Holding your breath at depth ???

[jhelmuth]

This is probably the same thing said another way...

I think what you'll find is that there is no advantage/disadvantage except when ascending/decending. You'll have a disadvantage with breathholds as you decend - IE needing to breath more quickly than normal; and an advantage as you continuously exhale gas as you ascend. Gas absobstion is relative to pressure, therefore if pressure remains constant, then all things are equal. It's the % of CO2 your body detects that sends the signal to breathe. THat makes sense and correlates to the dive tables reducing the time as depth increases due to the higher absorbtion rates at those depths.

 

[Oldenboldiver]

I'm not sure but I understand that CO2 production is dependent on metabolic rate rather than exercise etc. i.e. your body consumes O2 and generates CO2 at a set rate that varies between individuals. This why SCR rebreathers such the draeger dolphin have a choice of orifice size for the flow of nitrox into the counterlung and that doesn't need to be changed as exercise increases or decrease during the dive.

I also understand the ppCO2 in exhaled air is therefore a constant 0.04bar. I'm willing to be corrected however.

In essence holding your breath at depth is like a rebreather where the CO2 is not being scrubbed. There's plenty of 02 in the gas but its the CO2 that's the problem.

I think that in a CCR rebreather consumption of 02 from the O2 cylinder actually drops with depth. The rate your body consumes the O2 is constant but there are more 02 molecules in the gas in the loop because of the higher ambient pressure. Therefore any given injection of 02 from the 02 cylinder lasts longer. You'd have to ask a CCR diver to confirm or refute my conjecture though.

 

[Don Burke]

Oldenboldiver once bubbled...
I'm not sure but I understand that CO2 production is dependent on metabolic rate rather than exercise etc. i.e. your body consumes O2 and generates CO2 at a set rate that varies between individuals. This why SCR rebreathers such the draeger dolphin have a choice of orifice size for the flow of nitrox into the counterlung and that doesn't need to be changed as exercise increases or decrease during the dive.

I also understand the ppCO2 in exhaled air is therefore a constant 0.04bar. I'm willing to be corrected however.

In essence holding your breath at depth is like a rebreather where the CO2 is not being scrubbed. There's plenty of 02 in the gas but its the CO2 that's the problem.

I think that in a CCR rebreather consumption of 02 from the O2 cylinder actually drops with depth. The rate your body consumes the O2 is constant but there are more 02 molecules in the gas in the loop because of the higher ambient pressure. Therefore any given injection of 02 from the 02 cylinder lasts longer. You'd have to ask a CCR diver to confirm or refute my conjecture though.

Oxygen consumption and carbon dioxide production do vary with exertion. I'm an old submarine sailor and we had to vary what the atmosphere control equipment was doing depending on what the crew was doing.

I'm no rebreather expert, however the change of the orifice sounds more like a coarse adjustment so the solenoid valve doesn't have to cycle quite so much.

 

[Oldenboldiver]

AFAIK there is no solenoid on an draeger SCR. The orifice provides a constant flow of nitrox. Thus an SCR is limited to the MOD of the nitrox used.

There is a solenoid on some (or all?) CCR's as they have separate 02 and diluent cylinders and have top up the 02 to maintain the pp02 set point. Therefore CCRs are limited by the narcotic limit of the diluent.

I'm not sure about submarines but I do recall being told by a SCR user that O2 consumption was pretty much dependent on metabolic rate. That rate may vary depending on whether or not the person is awake or not. On a sub I daresay that depending on the condition set there will be more or less people awake at any one time.

Ultimately, I've always been told that it takes the same amount of energy to walk a mile as run one. If the same amount of energy is used then, logically I suppose the same amount of 02 is used.

 

[Uncle Pug]

... when you run at a sprint do you breath faster than when you walk briskly?

Do you breath faster when you walk briskly than when you sit in your easy chair?

Why do you think that is so?

Hint... CO2 blood levels are the respiratory trigger

 

[Don Burke]

Oldenboldiver once bubbled... I'm not sure about submarines but I do recall being told by a SCR user that O2 consumption was pretty much dependent on metabolic rate. That rate may vary depending on whether or not the person is awake or not. On a sub I daresay that depending on the condition set there will be more or less people awake at any one time.

