About contractions...
Hello Mark9989 Et Al,
Dive within your comfort zone. If you aren't comfortable and relaxed then what's the point? You want to feel natural in the water, so just relax. Don't be in a hurry to expand your comfort zone either, just enjoy diving. You might try apnea walking as a training tool as well. Try to alternate evenly between breathing and apnea; When that 'panic' feeling starts to creep up during the breath-hold, just tell yourself to relax--everything’s normal. Most importantly, always err on the side of caution.
If you're diving alone, then a good 'rule of thumb' is to be surfacing just as you get the urge to breathe; controlling contractions won't extend your O2 supply and you can't afford to push the envelope. Here's a brief description of Shallow Water Blackout and what happens when we dive to -10m, though you're probably already familiar with the concept:
At sea level, the air you breathe is .21 ATA oxygen and .79 ATA nitrogen (plus some trace gasses). Our brains require a PaO2 of .10 ATA to remain conscious, so if we dive to -10m we might have, say a PaO2 of .42 ATA in our blood (because the atmospheric pressure doubles between 0-10m) giving us the impression that all's well. If we use 60% of that O2 while at depth, then as we ascend the partial pressure will decrease to ~ .09 ATA and we'll pass out, probably around -4 to -3m -- hence the term Shallow Water Blackout. If you hyperventilate then you're blowing off co2, which is what triggers you to breathe; this can give you the false impression that your O2 levels are higher than they are. This, in combination with the diving physics described above, is one reason why hyperventilation is so dangerous. It doesn't sound like you are hyperventilating, but the idea of controlling the urge to breathe is similar.
Here are a few suggestions if you must dive alone:
1) Always dive conservatively, I can't emphasize this enough.
2) Do not focus on a goal while diving as most people die from SWB because of this.
3) Limit your bottom times to 1:30
4) Always give yourself at least 1.5-2 times the dive time to recover, this allows your body to replenish O2 and out-gas co2 which will build up over time.
5) Limit your depth to -20m
6) Remain aware of and pay attention to your body; how do you feel? If you're not ‘in the groove’ then don't push it--go home and dive another day.
Obviously, as we become more comfortable under water, tolerant to co2, and our bottom time increases, the likely hood of a BO increases as well. If you can dive with a buddy, please do so. The caveat here is this: wearing a seatbelt does not make it safer to drive fast, so don't let your buddy give you a false sense of safety--they may not be able to rescue you.
Hope this helps, I sound like my parents! :hmm
Safe and happy diving,
Ward
