Calc Pony Size?

diver-dad · Post by **diver-dad** » Tue Feb 12, 2008 4:37 pm

Folks, I remember reading in our forum somewhere about how to figure out the proper capacity of a pony bottle based on air use rates, depth, etc, but can't find it .. there are just too many entries when you search for "pony" on NWDC.
.... Can anyone point me in the right direction / link?
Thanks!

Sounder · Post by **Sounder** » Tue Feb 12, 2008 9:55 pm

You've got to get into one of Grateful Diver's gas management seminars.

In the meantime, if you'd like one of us to run through the math (and show the work), I'd be happy to as I'm sure others would be.

I have to run to bed right now, but the figures I'll use are ascending at 30fpm (max ascent rate taught by agencies), from 130fsw (max recreational depth), with a RMV of about 1 (estimating a realistic working/stressed RMV). It'll include time to sort out the problem and make stops.

If nobody else gets to it before I can tomorrow, I'll post my answer and show how I got there. It's an interesting question - I'll do it in bed while my wife does her crosswords. I hate crossword puzzles anyway!

Did I mention you should get to one of Bob's gas management seminars?!

cardiver · Post by **cardiver** » Tue Feb 12, 2008 10:10 pm

Crosswords and gas calculations in bed?

Penopolypants · Post by **Penopolypants** » Wed Feb 13, 2008 7:33 am

cardiver wrote:Crosswords and gas calculations in bed?

Huh, yeah, with all that excitement going on, how did they ever find time to make a baby?

spatman · Post by **spatman** » Wed Feb 13, 2008 7:58 am

Penopolypants wrote:Huh, yeah, with all that excitement going on, how did they ever find time to make a baby?

LOL. you are soooo bad!

Sounder · Post by **Sounder** » Wed Feb 13, 2008 8:19 am

Penopolypants wrote:
cardiver wrote:Crosswords and gas calculations in bed?

Huh, yeah, with all that excitement going on, how did they ever find time to make a baby?

Diving math really gets me going!!

Sounder · Post by **Sounder** » Wed Feb 13, 2008 8:47 am

Maximum depth for recreational diving is 130fsw (I'll use 132fsw for ease of math). Here is the math to show what you need ONLY for yourself (not sharing) to appropriately get from the maximum recreational depth to the surface, without other problems or hang-ups.

I'm figuring 1cu'/minute RMV (how much gas you use at the surface) working/stressed/etc. This is a higher, but realistic, RMV for a stressed diver or working diver (i.e. swimming into current).

Ascent rate is 30fpm - the rate taught by major training agencies.

Alright, here we go...

1 minute at 132fsw (5ATA) to sort out the problem, switch to pony, and prepare for the ascent.
1 minute x 1cu' RMV = 1cu' x 5ATA = 5cu'

Ascent from 132fsw to 66fsw (1/2 depth of your deepest depth)
Avg depth is 99fsw (4ATA)
2.2 minutes x 1cu' RMV = 2.2cu' x 4ATA = 8.8cu'

1 minute stop at 66fsw (3ATA)
1 minute x 1cu' RMV = 1cu' x 3ATA = 3cu'

Ascent from 66fsw to 15fsw (heading for safety stop depth)
Avg depth is 40.5fsw (2.23ATA)
1.7 minute x 1cu' RMV = 1.7cu' x 2.23ATA = 3.8cu'

3 minute safety stop at 15fsw (1.45ATA)
3 minutes x 1cu' = 3cu' x 1.45ATA = 4.36cu'

Ascent from 15fsw to surface
Avg depth is 7.5fsw (1.23ATA)
.5 minute x 1cu' = .5cu' x 1.23ATA = .61cu'

5cu' + 8.8cu' + 3cu' + 3.8cu' + 4.36cu' + .61cu' = 25.6cu'

So, from 132fsw, with proper ascent rate and stops, you need a BARE MINIMUM of 25.6cu' which means your tank will be DRY (i.e. ZERO psi) when you reach the surface.

A 30cu' tank will give you the needed 25.6cu' plus 4.4cu' reserve IF it's full when you start using it.
A 40cu' tank will give you the needed 25.6cu' plus 14.4cu' reserve IF it's full when you start using it.