I _am_ sure about submarines. That's a change in activity, isn't it? Why would there be a change from asleep to awake, but not from inactive to active?

Why does someone breathing hard after exercise benefit from oxygen?

Oldenboldiver once bubbled... Ultimately, I've always been told that it takes the same amount of energy to walk a mile as run one. If the same amount of energy is used then, logically I suppose the same amount of 02 is used.

I don't buy either supposition for a nanosecond.

I don't know where you're going with the narcotic depth and MOD discussion.

 

[Oldenboldiver]

I didn't make any comment about CO2 being the respiratory trigger. I thought everyone else had posted to make that point clear and I didn't think my repeating would contribute anything.

I did, however, comment on oxygen consumption, which appears to be a constant based on metabolism. This fits with SCR theory in that a constant flow orifice allows nitrox into the counterlung. The orifice is fixed and cannot vary with exercise. Thus if we were using more O2 under exercise the gas in the counterlung would quickly become hypoxic if the bleed rate into the was insufficient to keep up. The point about MOD and narcotic depth limits was simply to illustrate the difference between SCR and CCR units and the presence or absence of solenoids and what that may hint at in terms of 02 consumption rather than CO2 production. Perhaps if I knew enough about them I'd be able to say if CO2 absorbent life is effected by how hard the user exercises during a dive.

I'm sure someone with more physiological knowledge than I can confirm or refute the point of about energy being used. AFAIK the reason we pant if exercising and don't if sitting is to do with how efficiently our bodies deal with waste products rather than how rapidly they are produced.

To illustrate, if we lift a heavy object from the floor onto a table. Does the energy required to lift that weight vary if it is moved slowly or quickly? Alternatively, if you run a mile one one of those treadmills that show calories used, does that amount vary depending on the speed set on the machine? I don't know but I'd be happy to find out.

Finally, I've made some general points and raised queiries in what I hope is friendly enough manner. I haven't got any answers back but a few sarcastic comments instead. If it wasn't your intention to be so then perhaps I am over sensitive, for which I'm sorry. If however you did intend the sarcasm, why?

 

[cyklon_300]

it shows that an individual (170 lbs) walking (2 mph) uses 102 cal. to go 1 mile.

Running (10 mph) shows an expenditure of ~120 cal. to cover the same distance.

The cal/hr for these two activities is obviously quite different, but for an equivalent distance the caloric requirement is pretty similar.

Surprised me...

 

[Uncle Pug]

Oldenboldiver once bubbled...
perhaps I am over sensitive, for which I'm sorry. If however you did intend the sarcasm, why?

You are too sensitive. Sarcasm is something you will need to deal with if you intend to post on the internet.

However I wasn't being sarcastic. When activity is increased the increase in blood CO2 levels triggers increased respirations. This happens because more O2 is being used (the O2 part of the CO2.)

Think of your walking/running a mile illustration. Even though the amount of calories burned is only marginally higher running the mile vs walking... the rate is much higher since the duration is considerably shorter.

 

[Charlie99]

Oldenboldiver once bubbled...
I did, however, comment on oxygen consumption, which appears to be a constant based on metabolism. This fits with SCR theory in that a constant flow orifice allows nitrox into the counterlung. The orifice is fixed and cannot vary with exercise. Thus if we were using more O2 under exercise the gas in the counterlung would quickly become hypoxic if the bleed rate into the was insufficient to keep up.

If you just back off a bit and think about it you will see that it would be highly unlikely for O2 consumption to be constant, independent of exercise. Even without getting into medical texts, you probably recognize that various activities "burn" extra calories. Those extra calories need O2 to be "burnt" or metabolized.

A smart user of a SCR like the Draeger Dolphin will know his O2 consumption rate at maximum sustained effort level (aka VO2 max).

The simplistic operation of a mass flow SCR is to simply make sure that the added O2 exceeds the O2 usage.

This is also why the bottom times of the Dophin with the manufacturer's supplied tanks is relatively small ---- you are wasting most of the O2 that is added (in addition to continually adding N2 that isn't metabolized).