YD Scuba Diving Forums banner

1 - 15 of 15 Posts

·
Keyboard Worrier
Joined
·
4,155 Posts
Discussion Starter #1
Never really worried about air consumption - you use it, you get out - end of.
I'm not bad , not as good as many, but it'll do.

However my deep use seems far better than the 'dive physics for beginners' suggest...

ie we teach that a 12l tank at the surface lasting an hour - will only last 1/2 at 10m, 20 mins at 20m yadda yadda, we all know the pressure chart - it all makes sense from that point and we move along...

and yet - my consumption at 30m isn't far off that at 10m - perhaps an hour at 10m, 45 mins at 30m (ok incl travel etc but lots at 30m)... doesn't obey chart above...

So - where does pp come into air use - surely if you are breathing air at 30m, you have a pp o2 of .84 - you body doesn't NEED to breath so often as it is getting more bang per buck per breath?
Ie, you could only breath in 4 to have same effect on body? So back to square one with consumption.

So - if correct - gas use would be the same regardless of depth/pressure...body need more fuel, but
you are either getting dilute version or new improved super concentrated at depth ;-)

However, I understand breath is controlled by C02 content of body - but since THIS is at a higher pp than at surface does this mean you would feel urge to breath (4x) more or (4x) less?


confused.com

;-)
 

·
Nigel Hewitt
Joined
·
7,142 Posts
However, I understand breath is controlled by C02 content of body - but since THIS is at a higher pp than at surface does this mean you would feel urge to breath (4x) more or (4x) less?
Your body has no sensor for oxygen. In pure oxygen you do not breath less and in pure nitrogen you die painlessly never knowing there was a problem.

CO2 sets the blood acidity and that drives breathing. However to flush CO2 out of your lungs involves moving a volume of gas through your lungs regardless of its density. At depth that 'same volume' takes more grams of gas out of your tank every breath.

When assessing breathing rate be careful to read tank consumption at a fixed depth for a known period doing the same activity. It is so easy to miss out all the stages of a dive that consume the same amount of gas.
Doing deco and checking the tanks afterwards against the schedule I discover I am remarkably consistent and factoring by depth works for me.
 

·
Karlos or Karl
Joined
·
1,001 Posts
In a nutshell a 30m dive is probably easier than 10m, unless you stayed at the fixed depth (10 & 30) you would be using more gas to tune your depth (adding & dumping BCD & drysuit more at 10 as the pressure change is greater between 10 & 6 than 30 & 26 (both .4 bar but 1.25x dif in shallow only 1.1x at depth).

Try doing a surface air consumption rate (SAC) swim- hold a fixed depth ideally 10m or deeper, exact depth not relevant just maintained. Swim at constant steady swim at the same exercise rate (don't swim into or with a current) for 10 mins. Record your bar reading before & after the exercise (digital gauge is best, if twinned isolate) during do not inflate BCD or suit any.

With the readings do a quick calc on the surface:-

Bar used x cylinder size (18bar x 12L = 216 litres of air used)

Divide air used by pressure at depth (216 / 2 (bar pressure at 10m) = 108 (15m would be 2.5 & so on).

Divide this figure by mins at depth (108/10= 10.8) this will give you your SAC.

Repeat this frequently to get an average & in the kit you dive.

Most averagely fit divers will get between 15 & 25 at rest (deco maybe as low as 10m.

Anecdotally, in Cuba last year with a single, 3mm shortie, clear viz & a (head of vodka from night before - this is not condoned - still to drunk to realise I was still drunk) achieved a SAC of 8, when teaching some students their OW I have achieved 35. My computer tracks the dice air use & gives it on download.

In short if you are breathing less at depth than shallow its one if 6 things.

1) you get very anxious in the shallows
2) your buoyancy needs tuning
3) you are so narked at depth your uber chilled
4) your on a rebreather
5) you are diving a 7l in the shallows & 18l at depth
6) you have evolved gills which only work at depth.

I would say a combo of 2&3 (not being disrespectful of your buoyancy)

Good luck & enjoy

Karl
 

·
Registered
Joined
·
3,640 Posts
My missus suffers with sympton 6.
I kid you not. We dived with a bloke in Denmark who had double 10's, at the end of the dive he says, That was great, and I still have 90bar. My missus retorts that she has just over 90bar................single 10.
 

