Speed and gas consumption

[sydney-diver]

Interesting thread, and one I think some DM's I've dived with should read. Twice now I've tagged along on a dive run by the LDS with a newish DM where one of the clients got low on air at the turn-around (didn't signal 100bar because he didn't want to upset anyone). Both times the DM turned the dive and headed back to the exit point at maximum speed which I always assumed was the wrong response as I assume resistance and hence workload increases with the cube of the swimming speed. My air-consumption certainly held out that theory, it was highest on those two dives. Plus I thought it was a bit risky if you start lagging behind your buddy because you're struggling to keep up.

 

[Tigerman]

Question though..
If you keep filling and dumping 1L of air into your BC, isnt that as ineffiecent as inhaling and exhaling the same 1L of air in your lungs?
Im not saying that using the BC is infact as inefficient, but the degree of gain from it would be variable depending on how often you need to fill/dump air from the BC as opposed to your lungs, wouldnt it?

 

[Bubbletrubble]

Bubbletrubble, that goes along with what I have read, which is that the maximum buoyancy adjustment that can be done with the lungs is in the range of 10 lbs. But of course, that number is dependent on diver size. A 5', 100 lb woman is not going to have the buoyancy resources that a 220 lb ex-football player will have with his breathing. I've talked to a number of female diving friends about this, and we all agree that the guys can initiate an ascent much faster than we can, simply by inhaling.

Absolutely. People are not all the same size. Anatomical difference will occur. Generally speaking, smaller people will have smaller overall lung volumes.

Often you can just exhale and hold the exhale for a few second to start the ascent and then let inertia take it the rest of the way, then inhale normally and ride back up on the other side of the duck under with no need to add and then immediately dump gas from the wing.

However if you have to maintain the change in buoyancy more than a breath or two, it is more efficient to make the change using gas in the wing. If it is seriosuly disupting your breathing pattern or requiring you to take very small and frequent breaths, it is probably counter productive.

I interpreted a "sawtooth" profile as something that looks like a sawtooth when reviewing the depth profile recorded by a dive computer/logger. Often, the default sampling interval for such devices is 20 seconds. In order for a computer to record a sawtooth profile, assuming a normal respiratory rate, the diver would almost certainly have to take "more than a breath or two."

 

[Bubbletrubble]

Interesting thread, and one I think some DM's I've dived with should read. Twice now I've tagged along on a dive run by the LDS with a newish DM where one of the clients got low on air at the turn-around (didn't signal 100bar because he didn't want to upset anyone). Both times the DM turned the dive and headed back to the exit point at maximum speed which I always assumed was the wrong response as I assume resistance and hence workload increases with the cube of the swimming speed. My air-consumption certainly held out that theory, it was highest on those two dives. Plus I thought it was a bit risky if you start lagging behind your buddy because you're struggling to keep up.

@sydney-diver: I don't understand why you were at risk of lagging behind your buddy. Good buddy diving principles dictate that one moderates his speed so as to be in close proximity with his buddy. I'd just let the DM rush back to the exit point and make my way back (next to my buddy) at a leisurely pace. No need to hurry, right?

Question though..
If you keep filling and dumping 1L of air into your BC, isnt that as ineffiecent as inhaling and exhaling the same 1L of air in your lungs?
Im not saying that using the BC is infact as inefficient, but the degree of gain from it would be variable depending on how often you need to fill/dump air from the BC as opposed to your lungs, wouldnt it?

@Tigerman: Recall that there is a time delay between the addition of gas to your "system" and when the buoyancy effect of that gas addition is realized. If you're breathing normally, you should be able to use this time delay to maintain a neutral position in the water, i.e., as you begin to get a little floaty you exhale, as you begin to feel "heavy" you inhale. It's really not difficult, but it does take some practice. Dumping/filling your BCD with 1 L of gas to compensate for normal respiratory tidal swings is completely unnecessary.

 

[H2O 70]

I have no idea how much buoyancy change a person's lungs can produce. Ten pounds? Can anybody give some examples or ranges of typical lung volumes? How could I measure mine?

