Air Consumption Tables.

[Frog Dude]

What I am trying to find out if anyone has taken the time to develop a correction factor to compensate for the difference in pressure drop over time with different cubic footage tanks.
All of the dive manuals I have reviewed equate air consumption to the reduction of tank pressure. If you only dove AL 80’s or any other standard size tank and expended the equal amount of energy on each dive at a fixed depth this would factor out to a reliable average air consumption to plan from.
However, if you rent tanks, sometimes you will get a 72, 80 or maybe a 120, I think there may be a few 50s still out there. The size being the cubic footage at full fill.
The actual air consumption is a volumetric reduction.
Reduction of the volume of air in the tank results in pressure drop.
2 duplicate dive profiles, one with 80CF and another with a 72CF will result in a different pressure drop in the same period of time. One breath consumes X volume, the X volume of each breath will increase as you go deeper. There is a factor to adjust for the depth. Basically, you will double your consumption every 33 feet. This is covered in the dive manuals with the assumption made that you ALWAYS use the same CF tank.
If you have established that your pressure will drop 100 PSIG every 3 minutes at 60 feet with a 80 CF tank and your next tank is a 72 CF, you will be in for a surprise when you check your SPG. Some of you know, but all of us NEED to know that pressure drop over time WILL change based on the cubic footage of the tank you are using.
If anyone has info or has even though about this please post a link or your comments.
Just for my own edification I will plug in some formulas in XCELL just to see how dramatic the change in pressure drop will be with different CF tanks at a constant volumetric withdraw.

Frog Dude

 

[Doc Harry]

It's a simple mathematical relationship, easily programmed into spread sheets.

First calculate the volume of air consumed. Then convert the volume to pressure using the size & pressure of the tank.

Example:

Diver has a SAC of 0.5 cubic feet per minute
Diver instantaneously descends to 75 feet (3.27 atmospheres) for 10 minutes
Diver has an AL80 (77 cubic feet) and starts with 3,000 PSI

Volume of gas consumed:
(0.5 cubic feet/minute) x (10 minutes) x (3.27 atmospheres) = 16.3 cubic feet of gas

Converting to PSI:
(16.3 cubic feet of gas) x (3,000 PSI/77 cubic feet) = 635 PSI

Conclusion:
Diver started with 3,000 PSI in an AL80
After 10 minutes at 75 feet, diver has (3,000 - 635) = 2,365 PSI remaining

Folks, did I do this correctly?

With a spread sheet you can get more detailed and sophisticated with summations of incremental calculations. I usually make incremental calculations every 10 feet or every minute for my gas planning and rock-bottom calculations.

 

[spectrum]

FrogDude,

While you post is mosty a question you show more insight into gas planning than many divers ever attain. To make your project more comprehensive, intersting and challenging remember that you have cylinders with starting pressures of 3442/3500, 3300, 30o0 & 2640 just to cover the heavy hitters. Diving with somebody using a cylinder of a different max pressure makes for interesting comparisons when evaluating air supplies mid dive. Make the table water proof!

Pete

 

[spectrum]

Folks, did I do this correctly?

Without pulling the calculator out the calculation looks correct.

 

[MikeFerrara]

What I am trying to find out if anyone has taken the time to develop a correction factor to compensate for the difference in pressure drop over time with different cubic footage tanks.
All of the dive manuals I have reviewed equate air consumption to the reduction of tank pressure. If you only dove AL 80’s or any other standard size tank and expended the equal amount of energy on each dive at a fixed depth this would factor out to a reliable average air consumption to plan from.
However, if you rent tanks, sometimes you will get a 72, 80 or maybe a 120, I think there may be a few 50s still out there. The size being the cubic footage at full fill.
The actual air consumption is a volumetric reduction.
Reduction of the volume of air in the tank results in pressure drop.
2 duplicate dive profiles, one with 80CF and another with a 72CF will result in a different pressure drop in the same period of time. One breath consumes X volume, the X volume of each breath will increase as you go deeper. There is a factor to adjust for the depth. Basically, you will double your consumption every 33 feet. This is covered in the dive manuals with the assumption made that you ALWAYS use the same CF tank.
If you have established that your pressure will drop 100 PSIG every 3 minutes at 60 feet with a 80 CF tank and your next tank is a 72 CF, you will be in for a surprise when you check your SPG. Some of you know, but all of us NEED to know that pressure drop over time WILL change based on the cubic footage of the tank you are using.
If anyone has info or has even though about this please post a link or your comments.
Just for my own edification I will plug in some formulas in XCELL just to see how dramatic the change in pressure drop will be with different CF tanks at a constant volumetric withdraw.