Sounder · Post by **Sounder** » Wed Feb 13, 2008 9:03 am

Take this a step further... let's look at how much gas you should have in your tank for your buddy and you to make it to the surface while sharing air...

25.6cu' for you + 25.6cu' for your buddy (assuming the same stressed and working RMV) = 51.2cu' needed.

In an aluminum 80 (remember an Al80 only has 77cu' of gas in it) 39psi = 1cu' of gas.

39psi for every 1cu' of gas
51.2cu' of gas needed
39 x 51.2 = 1996.8psi (we'll call it 2000psi to keep it easy to remember) to get you and your buddy to the surface with NOTHING left in the tank.

So, you need 2000psi in an Al80 to get you and your buddy to the surface...
You need some reserve in the tank to inflate your BC and have gas left on the surface... let's use 300psi as the reserve pressure realizing that 500psi is the recommended reserve by training agencies.

2000psi + 300psi (7.7cu') = 2300psi

Thus, 2300psi is the absolute lowest pressure you can safely start your ascent from 130fsw on an Al80 tank.

This means that if you're diving with a buddy who is wearing an Al80 tank, and you're diving to 130fsw, if you're still at that depth when your buddy's pressure dips below 2300psi, he/she does NOT have enough gas to get the two of you back to the surface safely and appropriately.

Sounder · Post by **Sounder** » Wed Feb 13, 2008 9:08 am

For a HP100 tank, figuring the same volume of gas (51.2cu' + 7.7cu' reserve = about 59cu'), the minimum pressure you need in your tank for a safe ascent from 130fsw is about 2030psi.

I highly recommend everyone attend the FREE gas management seminars offered by Grateful Diver and BDub. The math isn't difficult, and it will change the way you look at diving.

kwbyron · Post by **kwbyron** » Wed Feb 13, 2008 9:19 am

Penopolypants wrote:
cardiver wrote:Crosswords and gas calculations in bed?

Huh, yeah, with all that excitement going on, how did they ever find time to make a baby?

I can see how 3 down can lead to an 8 up resulting in 4 across.

jcander · Post by **jcander** » Wed Feb 13, 2008 2:33 pm

Are they ever offered in Portland? If not, when are my options for a Seattle trip over the next month or so?

Thanks!

Penopolypants · Post by **Penopolypants** » Wed Feb 13, 2008 2:54 pm

kwbyron wrote:
Penopolypants wrote:
cardiver wrote:Crosswords and gas calculations in bed?

Huh, yeah, with all that excitement going on, how did they ever find time to make a baby?

I can see how 3 down can lead to an 8 up resulting in 4 across.

Sounder · Post by **Sounder** » Wed Feb 13, 2008 7:57 pm

jcander wrote:Are they ever offered in Portland? If not, when are my options for a Seattle trip over the next month or so?

Thanks!

I imagine that Bob would offer one if there was enough interest and a dive shop willing to host it. More likely though is having them offered in Federal Way (at Bob's house) or local Puget Sound dive shops.

There are lots of places to stay up here - make a weekend of it and come diving!! If you must spring for a hotel or make the late drive home, it's still WELL worth it. The key is practicing it after you learn it, and asking questions on things you didn't quite catch the first time. Many people attend the courses several times to ensure they catch everything, and Grateful Diver sends you home with notes and reading material... I think BDub does too, but I haven't been to one of his. BDub and Grateful Diver worked on the presentation together and both give a great presentation... oh yeah, and you might even laugh and have fun too!!

In the meantime, look at how I calculated it (follow my format) and you can figure out what the minimum amount of gas you need is (or how much your buddy needs for YOU!) in any situation. Just remember to work in cu' of gas and not psi because the amount of gas for each psi in a tank varies with the size of the tank. Figure a stressed or working RMV (respiratory minute volume - how much gas you consume) for 1cu'/minute... and remember that this means 2cu' per 1 minute for you + buddy.

diver-dad · Post by **diver-dad** » Thu Feb 14, 2008 2:08 pm

Sounder -

Perfect!!! ... Exactly what I was looking for.

Thanks a bunch!

Northwest Dive Club

Calc Pony Size?

Calc Pony Size?

Gas Management Seminar

Re: Gas Management Seminar