You can get a Pulmonary Function Test from your Dr.

If I'm reading my results right, mine was 5.23L.

 

[ucfdiver]

If you're cutting your gas reserves so thin that you have to think about what speed you swim (and can't just swim however "feels right"), you're already cutting it way too close. This exercise might show your SAC difference in CALM situations at best. Go buy a scooter or extra stage bottle, why dictate your exit speed by cutting reserves that thin?

 

[Sas]

I haven't done any kind of testing as stringent as described by the OP but I have done a few calculations regarding SAC. My base SAC rate seems to be 9L/min, usually this works out to 11L/min on a calm dive with no exertion, 15L/min on a stressful or deeper dive. When I am swimming as fast as I can this jumps to 33L/min. (I tested these in about 5m of water) I think I need to redo these with distance covered too

 

[Tigerman]

...

@Tigerman: Recall that there is a time delay between the addition of gas to your "system" and when the buoyancy effect of that gas addition is realized. If you're breathing normally, you should be able to use this time delay to maintain a neutral position in the water, i.e., as you begin to get a little floaty you exhale, as you begin to feel "heavy" you inhale. It's really not difficult, but it does take some practice. Dumping/filling your BCD with 1 L of gas to compensate for normal respiratory tidal swings is completely unnecessary.

I think you may have missinterpreted me or me not being clear enough..
Im not talking about normal bouyancy swings from breath. Im talking about compensating for bouyancy as you ascend/descend, which is where youd need to take bigger breaths, exhale more or use the BC.
In theory filling 2L extra air in your BC or 2L extra air in the lungs should mean youve removed the same ammount of air from the tank. If you dump it from the bc at the same rate as you would breathe it out, the total air consumption shouldnt be different?
The benefit is of course that with the BC you normally wouldnt exchange the air at the same time, but if you did it should be the same effect on the air consumption?

 

[Bubbletrubble]

Im not talking about normal bouyancy swings from breath. Im talking about compensating for bouyancy as you ascend/descend, which is where youd need to take bigger breaths, exhale more or use the BC.

Actually, you should never have to take "bigger breaths" unless you want to transiently affect your buoyancy. One should try to breathe normally during all phases of a recreational dive.

In theory filling 2L extra air in your BC or 2L extra air in the lungs should mean youve removed the same ammount of air from the tank. If you dump it from the bc at the same rate as you would breathe it out, the total air consumption shouldnt be different?
The benefit is of course that with the BC you normally wouldnt exchange the air at the same time, but if you did it should be the same effect on the air consumption?

The BCD is there to compensate for changes in:

• The buoyancy of your exposure protection due to changes in ambient pressure.
• The buoyancy of your tank as gas leaves it.

It's that simple.

If you are properly weighted, your BCD should be, for all intents and purposes, completely deflated as you get very close to the surface on a near empty tank.

A buoyancy swing of 2 L is greater than the normal tidal volume of the average person.
Gas/air consumption, as I use the term, refers to gas/air as it leaves the tank.
I find your comments confusing. Can you clarify your last post?

You may be talking about oral inflation of one's BCD vs. using the power inflater and the effect of this on overall gas consumption. If so, then, yes, in theory, that's a legitimate method of conserving a little gas. In practice, it saves only a very modest (probably negligible) amount of gas. It takes more effort to exhale against resistance into a BCD than it does to exhale into the surrounding water (as you would normally).

 

[Bubbletrubble]

In theory filling 2L extra air in your BC or 2L extra air in the lungs should mean youve removed the same ammount of air from the tank. If you dump it from the bc at the same rate as you would breathe it out, the total air consumption shouldnt be different?
The benefit is of course that with the BC you normally wouldnt exchange the air at the same time, but if you did it should be the same effect on the air consumption?

OK. I re-visited your post and I think I understand what you are trying to say now.

If you modify your inspiratory reserve and expiratory reserve volumes, then you will be sacrificing optimal gas exchange in favor of holding a larger expiratory reserve volume inside your lungs for buoyancy compensation at depth. This is not a good trade-off over the course of the dive; you will be wasting significantly more gas.