Frog Dude

This statement of yours from above is not correct.

There is a factor to adjust for the depth. Basically, you will double your consumption every 33 feet.

you do not double your consumption every 33 feet. From the surface to 33 feet it will double because the ambient pressure has doubled. However, from 33 feet to 66 feet you go from 2 atmospheres of pressure to 3 66-99 feet, you go from 3 atmospheres to 4. It only double over the first 33 ft.

You don't need a table. Do all your planning based on volume which simply relates to pressure. For instance a 80 cuft tank rated for 3000 psi yields about 2.5 cuft/100 psi (80/3000 (100). My double 104's is about 8 cuft/100 psi, my wifes HP 100's is about 6 cuft/100 psi. This is the key.


An example...
So if you have an RMV of 0.5 cuft/minute you'll use 100 psi from an al 80 in 5 minutes at the surface. How much gas will you use in 5 minutes at 99 ft? 2.5 cuft X 4 ATA = 10 cuft or 0.5 cuft/minute X 4 ATA X 5 minutes or just the 2.5 cuft used at the surface X 4 ATA.

What will the pressure change be in an al80? (10 cuft)/(2.5 cuft/100 psi) = 400 psi Or just the 100 psi used at the surface multiplied by the number of ATA.

Knowing that I'll use 10 cuft, I also know that the pressure in my double 104's would drop by about 125 psi.

With a little practice it's all doable in your head and on the fly before or during a dive and without a table. Some people write down the volume/pressure conversions for some common tank sizes in their wet notes for quick reference.

In most cases, however, knowing how long a dive will last isn't as important as knowing how to (when) to end a dive while you still have enough gas to get you and a buddy (who lost his gas) back to the surface.

 

[Frog Dude]

I was reviewing this post in my head this morning and realized the error, volume/ATA factor. For a quick calculation I use 2 times more air for the first ATA, 3 times more air for the second and so on.
Hey, it was 1 AM, after a 12 hour day and this thought popped into my head. I wanted to post a correction this morning before getting busted. I wasn’t quick enough.

Doc,
Your formula looks good. You start with a cubic volume consumption rate (CVCR). I wanted to start with the pressure drop method to establish a personal CV rate. Basically working your formula backwards.
I think the key to this computation is establishing your personal CVCR using the pressure drop method first. This will become the constant (K). Your personal K value with the CORRECT adjustments for depth (Kd) should remain fairly uniform. Adjustments can be factored in for exertion levels but that can come later.

Pete,
You hit the nail on the head.
I started with steel 72s when 2200 was a hot fill.
There are so many different size and pressure tanks out there, unless you lug your own around, you don’t know what size or pressure you will be able to rent.
That is what prompted this thread.
The only part of the gas issue we can depend on is our own K.
With this K value established and a simple formula we should be able to quickly determine what to expect from any size or pressure tank happens to be available.
It seems to me that this would be very good to know.

I will post what I come up with and subject myself to further slings and arrows.
Doc, Pete, thanks for reading the intent of this post. I may just PM you both the formula for review.

Frog Dude

 

[rjack321]

The advice to work in cubic feet per 100 psi is sound.

You need to make this simple enough to use it. An waterproof excel spreadsheet with a zillion numbers won't cut it.

SAC x time x ATA = cf used

cf used / cf per 100 psi = psi used in 100s.

cf per 100 psi is calculatable for any tank via:
cf / psi in 100s
(e.g. 77/30 = roughly 2.5 cf per 100 psi in an AL80
or 100/34 = 3 cf per 100 psi in an HP100)

Don't get hung up on the fourth decimal places, etc, not important. The key is to be able to use the formulas in the field. You can work all this out on the boat on your way to the dive site. I recommend wetnotes.

 

[fire_diver]

The thing to remember here is "simplicity." Don't make it more than it needs to be. Fir thing every diver should do is figure out thier ballpark SAC (Surface Air Consumption) rate. Your SAC won't be the same every dive, but you should know your general range. Then you can use that to plan your dives. As posted above, SAC x ata x time. Thats all you need. You should know the cubic feet your tank holds. Then it's sinple to figure out how much air should be used at any point of the dive. Of course this is based on basically flat dive profiles. If you are all over the place (like I am many times) you can't keep track as accurate.

FD

 

[Doc Harry]

You don't need a table. Do all your planning based on volume which simply relates to pressure. For instance a 80 cuft tank rated for 3000 psi yields about 2.5 cuft/100 psi (80/3000 (100). My double 104's is about 8 cuft/100 psi, my wifes HP 100's is about 6 cuft/100 psi. This is the key.

Cool man! Thanks!