How Much Air?

[boulderjohn]

Perhaps it has not been made sufficiently clear, but the only reason you would ever want to do a 30 second exhale is while doing a CESA. It is definitely NOT something to do while diving normally.

Will a 30 second exhale affect your buoyancy? I can guarantee that. When I had to do the horizontal CESA for my instructor certification, we had to go 30 feet in a pool with a sloping bottom, starting with a maximum depth of 4 feet and ending at less than 3 feet. I had been trained to do that in 30 seconds, which meant I was exhaling for that length of time. All the candidates in that session lost a point on our score because by the time we reached the end of the exercise, we had lost a foot of depth, and our knees scraped the bottom. That loss of buoyancy was deemed to be unacceptable, but we found it close to unavoidable.

 

[outofofficebrb]

I have seen some divers do some yo-yo diving (up, down, up, down) constantly deflating and inflating BCs and burn through air quickly that way. That and also being overweighted, sometimes together with the yo-yo diving, and not being trim will all have an effect on the air.

 

[Colliam7]

I have seen some divers do some yo-yo diving (up, down, up, down) constantly deflating and inflating BCs and burn through air quickly that way. That and also being overweighted, sometimes together with the yo-yo diving, and not being trim will all have an effect on the air.

A very good point. I will add an element to that scenario that further contributes to (respiratory) air consumption.

Not uncommonly, the over-weighted diver (for that matter, even some properly weighted divers) who ends up with all of their weight on their waist (a belt, or in the integrated weight pockets commonly found on many BCDs - i.e. in a position that is not aligned with their center of lift - their thorax), moves through the water at an angle - head slightly up, feet slightly down - instead of horizontally. I describe them to students in my OW / AOW classes as the 'Forty-fivers'. When they fin, part of their thrust is directed downward, not forward. So, instead of getting maximum forward motion and efficiency with each fin cycle, part of their effort, and associated air consumption, is directed toward maintaining their position in the vertical water column (this is why they also start to sink if they stop finning, which causes them to add air to their BCD, which they then have to purge once they start finning again, because they then start to ascend). If that diver is swimming with other, more experienced - or, at least, more horizontally trimmed - divers, the 'Forty-fiver' will necessarily use more finning effort, and air, to 'keep up', even if the other divers are swimming slowly. They are finning harder to match the forward movement of the other divers because they are not getting 100% thrust moving them forward, plus they are presenting a larger cross-sectional area to the vertical water plane in front of them, increasing drag / resistance.

So, for the OP: in addition to a) getting used to, and more comfortable with, breathing underwater (something we all have gone through, so you are not alone), and b) controlling your breathing pattern (slow and deep, not fast and shallow), and c) working on your weighting and buoyancy control, if you also d) work on your horizontal TRIM you will find that your air consumption decreases over time.