I think I'm going to cry...

[xvwesvx]

I am taking a scuba class right now and its amazing...

But...

I don't understand at all how you firgure out how much air you would need to do a dive.

All I know is that my SACR is 0.4 cu ft/min
and that you find the ATM by (Depth/33)+1

I know if I did a dive at 106ft the ATM would be 4.22.

So do I just multiply 4.22 by .4 to get amout of air I am using a min at 106 ft? Then take that number and multiply it by number of mins my bottom time is? But, then do I have do this all over again at 15ft for my safety stop for 3mins and add it together?

Please help me...

 

[Ironman]

So do I just multiply 4.22 by .4 to get amout of air I am using a min at 106 ft? Then take that number and multiply it by number of mins my bottom time is? But, then do I have do this all over again at 15ft for my safety stop for 3mins and add it together?

Yes. Factor in descent time, and ascent time air consumption as well. Add the descent time to your bottom time, at your bottom air consumption rate, for simplicity. Set it up as a spreadsheet, with coulmns for Depth, Time, Total time, SACR, Converison factor (e.g. the 4.22 for 106 ft), Volume Consumed. It seems like more work than necessary, but it gives your practice in planning.

 

[don Francisco]

Nothing to cry about, you had it. Ironman gave you a simple way to include the ascent & descent consumption. Alternatively you can calculate a constant to include ascent, safety stop & reserve & use it for future calculations.

I suggest that you ask this question in class. Odds are that you're not the only one who'd like this explained better. When I teach, my favorite student is the one with all the questions. He breaks up the monolog, gives me feedback on whether the material is understood, and helps the others who like yourself aren't getting it, but are too shy or self conscious to let me know. Good questions help flesh out the subject, stimulate discussion and make the class more interesting for everybody. This is your class, so make sure that you get the most out of it by speaking up when your lost.

 

[spectrum]

xvwesvx

You have it!

Average the distance between surface to depth for descent and depth to 15 feet for ascent and decide on a rate. You can also factor in the 15 to surface leg.

If it's a straight drop from a boat then the times can be easy to come up with. Ascent will be no faster than 30FPM so coming from 45 to 15 requires 1 minute minimum. Unless fussy ears are involved you can descend as fast ot much faster so using the same time is a good bet for planning.

You might want to use an even slower rate from the stop to the surface but for a test use what you have been taught. This is not a huge planning item since in <15 feet your actual consumption will be slight. Understanding how hast you will consume at depth is the big part of the lesson.

What works well is to sketch or chart the dive. Create each portion, descent bottom time, ascent safety stop & final ascent. Assign depths & times, calculate ATA then consumption, add them up and then if you know the cylinder involved you can determine how many PSI you will have left and if that represents a good safety margin.

BTW don't cry, it sounds like you are getting a more comprehensive program than many.

Pete

 

[ItsBruce]

If you search for threads referring to "Rock Bottom Time," you will find some very fascinating discussions of the subject of calculating air usage.

The basic concept is that to be safe, you need to head for the surface when your remaining air pressure drops to a certain point. The 500 pounds figure often taught in OW is not necessarily it. RBT takes into account not only the time it takes to safely surface and the air you use during that time, but the fact that your buddy may have had an equipment failure and may need to be sharing your air.

Working backward from this, your computation for possible maximum dive time should include (1) descent time, (2) bottom time, (3) ascent time, (4) safety stop time, (5) a safety margin (because if something goes wrong, it may take a minute or two to get things straightened out before heading for the surface), and (6) ascent time for your buddy (who may be sharing your air).

 

[ronbeau]

I am taking a scuba class right now and its amazing...

But...

I don't understand at all how you figure out how much air you would need to do a dive.

All I know is that my SACR is 0.4 cu ft/min
and that you find the ATM by (Depth/33)+1

I know if I did a dive at 106ft the ATM would be 4.22.

So do I just multiply 4.22 by .4 to get amount of air I am using a min at 106 ft? Then take that number and multiply it by number of mins my bottom time is? But, then do I have do this all over again at 15ft for my safety stop for 3mins and add it together?

Please help me...

This "Rock Bottom" paper might be a little overkill for an Open Water class. Since another member mentioned "Rock Bottom" gas planning Bob ("NWGratefullDiver" on this forum) has produced an excellent paper that explains this subject in simple easy to understand terms.

NAUI should really talk to Bob about incorporating his paper into the NAUI training manuals for "NAUI Advanced Open Water" and/or the "NAUI Master Diver" manual.

