 12:01
AI just ordered a 13cf from ST, now the fun part, how to figure out how to carry it :smiley5:

huvrr
05-20-2008, 21:34
Yeah, I finally got me a 19cf pony and rigged it up to sling it. I sure like not counting on others.