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

[Cave Diver]

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?

There are a lot of things, IMO that you're leaving out here Lynne.

I bolded the parts that stand out to me. What is a "big dive?" Multiple bottles? If so, I could easily see that the extra drag would have an impact on gas consumption vs. your normal recreational dive numbers. Seems like an apples to oranges comparison to me.

Another thing to take into account is if you're calculating gas usage from multiple bottles. More opportunities for a gauge variance of a mistake in calculations.

 

[nadwidny]

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.

Wow. You got that out of my post?

Ya know, sometimes a dog is just a dog.

 

[Kevrumbo]

I always have a consistent 30% increase in my nominal RMV in cold water versus warm tropical water. Typically it's 15 litres/min in 28deg C waters, while here in SoCal temperate waters 15-18deg C, it increases to 22 litres/min.

Respiratory and other responses in subjects immersed in cold water -- Cooper et al. 40 (6): 903 -- Journal of Applied Physiology

 

[BabyDuck]

not gonna lie - it would have to be a huge difference for me to notice.

 

[TSandM]

No, it was just one bottle (that's all I do) but it was a square profile dive on a wreck in very dark and murky water. Although the point about drag is a reasonable one -- I'll have to look back over the dives I've done carrying a deco bottle and see if it makes that much difference. But even then, what surprised me was that I wasn't aware of breathing any harder. I'd have thought a 30% increase would be noticeable.

 

[rjack321]

I find that my consumption increases the closer I am to my depth "limits" and the novelty of the dive. Also midwater ascents can chew up some extra gas for me esp. if I have alot of bottles - I tend to continue to do more lung control than I should. I usually notice my breathing way moreso than my guage so I don't really calculate 20% vs 40% more etc. In fact I rarely calculate "SAC" at all anymore. That said, I also did the PB4Y a few weeks ago for the first time in a few years and my consumption was <finally> down to "recreational". In fact it was so low esp on deco that I mentally pinched myself to remind myself that it wasn't really a recreational dive.

The fact that this was your first dive on the PB4Y, second(?) dive to 150ft with EAN50, in Lake WA, free ascent, etc etc basically its all expected & normal. That's probably where Brian's coming from even though I don't think he knows the dive.

I'd have thought a 30% increase would be noticeable.

I would be surprised if you were able to notice this during this dive.

So I'm wondering how much my breathing has to increase before I'll become aware of it during the dive.

Only you know the answer to this.

Anybody have any ideas of where they notice increased consumption?

I'll know its gone up but by how much I won't know until afterwards. When its gone up, really the last thing on my mind is recalculating SAC. You can't make any more gas on the dive anyway, you're better off not worrying about exactly how much its risen and addressing the root causes of why its gone up.

 

[giles45shop]

I suspect that a 30% difference in RMV is probably within the normal variation of diving, so you probably wouldn't notice it. To take Joel's example above, just the difference between 0.3 & 0.4 is 25%, and between his extreme examples you are talking 300%. Obviously, you would notice that, because you are doing work.

I would be suprised if you did the "exact" same dive multiple times and did not have a variation of +/- 0.05 to 0.1 RMV, and if your RMV is normally pretty low you are still dealing with a 20-30% change.

Adding to that, 25 minutes is a relatively short time period, which also further exacerbates the "averaging"

John

 

[MXGratefulDiver]

Knowing you, Lynne, that 30% increase was due to a proportional increase in brain cell activity during the dive ... plus a bit more for the additional head-turning you put into the dive trying to keep track of everything ...

Seriously ... if you perceived it as a "big dive", then almost by definition you were putting some additional stress on yourself as opposed to your "usual, cruising recreational dive". Stress alone could account for a 30% increase in gas consumption. That you didn't notice it is a symptom of task-loading ... you had more important things to worry about.

Do those dives a little bit more often and soon there won't be any noticeable difference in your consumption rate between the "big" dives and the "usual" ones.

... Bob (Grateful Diver)

 

[DA Aquamaster]

Like others, I can measure a slight increase in RMV on new/stressful dives. But as you noted, actually noting it on an SPG will occur fairly late in the dive unless the increase is huge - and since I norally fundge the gas plan a bit on "bigger" dives, I notice it as I am on plan instead of running a little fat on gas.

I know that I breathe more in very cold water, a breathe more unders stress, and obviously I breathe more when I am working harder, so all three will cause be to pad the gas plan a bit.

The other thing to consider is all the other things that may happen on the dive that could increase consumption. For example, lauching a 100 lb lift bag from depth will require about 12 cu ft of gas. If you are diving double 100's and using around 2/3rds, that 12 cu ft is nearly a 10% "increase" in gas consumption all by itself.

 

[lamont]

isn't a 30% increase in your RMV still like 1/3rd of my normal RMV?

i'd expect to see a 30% increase on a big-to-me dive, but it'd just make me more conscious of checking gas and staying within gas limits, i wouldn't expect to really be conscious of that difference. generally i don't start worrying about gas consumption issues until i encounter some significant stress on a dive -- then you want to try to fix the stress and change the dive plan to compensate.