Average Gas Consumption?

[RTee]

Just came back from a dive in 66F water. A nice drift dive in the St-Lawrence river going through a flooded lock system using a 100 cft tank. I wore a 7mm + Bare Chicken vest. Average depth was 23 ft, dive time was 63 mins, last 10 mins swimming through weeds to get back out. For the entire dive I used slightly less than 54 cft and got out with 1500 psi remaining...enough for an additional 30 to 40 mins at that depth.

 

[temp1e]

if you're using more air it doesn't come without a reason. you're certainly making mistakes. Ask a veteran diver to watch you closely. He'll certainly point your errors being metal or lacking a skill.
One thing that I feel is the most important is you need to be very well balanced. If your jacket air level is not balanced you compensate the fact with the fins which is a lot of energy and gas consumption respectively.

 

[LarsB]

Here's an interesting kink. From a dive log of mine (from an air integrated computer, regular air) showing the depth profile and varying sac rate, it appears that SAC (red line in pic) may be a function of depth (purplish area). Moreover, SAC rate improves as I go deeper.

Are you diving with Nitrox? The increased O2 partial pressure might explain why you consume less volume of gas.
Just my $0.02.

Another important factor which influences air consumption is body size/weight. Or what do you think?

I am male, 189 cm, and currently weigh around 110 kg. I attached my SAC calculations for your convenience.

Cheers,
LarsB

 

[ianr33]

Are you diving with Nitrox? The increased O2 partial pressure might explain why you consume less volume of gas.

Nitrox will not affect SAC rate.

 

[DA Aquamaster]

Are you diving with Nitrox? The increased O2 partial pressure might explain why you consume less volume of gas.
Just my $0.02.

Another important factor which influences air consumption is body size/weight. Or what do you think?

I am male, 189 cm, and currently weigh around 110 kg. I attached my SAC calculations for your convenience.

Cheers,
LarsB

Way back in the day when nitrox first started being accepted as mainstream I heard this argument alot, particularly among the PSDs we worked with who had just enough EMT traiing on O2 use to be dangerous in terms of not understanding the CO2 side of the equation.

The problem is that regardless of the FO2 or PO2, your body still has the same metabolic requirement and for every liter of O2 it uses, it produces about .8L of CO2. That is significant as it is the CO2 levels that drive the urge to breathe. If you supress that urge and artificially reduce your respiration rate to reduce your gas consumption, you will succeed only at the expense of elevated CO2 levels. Elevated CO2 levels tend to increase the effects of narcosis (as CO2 is quite narcotic at depth) and in addition CO2 levels are associated with increased risk of ox tox.

In the 1950's when the US navy first experimented with enriched air mixes, they used a sample composed of divers with extensive hard hat diving experience who for the most part had developed a high tolerance to high CO2 levels in the sometimes poorly ventilated Mk 5 helmet. The end result of that, combined with higher PO2s than we consider prudent now, was an unacceptably high rate of somewhat unpredictable incidences of oxygen toxicity. The navy concluded that enriched air was not worth the trouble in terms of US Navy operations and dropped it - delaying the adoption of nitrox in civilian diving for about 35 more years.

----

In short, even apart from nitrox diving, artificially lowering your respiration rate to improve your SAC will increase your CO2 levels. In addition to the narocis effects noted above and any anxiety that may come with a "dark" narc, high CO2 levels can significantly increase anxiety since you are in effect trying to short circuit an automatic response in your brain, and that anxiety creates muscle tension and increases in metabolism that can increase O2 use and CO2 production. It's all bad news with no upside, so don't do it.

You are far better off reducing SAC by reducing your metabolic requirements and that can be accomplished by reducing all uneccesary muscle movement or tension and by just relaxing on the dive.

 

[matthewmark_john]

I wonder whether an overcompensated first stage might be the reason for this behaviour?

I don't think so, as my reg is an Atomic B2, which I'm pretty sure is a true balanced 1st. Also, if I understand it correctly, overcompensated/overbalanced 1st stages deliver more air at depth, which is the opposite of what happens in my observations.

 

[matthewmark_john]

Are you diving with Nitrox? The increased O2 partial pressure might explain why you consume less volume of gas.
Just my $0.02.

Another important factor which influences air consumption is body size/weight. Or what do you think?

I am male, 189 cm, and currently weigh around 110 kg. I attached my SAC calculations for your convenience.

Cheers,
LarsB

LarsB, thanks for your numbers. To answer your question, yes and no -- I only use nitrox on about 10% of my dives. The pattern I mentioned shows up regardless of gas mix, however.
Certainly I agree that metabolism, which is affected greatly by body size (and especially tissue composition proportions, i.e., fat to muscle), accounts for a good percentage of the variance in SAC rates. I'm 5'10" (178 cm) and 175 lbs (79 kg), if that helps.

Just to be clear, overall SAC rate is not the point of my observation. It's the reliable change in SAC rate within a single dive -- specifically, SAC rate gets lower or higher as depth decreases and increases, respectively -- that I found noteworthy, and I'm wondering a) if this happens to others, and if so, b) if anyone has any clues to explain it.