Grateful Diver’s
School of Diving

Bob (Grateful Diver) Bailey

NAUI Instructor 41751

Understanding Gas Management

By Bob Bailey

People often ask me what I mean when I use the term “Gas Management”. My answer is that it means you’ve developed diving habits that enable you to base your dive plan on the amount of breathing gas you’re bringing with you. Often they’ll then ask why that’s necessary. Isn’t it enough to just watch your SPG and start your ascent when you start to get low? Well, that might work … but it really depends on the type of dive you’re doing, and how comfortable you are “winging it”.

To make the point, let’s consider an analogy. When you’re driving in your car, you monitor your gas supply by occasionally glancing at your fuel gauge. When it reaches a certain point you start looking for a gas station. Now, imagine that you’re driving along and see a sign that says “Next Gas 100 Miles”. Your first instinct is to look at your gas gauge. But does that really tell you enough information to know that you will make it? You’ll also need to know how many gallons your tank holds, and how many miles per gallon your car will get, on average. You might even have to consider that your miles per gallon will vary with terrain and driving conditions.

Diving is like that. Simply checking your gauge doesn’t give you enough information. You also need to know the rate that you consume the air in your cylinder, and possibly factor in conditions that will cause your breathing rate to vary. Only by considering all of these factors will you know that you have enough gas to dive your plan.

This is what we mean by “Gas Management”.

The elements of gas management include knowing:

Thinking Ahead

A typical recreational dive briefing ends with “End the dive with 500 psi”. But what does that mean? How do you do it? And is it enough gas in reserve if you or your dive buddy has a problem?

Instead of thinking in terms of how much gas we have in our tanks after the dive is over, let’s think in terms of whether or not you have enough gas during your dive. During the dive ask yourself “If I need to share my gas with my buddy right now, would I have enough to get both of us to the surface safely?”

Even if you are diving with a DM or other dive professional, it is your responsibility to know how much gas you need to dive your plan. Remember that the guide is there to show you the dive site, not manage your dive for you. It’s your responsibility to stick to your plan and manage your gas. In diving, we are each responsible for our own safety.

Why it is important

Gas management is important because we carry with us a limited supply of gas, and … well … none of us can breathe water. We cannot manage more than a few seconds without breathing if we run out of gas. And if we allow ourselves to run too low we might have to ascend at a rate that increases our risk of DCS or lung overexpansion injury. So it’s important to do everything we can to keep ourselves out of a situation where we are either too low on gas or out of gas altogether.

It’s also important to consider that one reason we dive with a buddy is so that if an emergency does occur, you and your buddy are able to provide each other with a reserve supply of gas in order to be able to ascend to the surface in a controlled manner. For this reason, each of you should manage your gas supply with both divers in mind.

How to develop good gas management skills

We start by finding out how much gas we breathe under different circumstances. Everyone is different, and the rate at which you consume the gas in your scuba cylinder will vary from dive to dive, depending on factors such as your dive profile, your state of mind, your physical condition, and events that occur during the dive.

To help us get an idea of how much gas we breathe, we use a standardized form of measure known as Surface Air Consumption (SAC) rate.

Your SAC rate is defined as the amount of gas you breathe in one minute at the surface. It can be expressed as pressure (PSI) or volume (cubic feet). For the purpose of this discussion, and to avoid confusion, we will refer to your SAC rate in terms of pressure. When expressing your air consumption rate as volume, we will refer to it as Respiratory Minute Volume (RMV).

You will often see someone describing their SAC rate as a number … this is not exactly correct, as it is only expressing your air consumption as calculated for a single dive. Your SAC rate is really a range, because it’s based on your breathing rate … which does not remain constant over a period of several dives, or even during a single dive.

At the low end of the range is your resting SAC, which is the rate you breathe when you are very relaxed, and at the high end is your working SAC, which is the rate you breathe when you are working hard. Many factors can affect your air consumption rate such as depth (due to narcosis effects), current (because of exertion), stress (because it psychologically causes you to breathe faster), fatigue, excitement, or swimming with a sh-sh-sh-sh-shark.

Because of this, the best way to calculate your SAC rate for dive planning purposes is to track your gas consumption over a number of dives, watch the trends, and consider what factors are affecting your air consumption, and by how much they are affecting it. Then, when planning a dive, you will have a better idea of how to calculate your gas consumption for the anticipated conditions of the dive.

But something else factors into our gas consumption rate … depth. Water pressure affects our air spaces. The deeper we go, the more squeezed our air spaces become. But water pressure also affects the gas we breathe. When we take a breath, our regulator delivers the gas at a pressure that equalizes the pressure of the water around us. Without this equalization our lungs would not be able to function properly, and our breathing would be inhibited. So the deeper we go, the more gas we remove from our cylinder when we take a breath. That is why you can kick around a shallow reef for an hour, while at 100 feet the same cylinder might only last you 25 minutes!

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