 

[xvwesvx]

Ok, first I will find the ATM for 106ft
((106/33)+1)= 4.22

And I want my bottom time to be 20mins and my SACR is 0.4 cu ft/min. So I multiply 0.4 by the ATM(4.22) Which give me the rate I use my air at 106ft, which is 1.69 cu ft/min So, then I multiply my bottom time to my SACR for 106ft, and get 33.8 cu ft

Then I do the same thing all over again for my safety stop of 15ft for 3mins
((15/33)+1)= 1.46 ATM
.4 x 1.46= 0.59 SACR for 15ft
.59 x 3 = 1.78 cu ft

Air used for dive 33.8 cu ft
Air used for safety stop 1.78 cu ft

Total air used 35.58 cu ft

And, if I was using a AL80(actual capacity 77.9 cu ft)
77.9(tank size) - 35.58(air used) = 42.32 cu ft(air left)
42.32 / 77.9 = 54% of the air left in the tank
3000psi x 54% = 1620 psi
3000psi-1620psi = 1380 psi left



I hope this is right. By the way I always rounded my numbers up, I feel its better to say I use more air than, less air. Also, after looking this over. I don't have the air I would use for my ascent. Which would be another 8mins under, if at 106 ft. Like would what i pick for my depth?

Thanks so much!

-Wes

 

[bleeb]

Ok, first I will find the ATM for 106ft
((106/33)+1)= 4.22

And I want my bottom time to be 20mins and my SACR is 0.4 cu ft/min. So I multiply 0.4 by the ATM(4.22) Which give me the rate I use my air at 106ft, which is 1.69 cu ft/min So, then I multiply my bottom time to my SACR for 106ft, and get 33.8 cu ft

Then I do the same thing all over again for my safety stop of 15ft for 3mins
((15/33)+1)= 1.46 ATM
.4 x 1.46= 0.59 SACR for 15ft
.59 x 3 = 1.78 cu ft

Air used for dive 33.8 cu ft
Air used for safety stop 1.78 cu ft

Total air used 35.58 cu ft

And, if I was using a AL80(actual capacity 77.9 cu ft)
77.9(tank size) - 35.58(air used) = 42.32 cu ft(air left)
42.32 / 77.9 = 54% of the air left in the tank
3000psi x 54% = 1620 psi
3000psi-1620psi = 1380 psi left

Looks about right. I didn't check that you operated the calculator correctly though.

Here are another couple of concepts that you may want to consider, if you haven't been introduced to them already. Hopefully, they won't overload you, which is why I said "may".

What you've calculated is nominal consumption. For emergency situations, it's common to assume a SACR of 1 cfm (or more if you know you're an air hog when working hard). Also, as other posters have touched on, for planning RBT, you may also want to multiply total air consumption by 2, to take into account the fact that your buddy may be breathing from the same tank in an emergency.

I hope this is right. By the way I always rounded my numbers up, I feel its better to say I use more air than, less air. Also, after looking this over. I don't have the air I would use for my ascent. Which would be another 8mins under, if at 106 ft. Like would what i pick for my depth?

For the ascent, you can use the depth half way between the lower and upper depths, i.e. 60.5 ft, in your example. One thing I'm a bit surprised at: 8 min from 106 to 15 ft seems a bit slow (11 ft/min). This would increase gas requirements and incur extra nitrogen loading. Any particular reason for this ascent rate?

 

[SelkieDVM]

I just wanted to say congratulations on taking these considerations into your dive planning. Unfortunately most recreational divers never take it this far.

Cheers,
L

 

[xvwesvx]

Looks about right. I didn't check that you operated the calculator correctly though.

Here are another couple of concepts that you may want to consider, if you haven't been introduced to them already. Hopefully, they won't overload you, which is why I said "may".

What you've calculated is nominal consumption. For emergency situations, it's common to assume a SACR of 1 cfm (or more if you know you're an air hog when working hard). Also, as other posters have touched on, for planning RBT, you may also want to multiply total air consumption by 2, to take into account the fact that your buddy may be breathing from the same tank in an emergency.



For the ascent, you can use the depth half way between the lower and upper depths, i.e. 60.5 ft, in your example. One thing I'm a bit surprised at: 8 min from 106 to 15 ft seems a bit slow (11 ft/min). This would increase gas requirements and incur extra nitrogen loading. Any particular reason for this ascent rate?

Its was my mistake. I screwed up my math.
I want my ascent to be 30ft per min

106-15=91ft
91ft/30ft= 3.1mins

to find the ata
91/2=45.5ft
((45.5ft/33)+1)= 2.38
My SARC for my ascent
2.38 x 0.4= 0.96
The amount of air I will use
.96 x 3.1mins = 3 cu ft