Tank size and time available

[Lord Northern]

Hey!
I keep looking at scuba tanks and I was wondering: how do you tell for how long a tank can last?
I know it all depends on the size, and the pressure, but isn't there a way to tell the time available for diving?

 

[spectrum]

First the cylinder factor is the capacity, usually expressed in cubic feet. 80 cubic feet is the most common size. The air in this size cylinder would fill an 80 cubic foot bag if released in that way. Physical size and the rated fill pressure are design and performance factors.

As for dive length it is based upon your cylinder size, the actual pressure to which it is filled. The depth to which you are diving since the increasing pressure will require your regulator to deliver air at air at higher pressures depleting the cylinder quicker. Your personal consumption rate which you will learn over time. Your rate will vary with things such as stress, currents, temperature and the pace of the dive. What you choose to set for an air reserve will also enter into the result.

Some moderately deep dives will be a limiting factor in themselves due to nitrogen loading and the deseire to avoid mandatory decompression.

So you can see that many many factors add up to the length of the dive. That being said most dives seem to end up falling in the 45-75 minute window.

Larger or dual cylinders, conditioning, shallow depths and so forth can result in dives of 2 hours or more.

There are methods of gas planning that let you calculate these factors but for most recreational diving it becomes intuitive.

Pete

PS If you add some meat to your profile responses can be better targeted at you and your diving.

 

[alpenadiver]

you use your SAC (Surface Air Consumption) rate, which will give you PSI / minute, use this formula:
Total Gas Consumed / Depth (in ATA)*time. (ATA = depth / 33 +1).

Or you can find your RMV which is cu. ft. / minute, and can be found with this formula:
RMV = SAC / (rated cylinder working pressure / rated cylinder volume)

Gas supply lasting = Gas Supply Volume / (RMV*ATA) (ATA = depth / 33 +1)

be sure to do the ( ) first.

Mike

 

[Dive-aholic]

Air consumption is the biggest factor in how long a tank will last. And air consumption is based on a lot of variables - depth, currents, water temp, comfort level, work load, etc. Experience will be the best aid in telling you what you want to know.

 

[Charlie99]

Here's a simplified method of estimatiing dive times.

A new diver will consume air at the rate of about 0.75 cubic feet per minute.

An AL80 tank has about 65 cubic feet, excluding the traditional 500psi reserve.

At the surface, this 65 cubic feet, when consumed at 0.75cfm will last 65/0.75 = 90 minutes.

Of course, floating on the surface for 90 minutes isn't a very exciting dive and you'll be somewhere deeper. Surface pressure is 1ata. At 33' it is 2ata, 66' 3ata, etc.

You just need to divide the 90 minutes of time you'd have at the surface by the number of ata's at the average depth of your dive ---- i.e. you'd have 90/2 or 45 minutes at 33' average and 90/3= 30 minutes at 66' average.

 

[Lord Northern]

Ok, I see. It was pretty much the info I needed. Thanks.

 

[pacchill]

GUE has a good read about gas consumption in their Tech-1 manual, might be in the Fundy book too. Really good info in there. Before this gets flamed, I'm endorsing GUE or DIR here, just passing some knowledge. lol Their manuals are really good. IMO
PacCHill

 

[spectrum]

Ok, I see. It was pretty much the info I needed. Thanks.

Remember too that the dive may end due to the air supply of your buddy or another team member. In other words, a big cylinder does not guarantee extended bottoim times. This is where haveing some idea about who you are diving with and what sort of diving you are doing is handy. After diving for a little while on rental cylinders you can get some idea where you fall with your SAC rate. With experience that will continue to fall over time.

Pete

 

[rawls]

Start with Boyle's law...read it, understand it, then you may be able to comprehend what all this SAC stuff is about. Understand the science...

 

[Meng_Tze]

Hey!
I keep looking at scuba tanks and I was wondering: how do you tell for how long a tank can last?
I know it all depends on the size, and the pressure, but isn't there a way to tell the time available for diving?

Bottom line is that a tank last as long as it takes to hit rock bottom or someone calls the dive.

The rest has been explained in this thread and I would suggest to read up on gas laws and figure out your SAC (this takes a bit of time). But dont be drawn into the mindset of 'I know I can get .... minutes and am going to make that time'. It is a potential set up for dangerous practices (skip breathing, Co2 buildup, cutting it close etc, etc). A tank of breathing gas under water is not similar to driving a car on a full tank of gas. There are many more variables and physiology.