WARNING: This post, or any on this blog for that matter, doesn’t constitute and is no subsitute for proper training and experience. We encourage you to seek the proper training before you attempt anything our posts discuss.
If you’ve spent any time on dive boats you’ve probably heard buddies or teams plan the next dive. It often goes something like this:
Buddy One: Let’s head out at 215 degrees.
Buddy Two: Okie Dokie.
*splash*
Hardly a comprehensive dive plan. At the very least they are missing a good gas plan. A solid gas plan tells us how much gas you should reserve to get you and your buddy back to the surface completing all the scheduled deco obligations. Remember, every dive is a decompression dive and we need to let our bodies off-gas the nitrogen we have built up in our tissues.
“But I have a computer! It tells me how much breathing gas I have left in terms of time! Neat huh?”
Computers are useful tools. However, letting them do all the thinking for us is not a good idea. First off, your computer is pretty dumb. You’re computer has no idea what your buddies air consumption is like. It doesn’t know what your ascent profile looks like, wether you plan to do deep stops at 75-50% of your depth or just a three minute “safety stop” at 10-20ft. It also hasn’t the foggiest what your air consumption is like during an emergency. This is often as high as 1cf/min or 25-50% faster than your regular consumption rate. When your teammate has an out-of-gas emergency is not the time to find out you’re going through your gas twice as fast as your computer thought you were. Long story short: You are smarter than your computer. There is a fast and easy way to plan your gas usage with your buddy to make your diving safer and more enjoyable. Let’s take a look:
Proper Gas Planning In Detail
There are a few things we need to know to make a proper gas plan. Firstly, we need to know how many cubic feet there are per 100 PSI in our tanks. This number is called a “Tank Factor.” Not all tanks are the same and 1500PSI in a HP100 is not the same amount of gas as 1500 PSI in a LP85. Second, we need to know what our ascent profile looks like. Third, how much gas will we be using per minute in an emergency and lastly, what will our average ATA’s, or atmospheres absolute, be during the ascent?
What the heck is a tank factor?
Different tanks have different service pressures and volumes. So how do we come up with the PSI per cubic feet in terms of a nice round easy to work with number like 100 PSI? The answer is determining our bottles “tank factor.” Let’s do a little math. I know, I know but it’s easy I promise.
How do we calculate our tank factor? We divide the rated volume by the rated service pressure and multipy that number by 100. Boom. Easy yes?
Let’s look at my shiny new LP85′s. They have a volume or 85CF and a service pressure of 2400PSI so my tank factor would look like this:
100(85cf/2400PSI)=4 Tank Factor
So my tank factor would be 4cf/100 PSI or just “4″. Remember, this is back-of-the-cocktail-napkin math so there is no need to be super precise. Rounding up to a whole number is just fine.
How long will our ascent profile take?
We need to determine how much time it will take us to deal with an emergency and ascend to the surface. For our example lets say we are on a 100ft dive. For this example we are going to using the ascent profile that PADI recommends: A direct ascent to the surface at 33ft/min and a three minute safety stop at 10-20ft. let’s also give ourselves one minute to deal with the emergency and get everything stowed away and ready to ascend. How much time will we need to get to the surface? The calculations look like this:
1min Emergency Time+(100ft Max Depth/33min Ascent Rate)+3min Safety Stop=7min.
So, at the very least we are going to need 7 minutes of gas to get us safely to the surface.
Great, 7 minutes. But how much is that in cubic feet?
We need to know our emergency air consumption rate. Hopefully, your not the type of diver who gets into emergency situations often enough to know this figure off the top of your head so let’s use the industry standard of 1cf/min.
7min(1cf/min)=7cf
So we need 7cf to get us back to the surface? Not quite. Our gas goes faster when we are at depth and we need to factor that into our plan. We can aproximate our average depth in feet for our ascent plan by dividing our max depth by 2, or…
100fsw Max Depth/2=50′ Average depth.
Still pretty easy? Feet doesn’t do us much good here though as we need to know how many cubic feet of gas we need. To calculate that number we need the average ATA’s of our ascent plan. To find our avergage ATA’s we divide our average depth by 33 and add 1.
50ft/33+1=Average ATA’s
Average ATA’s = 3
Multiplying the minimum amount of gas we need, 7CF, by our average ATA’s will give us the amount of gas we need to get a diver home.
7cf(3ATA)=21cf
But wait! That’s 21cf for one diver, not two. Let’s double that number.
21cf(2 Divers)=42cf.
“That’s great and all, but my gauge shows PSI not cubic feet.”
Still with me? Good. 42cf of gas minimum gets two divers safely back to the surface from 100′. But that number is not really that useful for us either. We need to know that figure in terms of PSI. How do we get that? By dividing our tank factor by minimum gas in cubic feet multiplied by 100.
(42cf minimum gas/4 Tank Factor)100=1050PSI
Well there you have it.
The magic number. 1050PSI is the bare minimum you need to leave the bottom with to get two divers safely back to the surface from 100′ on an LP85. Is it more than you thought? Keep in mind in an emergency situation, minimum gas means that you will have zero PSI in your bottle when you hit the surface. Because a tank with less than about 300 PSI may have gotten water in it, it will need a visual inspection or it might corrode. In an emergency, however, that will be the least of your worries. After some practice creating a solid gas plan shouldn’t take more than a couple of minutes to figure out in your Wetnotes.
I hope that helps even the most math phobic of you. Call or drop by the shop if you have any questions. and as always, safe diving.
