My 1st Diving Problem Ever!

[Meng_Tze]

Im going to share with you guys my dive plan for today, 60 foot dive with buddie.
I do not know my SAC rate so we are going to use a base .75 sac, Im going to start figuring it out right away here. anyways here I go.
60/33+1 = 2.82 for ata
75sac X 2.82 ata (Bar) = 2.12 x 35 = 74.2 Cubic Feet Used at 60 feet.

100 Divided by 3442 = 0.29 x 3450 = 100 cubic feet in tank.

Accent to S stop
15/33+1 = 1.45
.75sac x 1.45 = 1.11 x 10 = 11.1 Cubic feet at the safty stop for 10 mins.

Total of 85.3 cubic feet used
Totaly of 74.2 cubic feet used at 60 feet
Total of 11.1 cubic feet used at 15 feet used.
Tell me how I did?
Chris

You did well to start off. But you have calculated only bottom part of gas and your stop at 15ft. What about your time between bottom and 15ft while you ascend...... and what about your buddy? How much gas do you need to carry for both of you to make it to the surface safely? Also are you sure that .75 is the right number for you? I don't know where you are diving, but if it is cold, current , or you see an increased work load, 0.75 sac may not be enough....


Calculate what you need to get both yourself and buddy top side (RB)
subtract that from gas in tank (available gas -RB=usable gas).
Calculate gas requirements at bottom, during ascend and during hang.(gas needed)

If gas needed <= usable gas, then you are good.
If gas needed >= shorten bottom time or increase gas volume and recalculate.


make sense?

 

[LG Diver]

Sometimes it helps to use a predefined sequence and template to work through this methodically. I like this one:

http://www.5thd-x.com/xducation/pdf/Diveplanningworkbook.pdf

 

[HybridDiver]

Hey guys, Well i thought to do a 10 min Safety stop reason being if I had extra gas why not right. The water is going to be about a big 45F which is not to bad for this time of year. Anyways, I will re do my gas planning and get back to you guys. Im off to a better start for planning though I think.
Chris

 

[TSandM]

Yes, you are. At least you are thinking about the important stuff, like whether you have enough gas to do the dive you're contemplating.

When you do this dive, keep track of your starting and ending pressures, and check your computer for your average depth. That will allow you to calculate your OWN sac rate, so you can do this planning more accurately.

 

[Charlie99]

Yes, you are. At least you are thinking about the important stuff, like whether you have enough gas to do the dive you're contemplating.

Even more important is the gas to do the ascent at the end of the dive.

Chris -- the basic rule is that you should have enough in your tank to get both you and your buddy safely to the surface at any time. Obviously, he should always have enough in his tank to get both you and him to the surface also. This way, if a reg ices up and has an uncontrollable freeflow, you can still safely end the dive while sharing air.

There have been multiple threads on "rock bottom". Different people make different assumptions, but typically one plans on having enough gas to support two divers breathing at an elevated rate of 1cfm each. Let's assume that you might take 1 minute at depth to sort stuff out, then do 30fpm ascent, and a 3 minute stop at 15' (Or 1 minute at 30', 1 at 20', 1 at 10' -- that comes out pretty much the same).

For a 60' dive that means:
1min at 60'
2 minutes at average depth of 30' for the ascent
3 minutes at average depth 15' for the stops.

1 min * 2cfm * 2.8ata = 5.6cf
2min * 2ata * 2ata = 8cf
3 min * 1.5ata * 2ata = 9cf
for total of 23cf rock bottom reserve.

Convert that to whatever psi that is in the particular tanks you and your buddy are using and you will know your "rock bottom" ascent pressure. The above calculations assume that you can go directly to the surface at any point in your dive.

 

[amascuba]

Im going to share with you guys my dive plan for today, 60 foot dive with buddie.
I do not know my SAC rate so we are going to use a base .75 sac, Im going to start figuring it out right away here. anyways here I go.
60/33+1 = 2.82 for ata
75sac X 2.82 ata (Bar) = 2.12 x 35 = 74.2 Cubic Feet Used at 60 feet.

100 Divided by 3442 = 0.29 x 3450 = 100 cubic feet in tank.

Accent to S stop
15/33+1 = 1.45
.75sac x 1.45 = 1.11 x 10 = 11.1 Cubic feet at the saftry stop for 10 mins.

Total of 85.3 cubic feet used
Totaly of 74.2 cubic feet used at 60 feet
Total of 11.1 cubic feet used at 15 feet used.
Tell me how I did?
Chris

Good start. I would have planned it as a minimum deco dive.

Something like this:

60/2.81 x .75 x 35 = 73.76 (74)
**40ft would be the average depth for the ascent**
40/2.21 x .75 x 1 = 1.65 (2)
30/1.90 x .75 x 1 = 1.42 (2)
20/1.60 x .75 x 1 = 1.20 (2)
10/1.30 x .75 x 1 = 0.97 (1)

Estimated Gas Usage in ft3:

74 + 2 + 2 + 2 + 1 = 81ft3

Estimated Gas Usage in PSI:

100/3442 = .029ft3

81/.029 = 2793psi (2800psi)

Minimum Gas Calc:

60/2.81 x 2 x 1 = 5.62 (6)
**40ft would be the average depth for the ascent**
40/2.21 x 2 x 1 = 4.42 (5)
30/1.90 x 2 x 1 = 3.8 (4)
20/1.60 x 2 x 1 = 3.2 (4)
10/1.30 x 2 x 1 = 2.6 (3)

Minimum Gas Reserve in ft3:

6 + 5 + 4 + 4 + 3 = 22ft3

Minimum Gas Reserve in PSI:

22/.029 = 758psi (800psi)

How to Calculate your SAC rate:

I like to break the SAC calculation down into five parts.

1. PSI Used / Elapsed Time = PSI/min.@Depth
2. DepthinFeet / 33 + 1 = DepthinATA
3. PSI/min.@Depth / DepthinATA = PSI/min.@Surface
4. TankVolume / TankRatedPressure = TF(ft3)
5. PSI/min@Surface x TF(ft3) = SRC(ft3/min.)

For Example:

A diver has an AL80 filled to 3000psi. He at dives to 100ft. for 8 minutes and consumed 700psi. What is his SAC?

1. 700 / 8 = 87.5psi/min@depth (round up to 88psi/min@depth)
2. 100 / 33 + 1 = 4.03ATA
3. 88 / 4.03 = 21.84psi/min.@surface (round up to 22psi/min@surface)
4. 80 / 3000 = .026ft3
5. 22 x .026 = .57ft3/min.

In this case the SRC = .57ft3/min.

 

[HybridDiver]

Thanks guys, This is wicked. Anyways done the dive went really good. Buddie was lacking bouyancy so we stayed at 60 and just practice bouyancy and trim, We did good. Reg preformed alot ALOT better when the breathing ajestedment all the way in, Im happy this dive went better. Diving tomorrow afternon, Im going to do a basic 30 foot dive by myself to practice bouyancy tomorrow, and practice gas planning.
Chris

 

[HybridDiver]

Well good news, I found a buddie for tomorrow. So no diving solo, which is wicked. Feel much safer, Thanks for all the time you have taken out to help me with gas planning.
Cheers

 

[amascuba]

Read up on Lamont's Rock Bottom reference guide:

http://www.scriptkiddie.org/diving/rockbottom.html

And definitely hit up NWGratefulDiver on his gas planning guide. Both of those should help out allot.

 

[HybridDiver]

I hit NWgratefuldiver for the planning guide and man oh man that thing is wicked. Learned alot off that.
Chris