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Grateful Diver’s
School of Diving

Bob (Grateful Diver) Bailey

NAUI Instructor 41751


Understanding Gas Management

(continued)

Calculating how much gas you will need for your dive plan

For most recreational dives, knowing your turnaround pressure and rock bottom pressure are all the dive planning you will need. The nature of your dive dictates that you will get in the water and conduct your dive until either the pressure in your cylinder or your no-decompression limit dictates that you begin your ascent.

However, for some dives, where a particular destination or goal is the objective of the dive, it may be useful to plan in advance how much gas you will actually need for the dive. In this case, you can estimate your dive profile and calculate your gas requirement in a manner very similar to what we just did for calculating rock bottom pressure … by breaking the dive into segments, calculating how much gas we will need for each segment, and adding them together.

For example, one segment could be your descent to your target depth. By estimating how many minutes it will take, and how deep you are going, you can use your average depth over that interval to determine a reasonable estimate of how much gas you will use for the descent. Multiplying the pressure (ATA) at your average depth by your RMV will tell you approximately how many the cubic feet of gas are required for that part of the dive.

Likewise, you can do the same thing for the time spent at depth, for the ascent to your deep stop, your ascent to safety stop, and the time spent at each of your stops.

Figure 2 illustrates a planned dive profile superimposed over an actual dive profile. The red dashed line is the planned profile, the callouts along the top are the planned “segments” of the dive, and the yellow X’s represent the average depth for each segment. By using the relationships we have learned so far, we can estimate our gas consumption within a reasonably close limit.

You can see that while slight variations exist over the course of the dive, the actual dive is fairly close to the planned dive. Actual gas consumption on this particular dive was within just a couple of cubic feet of the planned consumption … well within the limitations of your expected reserve.

This skill can be very valuable for the dive team who has a specific goal in mind, or is planning a deep dive, and wants to be assured that they are carrying enough gas to execute their dive plan.

Figure 2. Planned vs. Actual Dive Profile

Dive profile

For most of us, this is a skill we will rarely, if ever need to use. It depends entirely on the nature of the dive we are planning. However, from a conceptual perspective it is a useful skill to have as an aid to understanding how much air you will actually use on a given dive.

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