So How Long Does a Tank of Air Last? (part 2)

Okay so this tip of the week thing is a little aggressive for my schedule but hey I'm trying. Not to mention you are getting my wisdom for free so you should be able to find it in your heart to cut me a little slack about how quick I get these done, right?

Well without any further delay lets get to it.

So we covered all the pressure/volume stuff I wanted to cover last week, having flashbacks from high school physics yet? Okay, Okay so how long can you stay down... well the first thing we have to do is calculate what we call our surface consumption rate (SCR). The rate at which we breath air at the surface. How do we do that? Well the best way to do it is to pick a specific depth in the ocean, lets say 15 feet. Then stay there at 15 feet and breath normally for 10 minutes. Record the tank pressure at the start of the 10 minutes and again at the end of the 10 minutes. Once we have all that information we can find out our SAC (Surface Air Consumption) rate.

The next thing we need to do is convert our depth into atmospheres. That is done by using the formula:

so in our case:

Now we must figure out how much air we used in the span of ten minutes. For arguments sake let's say we used 300 psi. Our equation now looks like this:

Now that we know these two values we can calculate our actual surface consumption rate. The above number, 30 psi/min, is our consumption rate at 15 feet not at the surface. So in order to find out our surface consumption rate we divide our breathing rate (30 psi/min) by the depth that we were at in atmospheres (1.45 ATM).

Okay great now that we have the number what the heck do we do with it? Hey would I leave you hanging...okay forget that, let's work out a real world problem and see what that number does for us.

We are going to Bonaire next week and we hear all they have is Aluminum 80 cylindars with a working pressure of 3000 psi. We also know there is a great boat wreck down there called the Hilma Hooker located in 100 feet of water.

Now knowing that we only have a max time of 20 minutes in 100 feet of water, according to our trusty dive tables) let's see how much air we are going to need and if we will even have enough to do the dive.

Okay don't get nervous now, the first thing we want to do is convert our SCR (21 psi/min) into cubic feet/min. How do we do that? Well we need to go to our old friend the tank chart and look up the size of an Aluminum 80 tank. As you can see on the chart an Aluminum 80 tank has an actual capacity of 77.4 cu.ft. By doing a little cross multipliction we can figure out what our breathing rate is in cu.ft./min.

Okay now that we know that we have to figure out the depth we are going which we said was 100 feet. Remember we need to convert that into atmospheres (ATM).

So far so good. Now we take our depth in atmospheres and multiply that by our breathing rate in cu.ft/min. This will give us the amount of air in cu.ft. that we breath per minute at 100 feet.

Okay last but not least we now know how much air we breath per minute at a depth of 100 feet and we know we can only stay down at 100 feet for 20 minutes so if we multiply our time by the amount air we consume at depth then we will know about how long a tank of air will last us...(finally!!!!!)

Hmmm that is very intersting. Let's do a little more figuring here. If we know that we will use 44 cu.ft. of a 77 cu.ft tank then we should be left with about 33 cu.ft.

So we have 33 cu.ft. to make it to the surface at the end of your dive right? Let's convert that back into minutes to see what that will by us for time.

Okay so we have about 15 minutes worth of air to make it to the top. If we ascend at a normal rate, 30 feet/minute, it should take us about 3 minutes and 20 seconds to get to the surface. Well that leaves us with about 11 minutes and 40 seconds to spare right? Hey don't forget about that 3 minute saftey stop at 15 feet! So that leaves us about 7 minutes to spare.

You want my advice, as long as you are truely comfortable at 100 feet then you should have no problem doing this dive..

Final Thought:

It's great to know your SAC rate but keep in mind that no two dives are ever going to be alike. Depth, current, time, temperature, physical fitness, comfort level and many more things enter into every dive. While you can get an approximate SAC rate please understand that it is just that approximate. Remember a diver that is uncomfortable in 100 feet of water will breathe a lot more then their calculated SAC rate, the moral of the story, don't go down there until you are comfortable and properly prepared.

Over and out.

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