Air consumption

[ajduplessis]

That's if you can remember anything.

 

[leadduck]

So, getting back to the original problem ...

I go through my air faster than anyone else. I am always the first to come up and it drives me crazy.

Your air consumption is driven by your body burning calories, there are various factors and I think it's worth going over them to see how and why it may differ from your buddies.

1. basal metabolic rate (BMR). The amount of air you need even if you don't move. Depends on your muscle mass, age, size. Smaller, younger, slimmer buddies will consume less. Can make a big difference. Lose weight, stay fit.
2. Stress level. Adds on top of the BMR regardless of speed. Some people are already exhausted before going down, just from carrying their equipment to the water too quickly. Always move slowly and efficiently, calm down before descending. You may benefit a lot from taking a class in apnea diving.
3. Finning efficiency. How efficiently do you transform muscle force into thrust? Think about well done frog kick vs flutter.
4. Water flow resistance. Are you well streamlined, how is your trim?

You may wonder why I don't list speed. You trivially minimize SAC by not moving at all like in static apnea. Your typical dive however is about swimming a certain distance along a route, and you asked why you need more than your buddy to do this at the same speed. Comparing SAC regardless of the dive doesn't make sense.

The energy you need to swim a certain distance grows with speed, due to the drag force growing quadratically with speed. Considering the BMR and stress level, there's an optimum speed that minimizes the air you need to swim this distance. (Note that this is very different to climbing a hill, where you gain a certain amount of potential energy regardless of speed and air's drag force is negligible.)

Your optimum speed for a given distance depends on what's your objective: lower SAC vs having more air left in the bottle at the end of the dive to brag about.

1. If your objective is not reducing total air consumption, but minimizing SAC while swimming a certain distance, then it's simple: swim as slow as possible, so that the air in your bottle is barely sufficient to finish the dive. You get out of the water with an almost empty bottle after a long dive.
   Improving finning efficiency, reducing water flow resistance, reducing BMR and stress level will reduce your SAC. Your speed objective however is always the same: as slow as possible, empty the bottle.
2. To minimize total air consumption, the optimum speed is faster than minimum, depending on the four factors above:
   • improving your finning efficiency and reducing water flow resistance increases your optimum speed; you can go faster (same distance, same SAC, but less total air consumption due to shorter dive)
   • reducing your BMR and stress level reduces your optimum speed; you can slow down (same distance, longer dive, but yet less total air consumption due to lower SAC)

Regarding another diversion in this thread:
BMI is not useful for evaluating your fitness or health risk. In a recent study with 10,000 participants (and more other studies) comparing various metrics, BMI did not correlate well with your risk of dying early from a stroke or heart disease either. A much better yet simple metric turned out to be the waist-to-height ratio (WHtR). It correlates better with visceral fat. WHtR shouldn't exceed 0.55 for a 40yr old.

 

[ScubaInChicago]

After rereading this thread, I must apologize and redact my earlier statement. I recommend the OP take a GUE Fundamentals Part 1 course as the GUE Primer course I originally mentioned has since transitioned.