PfcAJ
Contributor
Also consider this. Do you wanna be farther from the entrance of the (cave, wreck) with less gas?
How would my air consumption change on double 50s vs 85s?
- Thread starter CaveSloth
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OP
CaveSloth
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yikes! Hoover alert!
Why do you think it's so high? Have you ever done a SAC check while sitting on the couch to get a base line? That seems really high
OP
CaveSloth
Contributor
No. I have only calculated SAC & RMV on actual dives. My average SAC is 15.7.
I can tell you that my breathing is much improved from when I first started diving, if you can believe that.
It is probably so high because I am overweight. I am 5' 11" 240lbs.
?? A SAC of 15.7 psi/min would correspond to an RMV of about 0.41 cuft/min.
Which of your numbers is wrong?
ADDED: And that's the problem with SAC; I did not take into account he was diving double LP85s and had wrongly assumed a single AL80. His RMV based on 15.7 psi/min for his tanks would be 0.96 cuft/min.
100days-a-year
Contributor
Dive shop guy sounds like a real clown. No way anyone will get any significant improvement with such an insignificant change in surface area. Conditioning and other anatomical factors will play a much bigger role.
I’m 5’10 225-230 and my sac rate is just barely more than my 5’3 105lb wife. There are a ton of factors that go into it. Tanks are just a small portion. Comfort in the water is number one. Streamlining gear is another. By far and large being comfortable In the water is the key. That only comes with experience and time.
OP
CaveSloth
Contributor
Here are some numbers from a recent dive:
Duration: 29 minutes
Average Depth: 34 ft
Starting PSI: 2960
Ending PSI: 2058
Calculated SAC: 16 (ROUNDUP(((2960-2058)/29)/(34/33+1)))
Let's hope I calculated SAC correctly. My maximum calculated SAC is 19. Dual Faber 85s contain 162 CF of air at their service pressure of 2640 (according to @tmassey data). That means that 1 CF of air = 16.3 PSI. Since my maximum SAC is 19, that is over 1 CF of air per minute.
Did I calculate something incorrectly?
drk5036
Contributor
This is why using units when doing math helps a lot, it's hard to find the errors without them.
First think though, is we can do a mental check to see if this number makes sense. If you're breathing down 1 CF of air per minute, that would mean that with a standard AL80, on a OW dive to 60ish feet, you would completely drain the tank after about 25 minutes. Does that seem to match your experience? If not, there's a mistake somewhere.
Your calculation looks fine. You used a little less than 1/3 of your air (i.e. you used 900 out of 2960...about 30%) and you started with about 180 cu ft. So you used about 54 cu ft of air in 29 minutes.
You were at a depth of 34 ft, which is essentially 1 atm (I'm assuming you're in salt water.) So the total pressure was 2 atm.
Which means if you were at the surface you would have used only half as much air, or 27 cu ft... in 29 minutes. Which is right about 1 cu ft per min.
That's a rough calculation, but it should be pretty close. Hold on, I'll crunch the actual number... I got 0.94 cu ft per min
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