Bail Out Bottle Size - Advance Open Water

[peterbj7]

I first came across this issue when I was in Egypt preparing for a technical dive. In trying to plan my various gases I realised that some of my tanks were marked the American way, and I didn't know what their nominal pressures were. I therefore couldn't complete my calculations. The whole point IS trying to glean useful information.

I don't know what "Technically, the American system represents how much gas will come out if emptied at 1atm" means. Insofar as I do understand it it isn't much help to me, as I'm interested in knowing how much gas is in the tank at various intermediate points. To put the issue another way, if you take a tank marked "80 cu. ft." and fill it to 3000psi, how much gas will it contain?

 

[Blackwood]

Insofar as I do understand it it isn't much help to me, as I'm interested in knowing how much gas is in the tank at various intermediate points. To put the issue another way, if you take a tank marked "80 cu. ft." and fill it to 3000psi, how much gas will it contain?

As I'm sure you are aware, cylinders under DOT regulations are stamped with their full fill pressure.

example:

A prudent diver may elect to divide one number by the other to arrive at volume (CF) per unit pressure (PSI). This is directly analogous to L/bar.

The suggested implication in your post is that it's easier to multiply BAR by L/BAR (e.g. 230BAR * 12L/BAR) than it is to multiply PSI by CF/PSI (e.g. 3000PSI * 100CF/3442PSI). As far as I'm concerned, it's hardly a big deal. It would be nice if there was a world standard, but until that day, I'll remain flexible and look for the similarities in the systems (i.e. the basic physical principles) rather than focus on how to use one system at the expense of the other.



When it comes to actually diving, I work in pressure units since that's what my gauge reads.

 

[peterbj7]

I used to own a large number of cylinders of different sizes and materials from several manufacturers. I found, from those manufacturers, that there were different ways of determining the nominal pressure. It isn't necessarily the maximum safe fill pressure. And in any case, given that the tank probably isn't going to be filled to whatever pressure is stamped on it, how easily can you tell how much gas is inside it?

I think you'd find it very difficult and probably very error-prone trying to do a complete gas consumption forecast as part of a dive plan using only pressure units. Bear in mind you're in general using different tanks which may well be of different sizes and manufacturers. Do you have much experience in constructing gas forecast plans?

 

[Blackwood]

I used to own a large number of cylinders of different sizes and materials from several manufacturers. I found, from those manufacturers, that there were different ways of determining the nominal pressure.

How they determine it shouldn't really matter, right? Sure, there are aluminum cylinders that fill to 3000, 3300, steels that go to 2400, 2640, 3000, 3442, 3500, etc.. The end result is stamped on the cylinder (assuming it complies with DOT regulations).

I think you'd find it very difficult and probably very error-prone trying to do a complete gas consumption forecast as part of a dive plan using only pressure units. Bear in mind you're in general using different tanks which may well be of different sizes and manufacturers. Do you have much experience in constructing gas forecast plans?

I plan my dives in volumetric units. I execute my dives in pressure units (which is what meant by "When it comes to actually diving," which I may have edited in after you started replying).

 

[GrumpyOldGuy]

I am not sure why the OP wants Advanced Open Water Divers - this certification by itself (not knowing the instructor nor experience level) is next to useless in determining the validity of a user.

I am also not why why someone asking a question like this would advertise their bias towards the smallest solutions. It would be better to ask without adding the smile comments. You will just scare off someone using these because they don't want to start WW III on SB with another SA thread.

But to answer the questions,

1) my typical depths range from 60 to 115ft.

2) my pony size selection (6,13 or 30) is based on my depth. I typically don't bother with one in moderate depths IF one of my regular buds is with me. Solo or instant buddy does require one. I feel very comfortable with the 6 in known locations at 75' or less. The 13 will do most dives for me. The 30' at this point is overkill except for the 100+ deep dives.

3) My SAC is around .4

4) No real use. I drill 10x a year from 75'

 

[Fish in a Barrel]

Someone may want to check my math on this, but you should be able to directly compute your volumetric air consumption during ascent.

