Is my math right?

[skinerd]

Lets asume my SAC is 0.5cf/min. In order to determine my consumption at deapth, do I simply multiply that number by the number of ATA I am at? So @ 66ft would be 0.5 X 3 = 1.5 cf/min Or am I way off? Sorry if I totaly blew this, but I am trying to learn to better plan my turn pressures/rock bottom time.
Thanks

 

[gregor1234]

Yes, you have it exactly right

air consumption at depth = SAC X (1 + (salt water depth/33))
(the "1" for 1 ATM at zero depth)

1.5 = 0.5 X (1 + (66/33))

 

[The Kraken]

You're right on target, Skinerd.

But your SAC should be extrapolated from the average depth of the entire dive. The descent and ascent times, along with any safety stops or planned deco times will skew your SAC rate.

Another reason you should use the average depth of the dive is TO PLAN. Your SAC rate during the dive can change for a vast array of reasons, anxiety, stress, labor, excitement, depth, cold, etc.

So basically, a SAC rate is a historic datum that should be associated with a particular dive in order to plan gas management for a future dive of the same, or like characteristics.

the K

 

[Markdone]

Yes, SAC is surface, so you must multiply by 1 plus your average depth in ATM.

One way to simplify somewhat is to construct a profile - 10 minutes at 90 feet, then 15 minutes at 60 feet, etc.

Another good reason to get in a pool now and then. you can easily swim at a constant depth, and simplify your SAC calculation under different exertion levels.

As the K says very correctly, this baseline number is useful for planning, but you must always be aware these predictions are based on a number of constraints. For example, cold water will drive your air consumption up a lot, which you should note in your diving records to help refine your planning skills.

Here is a link to a little SAC calculator, remember your tank volume may not be equal to what it's called e.g. AL-80 is actually 77 cf.
SAC Calculator

 

[skinerd]

Ok so asuming that both my buddy and I have the same average SAC, an we are making a dive to 60Ft. In order for both of us to have enough air to ascened and have a 500psi reserve, I calculated that we would need 1157 to take 2.5min to ascend to 15ft with 2min saftey stop then surface. I came to that number by:


SAC: 0.5
ATA: 3

Time for ascent (TfA):
(ascent time + saftey stop) X 2
= (2.5+2)X2
=9min

Consumption at deapth (Cd):
SAC X ATA
=0.5 X 3
=1.5

Cuf used for ascent asuming breaths of 1.5 Cuf/min:
Cd X TfA
=1.5 X 9
=13.5

Tank vol - 13.5
=77 - 13.5
=63.5

Convert to a fraction of tank vol
63.5/80
=0.79375

0.79375 X (3000-500)
= 1984

There for, Rock Bottom Pressure is:
3000-1984
=1016

Ok phew, I think that is right... Is it? If you have any questions PM me plz!
Thanks

 

[The Kraken]

Well, let's see.

Descent = 0' to 60' @ 60'/min : average depth = 30' avg. for 1 min.
Dive time @ 60' = ?????
Ascent = 60' to 30' @ 60'/min : average depth = 45' avg. for 1/2 min.
Mid-depth off-gas stop @ 30' for 1 min. = 30' avg. for 1 min.
Ascent = 30' to 15' safety stop @ 30'/min = 22.5' avg. for 1/2 min.
3 min. Safety stop @ 15' = 15' avg. for 3 min.
15' to surface @ 30'/min. = 7.5' avg for 1/2 min.

(that's assuming that the depth/time relationships aren't logarithmic)

. . . the rest is yours!

the K

 

[skinerd]

K, I get what you are saying but how does that affect how I calculate my rock bottom pressure?
Andrew

 

[The Kraken]

One of the most important aspects of planning your "rock bottom" time is to adjust the amount of gas required in one tank to acommodate 2 divers having to make an ascent at the end of a planned dive with elevated SAC rates.

It takes X amount of gas to descend and complete your planned dive. Now, suppose Murphy comes along and applies his law and all of a sudden your dive buddy has some sort of catastrophic event occur that prohibits his ability to breathe from his regulator.

Now you have to be able to make a normal ascent and carry out any planned deco stops or safety stops in an elevated state of stress.

So figure out what you need at the end of your dive and add to that what you and your buddy would need in the case of an emergency.

I forget who posted it, but some time back there was an excellent post on "rock bottom" planning. I'll see if I can locate it for ya.

the K

 

[Charlie99]

Your math isn't quite right, in that the ascent and definitely the safety stop are not done at the max depth.

In addition, for emergency purposes you should assume something higher than your normal SAC.
-----------------

Here's a couple of calculations:

Normal air consumption, 1 diver, 0.5cfm SAC, 30fpm ascent from 60', 3 minute safety stop.

Ascent is 2 minutes (it doesn't matter that part is before, and part is after the safety stop0; average depth 30'. avg ata approx 2.
2ata * 2 minutes * 0.5cfm = 2 cu ft.

Safety stop depth of 15' is approx 1/2 atm, or 1.5ata.
3 min * 1.5 ata * 0.5 = 2.25 cu ft.

So normal ascent and stop at 0.5cfm is about 4.25 cu ft.

------------------------------------------

Now lets do you scenario. 2 people, 0.5cfm each, 2.5 minute ascent, 2 minute safety stop.

1cfm total. average depth for 2.5 minutes of 30', or about 2ata.
2*2.5*1=5cfm for ascent.

2 minutes at 1.5ata safety stop is 2 min * 1.5ata * 1cfm= 3cfm.

8cfm total. For an AL80 it is just under 40psi/cu ft, or about 320psi used. Add 500psi reserve and you get 820psi rock bottom using your asssumed 0.5cfm and no time fixing a problem at depth.

------------------
I don't agree with your assumptions however. I would assume a much higher consumption rate of 1 cfm per diver in an emergency, and assume that it takes a minute to sort out things on the bottom.

1 min x 2 divers x 1cfm/diver * about 3ata = 6 cu ft used before starting ascent.
Ascent from 60' at 30fpm is 2 minutes. 2 min x 2cfm x 2ata = 8 cu ft for ascent.
3 minute safety stop at 1.5ata is 3 * 1.5 * 2 = 9 cu ft.
6 + 8 + 9 = 23 cu ft. 23 cu ft x 40psi/cu ft = 920 psi + 500 reserve = 1420 rock bottom.

In reality, I wouldn't add in the safety stop ---- in real life, if we are so freaked out that we are both breathing at 1 cfm, then I'm heading on up to the surface rather than doing an optional safety stop. OTOH, if we aren't breathing at 1cfm each, then we'll have enough gas for the safety stop.

Dropping the 9cf requirement for safety stop just leaves 6 cu ft to get sorted out on the bottom, and 8 cu ft for the ascent. 14*40 = 560psi. Add in another 500psi for margin if you want, to get 1060psi.

 

[The Kraken]

WOW ! ! !
Sometimes I amaze myself . . .

Here ya go, Skinerd . . .
http://www.scubaboard.com/showpost.php?p=401869&postcount=65

the K