I think it is a swell idea! To those that say it is too much for new divers, I say that you might be correct.
However, the concept is what they need to buy into. Once that is done, the motivation is there to learn the method.
I agree, and I think Peter's cheat sheet is evolving into a useful tool. I posted the following thoughts a week or so back on a different forum ... I'll reproduce it here, perhaps to supply some support to the information he's presenting.
Gas management is a dear subject to me … because I believe it’s the most important topic that most recreational agencies don’t teach. And since I teach recreational divers, I’ve created an AOW class that puts heavy emphasis on proper gas management. I have my students take consumption measurements under varying conditions, and they’re required to calculate their expected gas consumption as part of their deep dive plan. I also teach a gas management seminar that I provide for free to any local dive shop or dive club that invites me.
My approach is to begin with concepts, and then introduce some basic math as needed … because most people really don’t see the need to work out calculations until they understand why they’re doing it. So the first step is to get divers to understand why it’s important to look at how much gas they need BEFORE the dive, rather than the standard approach of “end the dive with 500 psi”.
I start with an analogy … let’s envision that we’re driving across a wilderness area, and we see a sign that says “Next Gas 100 Miles”. What’s the first thing we do? The natural reaction is to look at our gas gauge. It provides us with a piece of information … but by itself that information won’t tell us if we have enough gas to make it. We also need to know how large our gas tank is … because the gas gauge only tells us how much gas we have relative to the size of our tank.
In diving, our SPG works the same way.
So once we determine how many gallons we have based on knowing the size of the tank, plus what our gas gauge is showing, we need another piece of information … how many miles per gallon we consume.
In diving, this is our surface air consumption (SAC) rate.
But we also know that in a car, our miles per gallon is based on certain idealized conditions … and that real-world factors like road condition, terrain, driving speed, and weather will change our actual consumption.
In diving, factors like current, fatigue, stress, task-loading, narcosis, and even something as simple as seeing a sh-sh-sh-shark will affect how we breathe. In diving, your air consumption rate … whatever you call it … isn’t a number, it’s a range that’s influenced by a host of factors on any given dive.
So let’s go back to that driving analogy for a moment. If we have an idea of what the conditions ahead of us are like, we can make a reasonable estimate of how much gas we’ll need to make it across to the next gas station … because when we purchased our car there were two numbers given for gas consumption … highway mileage and city mileage. What those represent are the high and low end of the continuum that comprises our actual gas consumption. And the conditions we encounter will determine where on that continuum our actual gas usage will lie.
In diving, the high end is called “working SAC” and the low end is called “resting SAC”. In my AOW class, I have the students calculate both of these … first by diving a timed course while working hard (kicking very fast), and then by diving the same course while barely working (kicking very slowly). The difference is dramatic, and varies from student to student. Typically your working SAC will be about twice your resting SAC … although depending on your individual physiology I’ve seen working SAC rates as much as four times higher than resting SAC rates. This is important information for dive planning, and something that any diver who wants to use actual gas management techniques needs to know.
Your actual consumption rate for any given dive will lie somewhere in between these two numbers.
But here’s where the analogy breaks down … driving a car is a two-dimensional activity, while diving is a three-dimensional activity. In diving we have a third factor that affects our consumption rate … depth. We know that as we dive deeper, we use the gas in our tank faster … and the rate is proportional to our depth. The reason why we calculate surface air consumption is because it gives us a baseline upon which we can apply a “depth” factor to calculate our actual consumption at any given depth. Let’s call that depth factor “P” … because it’s really the pressure at which we’re diving which is affecting us. Knowing our surface consumption, we can calculate actual consumption at any given depth by applying the equation …
P = depth/33 + 1
If there’s anyone reading this who doesn’t understand that equation, just ask … otherwise I’m going to assume that it was covered in your OW class, as it should have been.
You also need to know how to convert the contents of your tank from pressure (psi) to volume (cubic feet or liters) … because you’ll generally want to use volume for diving planning and pressure for dive execution. This is done using what we call the baseline (B), and applying the equation …
B = rated tank volume/rated tank pressure … which is a very small number
For example, the baseline for a standard AL80 is 77.4 cf/3000 psi = 0.0258 cubic feet per psi. What this tells you in practical terms is that for every 100 psi showing on your SPG you have roughly 2-1/2 cubic feet of gas.
Now let’s take a look at real-world diving. For most of us … diving recreationally … knowing our working and resting SAC rates provides us with enough information to make some reasonable estimates of how much gas we’ll need for a given dive. We start with the dive plan. It will generally be broken down into “segments” … so much time to ascend to a given depth, so much time at bottom depth, so much time to ascend to a shallower depth, so much time to remain at that depth, etc … right up to the point where we reach the surface. By breaking down the dive plan in this manner, we can apply the average depth for each “segment” as our “P” factor to our consumption rate and get some notion of how much gas we’ll need (in ideal conditions) for that segment. Adding up all the segments tells us how much gas (again, in ideal conditions) we’ll need for the dive.
Here’s an example dive profile … the one I use for my class and seminar. The red dashed line represents the dive plan. The yellow X’s represent the average depth used for calculating how much gas I’ll need for each segment. The blue shaded area represents the actual dive. As you can see, we don’t live in an ideal world.
Now, in order to make any meaningful calculations, we also need to consider what factors might increase our gas consumption, and by how much. Another analogy, if I may …
Let’s say you’re sitting at home watching TV. You’re breathing at a certain rate. You look out the window and decide it’s too nice a day to be indoors … so you go for a walk. Your breathing rate is no longer what it was while you were sitting on the sofa. After a few minutes, you decide you can use a little more exercise, so you pick up the pace … your breathing rate changes once again. All of a sudden this dog comes running across a yard, barking and growling … your breathing rate goes up dramatically …
Diving’s like that too … many factors affect your breathing rate. Some you can anticipate, some you can’t. In gas planning, you factor in the ones you can anticipate and add some gas reserves for the ones you can’t.
OK … so how do you determine how much to factor? Well, for the typical recreational diver, at first it’s a WAG. You need data … and everyone’s different. My recommendation is to track your actual gas consumption on every dive. Look at your SAC rate for each dive … it will go up and down according to the type of dive you did, the conditions you encountered, how you felt, what you were doing during the dive, and a number of other factors. Consider what you encountered during the dive that might have affected the consumption rate during that dive. Over time, trends will become apparent.
The important thing is to shift your thinking away from how much gas to end the dive with, and toward how much to begin the dive with. The reason I have my AOW students do a gas plan as part of their deep dive plan is because it becomes readily apparent to them why doing a deep dive on a small tank such as an AL80 isn’t a good idea.
… Bob (Grateful Diver)