Let
S = SACR in ft^3/min
D = max depth in ft
A = ascent rate in ft/min
C = total air consumed in ft^3

C = S * (D/A) * (1/2 * D/33 + 1)

Or if we switch to liters and meters for you metric folks:

C = S * (D/A) * (1/2 * D/10 + 1)

So for a diver who breaths at 0.5 ft^3/min, an ascent from 66 ft at 30 ft/min should burn 2.2 ft^3 of air on a normal ascent. From 130 ft the same diver would burn 6.4 ft^3.

Does that seem right?

 

[DaleC]

Peter, you're losing me here. Considering this is basic scuba and one is not planning technical dives the calculations for gas planning are pretty rudimentary. All I need to know is the actual volume of the tank at service pressure and the actual pressure it is filled at.

To calculate reserve volume by psi: (service pressure) / (volume at service pressure) x (desired volume for reserve).

If I know my desired reserve volume is (for example) 25 cuft...

and I use an Al80 that actually holds 77cuft. at 3000psi...

the calculation is: 3000/77x25=974.02psi, rounded up for practical application to 1000psi. At 1000psi I have 25cuft. in reserve.


To calculate a fill at other than stated service fill pressure: (volume at service pressure) / (service pressure) x (actual pressure).

In this case, an Al80 with a volume of 77cuft. at a service pressure of 3000psi is only filled to 2800psi. 77/3000x2800=71.86cuft., rounded down for practical application to 71cuft.


The reserve pressure as in the initial example would be the same (ie 1000psi for the Al80).

What else does one need?


As to the OP's question: I have used 19, 30, 40, 80 cuft. bottles and doubles. The choice has to do both with needed reserve volumes and practicality. If one plans to progress further into technical diving a 40cuft. cylinder, while perhaps overkill in some recreational dives, may save a repurchase of a stage bottle later on down the road. If one is traveling, a 6cuft or 13cuft. bottle might be a good compromise between gas requirements and luggage restraints.

Also, your SAC is not a total volume, it's a rate. You use your SAC, adjusted for depth, to calculate your neccisary reserve volume.

 

[peterbj7]

an Al80 that actually holds 77cuft. at 3000psi

How do you know that?

 

[Blackwood]

How do you know that?

Manufacturer data.

Really, does it matter? People don't plug into a flow meter to determine their consumption rate, they use cylinders, so errors in manufacturer ratings will be covered by experiment. Maybe my SAC rate is really .53, but if in practice it shows as .5, I can plan to .5, no?

Short of taking off the valve and filling the cylinder with water, the only way you can 'know' what a it holds is by trusting its maker (goes for any cylinder regardless of how it's rated). And tolerances suggest that if you took two 15L cylinders and filled them, they'll have different capacities.

Then there's the fallacy of a linear relationship between pressure of volume.

It doesn't matter because it's not that exacting of an application. At the end of the day, there's enough accuracy in doing some dives, noting pressure consumption, depth and time, and computing based on real cylinders.

 

[UaVaj]

Someone may want to check my math on this, but you should be able to directly compute your volumetric air consumption during ascent.

Let
S = SACR in ft^3/min
D = max depth in ft
A = ascent rate in ft/min
C = total air consumed in ft^3/min

C = S * (D/A) * (1/2 * D/33 + 1)

So for a diver who breaths at 0.5 ft^3/min, an ascent from 66 ft at 30 ft/min should burn 2.2 ft^3 of air on a normal ascent. From 130 ft the same diver would burn 6.4 ft^3.

Does that seem right?

The math looks correct however the formula is WRONG.
You formula is only good for constant depth air consumption where SAC rate is constant.
During ascend (or descend) - you need to account for the difference in SAC rate in relation to different depth. aka gas compression at depth.

Two way to figure this out is to either: (1) look at previous download computer dive log or (2) to note the pressure prior to asending and then to note the pressure at the surface. That difference in pressure in contrast to the tank size will equal AMOUNT of gas used for ascending (or descending).