Do you avoid people with bad sac rates?

[mikerault]

Does breathing rate stay constant? The total O2 contained in each breath as depth is greater increases due to compression, now, can our lungs take advantage of that and lower our "average" breath rate? If so what affect will this have on apparent SAC rate?

 

[berick]

SAC rates calcs shouldn't decrease(or increase) with depth unless one of the following happens:

1. You're incorrectly calculating the average depth of the dive -- it should be weighted by the time spent at each depth.
2. Your data was collected over time and depth is somehow correlated with a factor which has
an impact on SAC (As your experience increased you dove deeper)

The math and physics just don't allow for it.

 

[vondo]

I have to back up Rick Inman's post.

ALL OTHER THINGS BEING EQUAL

A person breathes at a relatively constant rate.
The rate at which a person breathes is generally expressed in cubic feet.
So, if a person, while at the surface (1 ATA), requires a volume of 0.5 cu. ft. of per breath, his SAC rate is said to be 0.50 - SAC=0.50 cfm.

So, if this diver were sitting at the surface breathing from his tank, he would require 0.50 cubic feet of gas per breath.

Now if this same person is doing a dive at 100' and is perfectly relaxed as he would be at the surface, then his SAC (read "surface") rate would still be 0.50 cfm.

BUT the VOLUME of gas he would consume from the tank each breath would be 2.0 cu. ft. instead of .50 cubic feet because the ambient pressure upon the body is 4 ATA.


the K

I don't think anyone is disputing this.

What I notice in my own (limited) data is that my SAC (defined exactly as you quote) increases slightly as I go deeper, even dropping out dives with lots of current and so forth. This is presumably due to nerves. I also see that my "worst" dives are often very shallow ones. This could be for any number of reasons: differences between the depths of my wrist and lungs (magnified), surfacing to look for the boat not being included in the dive time, surge, no relaxing SS, more effort spent compensating for less than perfect buoyancy. I don't know what the reasons are.

Back to the original topic. I said 1.00+ in the poll. My SAC is around 0.6, I think. I almost always am paired up as an insta-buddy. I would never "quiz" someone on their SAC and if I was stuck with an air-hog, I might try to switch insta-buddies so the two of us are better matched, but that's about it. Hasn't actually happened yet.

I did once, refuse to switch buddies with someone on a liveaboard. The person they were trying to "stick" me with was a huge airhog and a panicky diver with way more dives than me and no interest, apparently, in learning how to dive well.

So, "avoid," yes. "Refuse," no.

 

[under water]

SAC rates calcs shouldn't decrease(or increase) with depth ..
.... The math and physics just don't allow for it.

Ahhh , but it isn't just about math & physics, it's also about physiology, psychology and probably some other ology's. I know that my wife breathes harder when she is deeper (getting her some new regs to help with that) and I know that slowly hovering along a 20 foot sunlit reef is a lot more relaxing. It's hard for me to not believe that this factors into how efficient our breathing will be.

 

[ClayJar]

Does breathing rate stay constant? The total O2 contained in each breath as depth is greater increases due to compression, now, can our lungs take advantage of that and lower our "average" breath rate?

Normal breathing in normal, healthy people is not limited by oxygen. It's limited by carbon dioxide. Doing work (both the "background" work of being alive and the obvious work of moving around) consumes oxygen and generates carbon dioxide. For the same amount of work, you generate the same amount of carbon dioxide (regardless of ambient pressure).

SAC rates calcs shouldn't decrease(or increase) with depth unless one of the following happens:

1. You're incorrectly calculating the average depth of the dive -- it should be weighted by the time spent at each depth.
2. Your data was collected over time and depth is somehow correlated with a factor which has
an impact on SAC (As your experience increased you dove deeper)

The math and physics just don't allow for it.

Two of the most obvious ways SACs vary with depth (in real life situations) are depth-temperature and depth-stress correlations. SAC may vary based on ambient temperature, and ambient temperature may be a function of depth. SAC may vary based on stress, and stress may be a function of depth (due to narcosis, just feeling too far from the surface, or perhaps even inversely: if the deeper water is clear, with a top layer of limited viz, a diver may be more relaxed at depth).

I've actually seen (in myself and others) strongly temperature-dependent variations in air consumption rates when we hit that 50°F thermocline at 85 feet down. When I'm diving dry and my buddy's diving wet, I can actually almost catch up air consumption wise. Then we return to the relative warmth above the thermocline, she warms up, and I end up thumbing the dive. :biggrin:

 

[mikerault]

The averages for my plot (the biggest one) come from my dive computer, I hope it is doing it right, I wear two and compare them after every dive (a Citizen Cyber Aqualand NX and a Cochran Commander. Before you weigh in on this have you:

1. Calculated your SAC for over 50 dives for either your max depth, average depth or both
2. Arranged those by depth
3. Plotted those results as SAC verses depth and if you have it, average depth
4. Done a least-squares linear fit to the data to derive a trend line?

If not then you really don't have a clue what is going on with your SAC rate verses depth and I wish you wouldn't silt up the water with assumptions.

To paraphrase Mr. Gooding..."Show me the data!"

If you can't defend your statements with data then you really have no say. I've posted three divers data, using standard SAC and other calculations, each shows the same decrease in SAC with depth, what do you have to prove me wrong?

Mike

 

[ClayJar]

Calculated your SAC for over 50 dives for either your max depth, average depth or both

Using anything but average depth will give you an incorrect answer, by definition. That much is an absolute certainty, as maximum depth must be greater than average depth for any dive with finite time spent between the maximum depth and the surface.

 

[mikerault]

yep, I agree.

 

[Meng_Tze]

..I've posted three divers data, using standard SAC and other calculations, each shows the same decrease in SAC with depth, what do you have to prove me wrong?

Mike

Not disputing the trends, again, my question is Why is this so?

 

[berick]

Yes, I've calculated SAC rates for more than 50 dives. The variables that make my sac rates vary are temperature, effort, and anxiety.

Here are 2 fairly recent back to back dives: At an average depth of 62 feet in Cozumel (120' max) I had a .38 SAC rate. Next dive, at an average depth of 18 feet
(20' max) I hadd a .38 SAC rate. Water temperature the same 84F (minimum).

The question is whether SAC rates should vary by depth -- implicitly assuming all other variables are constant. I thought I pointed this out when I stated case 2 -- some other variable correlated with changes in SAC rates.\

Oh, just a little lesson on science. Logic (math) can prove something. Data can only suggest something but not prove it. As a computer programmer, you should know this.

If you feel confident your data is correct, I would suggest you look for some other correlated factor that varied by depth to explain the decreasing SAC rates by depth. If you can provide a theoretical basis for the SAC rates declining with depth, I'm sure you'll be able to become famous.