·
Registered
Joined
·
1,837 Posts
Ahh it's common to have a smaller sac rate at depth... Mine drops dramatically past the 10m line.
 

·
Registered
Joined
·
31 Posts
I tend to agree with the above posts.. I am always more relaxed at 30m than 10 (i dive in dry suit and twins) so bouyancy is a bitch on 10m dives.. but I also beieve the narcosis mellows u and u go into a relaxed state. When I was on twins i used to use 12LPM for working and 10LPM for DECO to calculate gas required for a particular dive... now on rebreathers,,, it doesnt matta~!
 

·
Keyboard Worrier
Joined
·
4,155 Posts
Discussion Starter #8
My bouyancy is fine at all depths, but agree with the 'much more mellow at depths' reasoning...crazy as it might be.
Just thought there must be some science including pp to it...
 

·
Nigel Hewitt
Joined
·
7,142 Posts
...but agree with the 'much more mellow at depths' reasoning...crazy as it might be.
Jacques-Yves Cousteau. The Silent World. p43 said:
I am personally quite receptive to nitrogen rapture. I like it and fear it like doom. It destroys the instinct of life.
You're narked. HTH. :)
 

·
Registered
Joined
·
653 Posts
Your body has no sensor for oxygen. In pure oxygen you do not breath less and in pure nitrogen you die painlessly never knowing there was a problem.

.
Not quite true, there are O2 receptors in carotid bodies in the neck, the neural output (up to the respiratory centre in the brain) decreases with increasing PaO2 (arterial blood oxygen partial pressure) and increases with decreasing PaO2. You are correct that under normal inspired Oxygen partial pressure and nothing wrong with heart or lungs then it is PaCO2 (partial pressure of CO2 in arterial blood) that controls depth and rate of breathing, but with increasing PaO2 there is a decrease in sensitivity to CO2 changes, the extent of this is highly variable from person to person but can be sufficient to influence SAC particularly under conditions of increased workload and increased work of breathing.
Individuals showing this effect may be more at risk of narcosis as CO2 and N2 effects are additive. The effect on respiratory centre of low O2 is relatively mild and I agree it is perfectly possible to loose conciousness due to hypoxia of rapid onset without realising the problem particularly during exercise.
Kind regards
Tim Digger
 

·
Registered
Joined
·
92 Posts
Not quite true, there are O2 receptors in carotid bodies in the neck, the neural output (up to the respiratory centre in the brain) decreases with increasing PaO2 (arterial blood oxygen partial pressure) and increases with decreasing PaO2. You are correct that under normal inspired Oxygen partial pressure and nothing wrong with heart or lungs then it is PaCO2 (partial pressure of CO2 in arterial blood) that controls depth and rate of breathing, but with increasing PaO2 there is a decrease in sensitivity to CO2 changes, the extent of this is highly variable from person to person but can be sufficient to influence SAC particularly under conditions of increased workload and increased work of breathing.
Individuals showing this effect may be more at risk of narcosis as CO2 and N2 effects are additive. The effect on respiratory centre of low O2 is relatively mild and I agree it is perfectly possible to loose conciousness due to hypoxia of rapid onset without realising the problem particularly during exercise.
Nice summary.

Respiratory agent consumption is well related to ventilation, based on the Interspiro SCR DCSC, IS-MIX.
It has proven to a depth of 150m in the diving DCSC, in the chamber simulated dive to 450m.
Of course, physiology complicates things again, this relationship work good for big efforts. In maximal and submaximal there is also anaerobic energy yield.

rc greet
 

·
Registered
Joined
·
1,619 Posts
find a buddy who has a nice air integrated computer ask nicely to borrow it for a few dives and some tell you what your sac is then you can know for sure i have never worked my sac out i will do one day probably.
 

·
All we wanted was a home... Manics
Joined
·
4,553 Posts
maybe you were a budgie in a previous life?: when it goes dark, you go quiet/sleep? try not to sleep when diving
 

·
Registered
Joined
·
3,374 Posts
I have noticed this effect too. Unfortunately my SAC has not been constant due to faulty regs. New ones seem better so far but still show a reduced SAC when below 30m. I also have a lower SAC when solo.
 
1 - 15 of 15 Posts
Top