How much can your gas consumption increase before you notice it?

[TSandM]

I did what was for me a big dive yesterday. I thought I was pretty relaxed, and the dive went well. But I sat down and figured out my ending gas consumption and it was about 30% higher than my usual, cruising recreational dive numbers. It wasn't high enough to trigger an "OMG what's going on?" reaction while the dive was happening, but it was a definite increase. What interests me is that I was completely unaware of it.

I HAVE done dives where I was stressed and knew it, and blew through gas (my first practice dive running line comes to mind). So I'm wondering how much my breathing has to increase before I'll become aware of it during the dive.

Anybody have any ideas of where they notice increased consumption?

 

[Rick Inman]

Not sure if this is a bit off topic. With my CCR, as I work harder I metabolize more o2, and so I have to add O2 into my loop more frequently, which I consider a gas consumption increase. This is normal, and I don't usually take notice of it much. But yesterday we were diving a new lake shallow, the vis was bad and the water was warm, so we were really just mosey-ing along real slow and relax, and I "noticed" how much my consumption had decreased. Since I fly my eCCR manually, I inject O2 as the PPO2 drops, so I keep a pretty good eye on it. And yesterday my PPO2 was so slow to decrease that I started wondering if I was having a sensor malfunction. Of course, there are 3 sensors and 2 unique computers giving me the same O2 reading, so that wasn't it. I was just metabolizing very little O2.

Made me wonder why it's not that low all the time...

 

[Cave Diver]

Not sure if this is a bit off topic. With my CCR, as I work harder I metabolize more o2, and so I have to add O2 into my loop more frequently, which I consider a gas consumption increase. This is normal, and I don't usually take notice of it much. But yesterday we were diving a new lake shallow, the vis was bad and the water was warm, so we were really just mosey-ing along real slow and relax, and I "noticed" how much my consumption had decreased. Since I fly my eCCR manually, I inject O2 as the PPO2 drops, so I keep a pretty good eye on it. And yesterday my PPO2 was so slow to decrease that I started wondering if I was having a sensor malfunction. Of course, there are 3 sensors and 2 unique computers giving me the same O2 reading, so that wasn't it. I was just metabolizing very little O2.

Made me wonder why it's not that low all the time...

How old are your sensors? Are you sure they're just not aging and getting slow to react? Were they all bought around the same time or do you stagger them?

 

[Rick Inman]

How old are your sensors? Are you sure they're just not aging and getting slow to react? Were they all bought around the same time or do you stagger them?

I stagger them sequentially. Right now one is at 9mv, one at 12mv and one at 14mv.

 

[Cave Diver]

Maybe you've been diving too much lately and you're just O2 saturated.

 

[TSandM]

Hmmm . . . nobody with some personal observations on the original question?

 

[Dsix36]

I know that when I do a "big" dive, my SAC is increased even if I am calm and relaxed. I think it is just a mental think floating in the back of my mind. My SAC rate will go back down to normal as soon as I begin to ascend/exit and get back into my comfort level.

 

[nadwidny]

Hmmm . . . nobody with some personal observations on the original question?

Maybe the silence is your answer.

 

[TSandM]

Maybe the silence is your answer.

Knowing you, Brian, there's some kind of sting in that . . . and I know the answer is, "Since you know what your normal consumption is, you should be able to to detect an increase when your pressure, when you check it, is less than you expect." However, I figured out that the increase I was looking at was about 10 psi/min -- so over a 25 minute dive, I'd have to be well into the latter portion of it before the difference was great enough to be more than the imprecision in my reading of my gauge.

 

[JS1scuba]

Hmmm . . . nobody with some personal observations on the original question?

Situational awareness. I can pretty much know when the RMV will go up depending on how much work I am about to do. But it does average back out. On a typical dive I might run .30 or .40 cuft per min just doing nothing but tooling around. If there is a current and I need to use the legs it will jump up to .50-.60 cuft per min -- then on a major hump it can move up to 1.0 cuft etc and the once I feel that ....... stop, pause, and reset.

If i have to work I will take a pause before the work and try to drop the RMV before starting the work.

But ....... even with a very low average RMV i will always plan the dive assuming 1.0 cuft per min / surface value. IT builds in a VERY big safety margin.

Cheers
JDS