how much air do you use?

[Don Burke]

nemo06489 once bubbled... Yes the tank i use is a AL80.

1400 psig off of 3000 in a 77 foot tank is about 36 cubic feet of air.

That's .61 cubic feet of air per minute at 35 feet for a surface air consumption of about .30 cubic feet per minute.

I'm guessing you were quite a bit shallower than you thought you were.

If you really averaged 10 feet, that would be .47 SAC, which would be very good.

If you really averaged 20 feet, that would be about .38 SAC, which is pretty phenomenal.

 

[erubic]

For example I did two dives today, the first to 90 feet and the second to 80 feet. I guestimated my air consumption so I ended each dive at 60 minutes with 500 psi of air. To me that makes an interesting game out of it- to see if I can accurately judge my air consumption over the course of the dives

drbill, i am a little confused here(new to diving) you dove to 90ft for 60 min and than 80ft for 60 min. I probably just don't understand correctly but these seem really long dives. Besides being decompression dives you are exceeding the limits by a lot. ex. 90ft is 25 min. max for a no decompression dive. Other than that I can´t believe that you could make a tank last that long at a 90ft dive without using nitrox or some other combination of gases. Also you would need a really long surface interval correct? please help me understand this!

eric

 

[DeepScuba]

erubic:

Assuming an ALU80 @ 90ft, his SAC would be around 0.30 (Consuming 2500PSI)

His plan is multi-level, which would make his SAC higher than 0.30, and no deco.

Just as an aside, using Nitrox doesn't change your SAC (Make your tank last longer), just your NDL, but I'm sure that is what you meant anyways.

60min @ 90ft would be one hairy deco time on air for a "rec" dive!

 

[DeepScuba]

AS far as people asking "How long does a tank last". I usually try to give them a good ball park when I know the dive they are about to do.

It helps in planning the dive, as most people haven't a clue how long, so me telling them they'll run out of air before they hit a NDL (Using Nitrox, is the one I'm thinking of right now) and then telling them they'll hit 500PSI in about 15 minutes (or whenever) seems to put it in perspective for them, instead of just jumping overboard, and figuring it out as they go.

You KNOW thats how a majority of divers do it.

I basically ball park a SAC, as most divers that don't know how to figure it out ALSO generally have SAC rates that I "pad" to about 1 CF/min/ATA. It makes the math easier and for cold, dark waters, it's probably pretty darn close.

 

[erubic]

Thanks for the answer deepscuba- - but I think I am more confused than before!!! lol
thats ok though, this is what I love about scuba- there is so much to learn and so much more to keep learning! anyway let me do a few searches to see if I can find out more about SAC than later in the day if I have anymore question I will post them!


thanks again!!

 

[torbaydiver]

nemo06489 once bubbled...
Hey every one. Ive asked a few things on this board and had some great replies. Id like to find out if my air usage is average or below average. My last dive averaged 35' for 59 minutes at a temp of 52. I used 1400 psi. My dive budy used 2300psi. I asked my dive shop how long a tank should last and he said thier are to many factors to guess at how long a tank should last.
So this is my question. If you dove a 35' for an hour how much air do you use.

On a 12 ltr tank, shallow shore dive 35ft I usually use about 2000psi in about 80-90 minutes depending on any currents!

 

[nemo06489]

Don Burke once bubbled...
1400 psig off of 3000 in a 77 foot tank is about 36 cubic feet of air.

That's .61 cubic feet of air per minute at 35 feet for a surface air consumption of about .30 cubic feet per minute.

I'm guessing you were quite a bit shallower than you thought you were.

If you really averaged 10 feet, that would be .47 SAC, which would be very good.

If you really averaged 20 feet, that would be about .38 SAC, which is pretty phenomenal.

I though any dive less than 35 feet should be considerd a 35 foot dive (New diver didnt know).
My computer dive profile was 33'max depth 13' average depth for 59 minutes at a temp of 52 using a al80 starting with 2800psi and ending with 1600psi.
I dont feel to smart about asking this but what does SAC stand for and how do you figure it out?

 

[scubafanatic]

Ave depth = 13 ft . so ave pressure in ATM's (atmospheres) = 1.39 ATM (33 ft. = 1 ATM)

AL80 holds about 77 cu ft. @ 3000 psi, so each 'psi' = .02566 cu. ft.

You used 1200 psi, which equals 30.8 cu. ft. ( .02566 cu. ft./ psi X 1200 psi)

You used 30.8 cu. ft in 59 min= 0.52 cu. ft. per min.

0.52 cu. ft. / min @ 1.39 ATM gives you a SAC of 0.37 cu. ft. per minute. ( 0.52 cu. ft/min X 1.00 ATM/ 1.39 ATM)

SAC = surface air consumption ( in cu. ft. per min.)

I would consider your SAC of 0.37 cu. ft. / min to be outstanding....mines in the 0.70--0.80 cu. ft. /min range, but I'm an admitted air hog!

Karl

 

[Warren_L]

0.37 is definitely outstanding, if that's what it really is. I average about 0.50 myself....

 

[FreeFloat]

The way my logbook pages calculate SAC is in PSI/min. In order to calculate it by cu. ft./min like you do, I need to know "how many cu. feet" are contained in a steel "72". I know the actual figure is less than 72, just as an AL 80 contains less than 80 actual cu.ft.

For the record my SAC rates (according to my logbook) are generally in the neighborhood of 29 psi/min from my last checkout dive, to 17.0 psi/min at the beginning of November, back up to 20.2 psi/min which I blame completely on cold water