How conserve air when necessary

[Michael Guerrero]

say your prayers or don't get yourself into stupid situations.

 

[dmaziuk]

I wonder: having enough air in my BCD to keep me going for any amount of time would mean I'm below 20m wearing a 7mm hooded suit. So here's a simple fix: shallow dives in the tropics.

 

[Altamira]

Oh, right. Then I'd expect one would have told him "this is not the industry standard but I like it better", right? I assumed he was talking about the junction between the hose and the BC.


Edit: Just looked up what you mentionned, the zeagle Bx Bx Power Inflator. Sidenote: someone should really teach the guys at Zeagle how to rinse a BC without wasting freaking 50l of water.

I just love it when people criticize something they have never used, and know nothing about the product. If you have not used the Zeagle Power Inflator, maybe you should not knock it. Personally, I find hooking the inflator hose up to a garden hose is a very good, quick and efficient method of rinsing the inside of the bcd. Probably use less than a gallon of water when I give the bcd and overpressure valves a thorough rinse after a dive trip.

 

[Patoux01]

If you could point me to where I critized the product? Just put it in bold in my message.

 

[Altamira]

If you could point me to where I critized the product? Just put it in bold in my message.

How is this?
"Edit: Just looked up what you mentionned, the zeagle Bx Bx Power Inflator. Sidenote: someone should really teach the guys at Zeagle how to rinse a BC without wasting freaking 50l of water."

Perhaps we just have a difference of opinion as what defines criticism. I do not have any interest in Zeagle other than the fact that I own one, think the Bx Power Inflator is a very nice feature, and did not want anyone to bypass looking at a Zeagle bcd because of your comment.

 

[Patoux01]

That's the user in the demo that I criticized. Not the product. And my message is very clear about it.

I just linked to the product the other guy mentioned, as it took me a bit of time to find it.

 

[crookshanky]

...The partial pressure of CO2 is essentially dependent on the number of molecules present....All other factors being equal it makes no difference if a diver stops breathing or hypoventilates (the scenario being considered here) at 10 feet or 300 feet. The rate of metabolism is the same and the partial pressure of CO2 will rise at the same rate. Thus, in this context depth makes no difference.

Forgive me, my educational background is not dive physiology so please correct me if I am wrong as I am always interested in learning more about this area of study. I agree the change in partial pressure would be the same as the increase in the mole percentage of CO2 would increase the same amount (given the same metabolic rate). However, the absolute value of the partial pressure would be higher at the greater depth (higher ambient pressure). From my understanding, the narcotic effect of a gas is dependent upon the absolute value of partial pressure so, for a given increase in CO2 mole percentage, the narcotic effect should be greater at the greater depth.

Extreme depth IS a risk factor for CO2 retention for a variety of reasons which are not really relevant to a scenario in which a diver is forced to hypoventilate because dwindling gas supply. That may be what you are thinking of, but it is not really relevant to this discussion...

Is this due to a change in the hematic pH level?

I am not certain about this number. You are completely correct to suggest that CO2 is narcotic (and irrespective of the relative potency, you are also correct to suggest that this is a good reason to avoid high levels). But to my knowledge, no one has ever experimentally defined the relative potency in respect of nitrogen for realistic exposures. Given that this is the subject of one of my current grant applications I would be interested in whether you can recall where you heard or read this?

Simon M

That potency number comes from this GUE article and it references this paper...

It's a common often quoted relative metric in the Tech Forums that comes from this:
Undersea Biomedical Research, Vol 5, No. 4 December 1978 Hesser, Fagraeus, and Adolfson.

The paper references two other papers that state narcotic potency ratios of CO2 to N2O and N2O to N2. The multiplication of these ratios is used to arrive at the 130 times narcotic potential.

Also, if I recall correctly, the CO2 limb of that experiment received a little criticism for combining high CO2 and oxygen at 1.7ATA. This is a combination of two potentially narcotic gases (oxygen and CO2), and raises the possibility of sub-clinical cerebral oxygen toxicity also affecting the results. A better design would have been to use a mix of oxygen (at an FiO2 producing an inspired PO2 the same as that at the surface), inspired CO2 sufficient to clamp the end tidal value (body) where you want it for the narcosis experiment, and the balance being helium. Then you really are isolating the effect of CO2. Or more simply, you could just do the experiment with inspired CO2 at the surface during air breathing.

Simon M

The paper did make a small mention of the effects of the elevated partial pressure of O2 but it would be great, as you mentioned, if the experiment could be reproduced with the elimination of the effects of oxygen. Fingers crossed for your grant application!

 

[Kevrumbo]

@crookshanky ,my take on CO2 narcosis based on Simon's opinion is that it's not as insidiously malignant or as dangerous as CO2 retention/Hypercapnia, which can occur at any depth including at 1ata (surface).

Whether excess metabolic CO2 is generated at the surface -for instance a physical & respiratory exhaustive surface swim against a current and through large swells forcing you to use your regulator- or breathing air under exertion at 90m deep -due to increased gas density Work-of-Breathing and negative static lung loads- the effect is the same if you cannot eliminate the CO2 fast enough through forceful exhalations: Dynamic Airway Collapse, leading into the vicious cycle of CO2 retention, CO2 poisoning, Hypercapnia and ultimately anoxic incapacitation, loss of consciousness -and death.

 

[Dr Simon Mitchell]

Hello Crookshanky

However, the absolute value of the partial pressure would be higher at the greater depth (higher ambient pressure).

This is not correct. If it were, then it would be impossible for us to dive because the PCO2 in our bodies would be multiplied many times as we descend and this would rapidly prove fatal. CO2 in the body is a dissolved gas in a non-compressible space (fluid) and its partial pressure is therefore unaffected by changes in ambient pressure.

From my understanding, the narcotic effect of a gas is dependent upon the absolute value of partial pressure so, for a given increase in CO2 mole percentage, the narcotic effect should be greater at the greater depth

Essentially correct. As the partial pressure of a narcotic gas increases, then so too will its narcotic effect. My point (as above) was that this does not occur simply as a function of ambient pressure in relation to CO2. For the CO2 partial pressure in the body to increase you either have to:

1. Not move enough fresh gas in and out of the lungs (ie, breathe enough) to eliminate the CO2 that the body is producing; or

2. Actually have CO2 in the inhaled gas so that breathing becomes very inefficient at removing CO2. This can happen in a rebreather.

The physiological basis for the first cause is complex, and is discussed in this lecture which I gave at the Southern Africa Undersea and Hyperbaric Medical Association Meeting several years ago.

That potency number comes from this GUE article and it references this paper...

Yes, the confusion arose out of what I thought you were implying. My point was that CO2 will never cause 130x the amount of narcosis of nitrogen even though that figure may represent an estimate of its relative potency. The narcotic effect of nitrogen and CO2 at plausible exposures are actually quite similar as was shown in the paper you cite.

Fingers crossed for your grant application!

Yes, thanks.

Simon M

 

[Magnus Lundstedt]

I wonder: having enough air in my BCD to keep me going for any amount of time would mean I'm below 20m wearing a 7mm hooded suit. So here's a simple fix: shallow dives in the tropics.

I think the suggestion is to breathe out into the BCD, than back in your lungs, like reabreathe the air some cycles before you breathe it out and take in new air. Not that the amount of air you carry in your BCD should act as a pony-bottle.