How Much Bottom Time?

[WaterFox]

Just finished my open water class and ready to make dive #5 with group of friends.

I used the AL63 size tank for the class. I'm trying to decide whether to rent an AL63 or AL80 for dive #5. I guess what I'm really asking is how long can I expect my bottom time to be at around 30'-40' depth with an AL63 tank. From my open water class dives my SAC rate is about .75 cf/min (is this good? can I expect to improve this number with more diving experience?). I'm at a loss as to how to do the calculation to see given these numbers how long I can expect my air supply to last at about 35'... Just want to know if I need to rent the larger tank. I don't want to make my dive buddies angry when they have to cut their dives way short because newbie diver has sucked tank almost dry.

Also, just out of curiosity, how long can the more experienced divers last on 63's?

 

[diver 85]

Seriously, you go with the 80, never can have too much air(a joke from here on---> & get your buddies to go with the 63, that way you won't make them 'angry'..lol)

Too many variables to actually figure out how much time a 63 will last(in minutes) ie water temp that day(will you suck the fool out of the regulator when you hit 70 degree water for example), currents that day, your 'fear factor' for the 1st dive etc etc etc...I would guess around 20+ minutes for your 1st dive????....---it's very easy to waste alot of air UW....

Would be interesting for you to report back your F/M for that dive.......good luck....

 

[The Kraken]

The bottom times stated are approximate; bottom times give include a residual 500 psi in the tank at the end of the dive, bottom times are based on square dive profiles:

Given: SAC rate = 0.75 cfm
Depth: 40'

Bottom Time:
63 cf = 31 minutes
80 cf = 36 minutes

Hope this helps,

Safe dives . . . . . .
. . . safer ascents !

the K

 

[TSandM]

The Kraken has given you the answers, but I'll take you through the process so you can do it for yourself the next time.

Your SAC rate is .75 cft/min; that's at 1 ATA.

35 feet is roughly 2 ATA (a little more, but nothing in diving needs to be that precise), so you'll use twice as much, or 1.5 cft/min.

An Al80 holds 77 cft at 3000 psi; its "tank factor" (the volume of gas represented by 100 psi) is 2.5. Rock bottom (safety reserve) at 35 feet is 500 psi, so you have 2500 psi usable, or 62.5 cft.

40 minutes at 1.5 cft/min is 60 cft, so you'll have roughly 40 minutes at 35 feet.

Knowing the tank factor for the tanks you use makes these calculations something you can do in your head, which can come in handy when you're trying to figure out if you have enough gas to make the dive you contemplate and maintain reserves, and if you're trying to figure out how much bottom time to expect, as in this case.

 

[WaterFox]

Given: SAC rate = 0.75 cfm
Depth: 40'

Bottom Time:
63 cf = 31 minutes
80 cf = 36 minutes

I'm short at 5'4" and I actually prefer the size of the 63's. I'm only gaining 5 minutes by going to 80 cf. Ok, well actually we plan to surface at 1000 psi. But not really gaining much time with my current SAC. hmmm... 5 extra mins or increased comfort level... seems like which tank to rent is a no brainer to me (perhaps my buddy will not think so!).

Also, still curious if SAC = .75 is a decent number and if I can expect to improve with more experience. How long do more experienced divers last with 63's and better SAC's than mine?

 

[The Kraken]

You may be one of those divers who might consider the purchase of HP 80's.

The tanks are about the size of an aluminum 63, hold a true 80 cf of gas, and are negatively buoyant when dived down to 500 psi.

Just a thought.

the K

 

[scarefaceDM]

Not to forget..your SAC is higher compared to the experienced diver. Also depends on the conditions and your buddies. AND on your mental state. Meaning how nervous or excited, how hard did you fin kick etc...

SO air on caution. Once you get more dives under your belt...your SAC rate will be much better. And those calculations will be very usefull in planning your dives.

Important thing is...relax and have fun.

 

[WaterFox]

You may be one of those divers who might consider the purchase of HP 80's.

The tanks are about the size of an aluminum 63, hold a true 80 cf of gas, and are negatively buoyant when dived down to 500 psi.

Yeah, I will definitely consider those when the time comes to purchase my scuba equipment. My LDS only offers aluminum rental tanks. I have to do some research and find out where I can rent an HP80. I want to figure out which setup works best for me before I invest in a full set of gear.

Thanks for your help!

 

[The Kraken]

Oh, by the way, that HP 80 will give you a 41 minute dive, given the same considerations as those above.

the K

 

[TSandM]

Your gas consumption will likely decrease with time, as you are more relaxed with your diving. Getting properly weighted, and distributing your weight so that you can effortlessly remain horizontal in the water, will also help reduce it.

I'm surprised that you prefer the 63s. I'm 5'4" and I like Al80s (but even more, I like LP95s or HP100s ). At your size, I would expect you to see some significant reductions in your SAC as you get more efficient and more relaxed.