SAC versus depth

[TSandM]

Buoyancy at shallow depths does use more gas . . . But I also wonder if the way the computers calculate average depth plays a role in this, also.

 

[rjack321]

You've seen a six-gill shark? Sorry... don't mean to hijack the thread... Just thought six-gills were *very* deep water animals... Extremely rare sighting even at depth a technical diver would take seriously...

They are deep water animals - but fairly routinely sighted in recreational depths (80-110ft) up here, depending on location and time of year mostly. Some places quite a bit shallower.

One lure is our local aquarium is tagging them and taking biopses. They use salmon frozen carcasses in ~60ft.

They are not violent towards divers, at least haven't been yet.

 

[pickens_46929]

Let's not forget the narcosis either... if you're anxious or something, a tad bit of narcosis just might relax you ... (kidding somewhat here)

I think the two biggest factors are 1) takes more gas at shallow depths for bouyancy control; 2) most of my deep dives aren't the first dives of the trip so I've had a bit of time to dial-in my bouyancy and comfort.

(oh and narcosis too... )

 

[Kendall Raine]

Let's not forget the narcosis either... if you're anxious or something, a tad bit of narcosis just might relax you ... (kidding somewhat here)

I think the two biggest factors are 1) takes more gas at shallow depths for bouyancy control; 2) most of my deep dives aren't the first dives of the trip so I've had a bit of time to dial-in my bouyancy and comfort.

(oh and narcosis too... )

SAC is a function of breathing rate and factors effecting breathing rate drive SAC. Stress, both physical and emotional, is far more important than buoyancy. In fact, buoyancy related SAC fluctuations are effectively non-existent for even reasonably experienced divers.

Factors effecting stress include:

1. Visibility
2. Depth
3. Work load
4. Temperature
5. Equipment load
6. Task loading

Autocorrelations between task loading, work load and equipment load are obvious. These are physical stressors. Inadequate thermal protection is another contributor to physical stress.

Low visibility, narcosis, and anxiety caused lack of familiarity with conditions and equipment are emotional stressors.

Point is complex deco dives in cold high flow current involve much higher stress, both physical and emotional than shallow dives in warm clear water.

Experience in the specific environment lowers stress and drives down SAC. Improved physical conditioning lowers physical stress. Lower physical stress lowers emotional stress.

 

[Bobbin-along]

Nicely said Kendall, it's what I was trying to say when I said SAC was not a function of depth but of other factors both physical and mental. You just said it better.

And yes we get Six gills in Seattle in the 110-90fsw range. They are so amazing that you can't help but get excited at seeing them.

 

[rjack321]

In fact, buoyancy related SAC fluctuations are effectively non-existent for even reasonably experienced divers.

I suspect you have misinterpreted TS&M and my comments...

An average depth of 33ft plus or minus 10ft will use more gas (normalized to 1ATA) than an average depth of 99ft plus or minus 10ft.

You use 2 ATA and 4 ATA for your SAC calculations but the relative standard deviation (RSD) around those average depths is more important at shallow depths.

Lets just assume the standard deviation of these averages is 10ft = 0.3 ATA.

RSD for 33ft dive = 100*0.3/2 = 15ft
RSD for 99ft dive = 100*0.3/4 = 7.5ft

Proportionally minor depth fluctuations are more significant in shallow water. So you are forced to make bouyancy changes more often. These use gas which increases your calculated SAC for shallow water dives.

Equivelant depth changes in deep water are not significant and do not force the use in the BC.

Deepish dives are better measures of your body's use of gas since there's less influence from your BC. All dives are a better measure of your typical SAC since this value incorporates more than just what goes in and out of your lungs.

 

[Kendall Raine]

I suspect you have misinterpreted TS&M and my comments...

An average depth of 33ft plus or minus 10ft will use more gas (normalized to 1ATA) than an average depth of 99ft plus or minus 10ft.

You use 2 ATA and 4 ATA for your SAC calculations but the relative standard deviation (RSD) around those average depths is more important at shallow depths.

Lets just assume the standard deviation of these averages is 10ft = 0.3 ATA.

RSD for 33ft dive = 100*0.3/2 = 15ft
RSD for 99ft dive = 100*0.3/4 = 7.5ft

Proportionally minor depth fluctuations are more significant in shallow water. So you are forced to make bouyancy changes more often. These use gas which increases your calculated SAC for shallow water dives.

Equivelant depth changes in deep water are not significant and do not force the use in the BC.

Deepish dives are better measures of your body's use of gas since there's less influence from your BC. All dives are a better measure of your typical SAC since this value incorporates more than just what goes in and out of your lungs.

I understood your point the first time.

I also understand the rudimentary physics involved.

As anyone with experience will tell you, minor buoyancy adjustments, either at depth or shallow, are made without using the BC at all-unless you're using a rebreather.

 

[rjack321]

I understood your point the first time.

I also understand the rudimentary physics involved.

As anyone with experience will tell you, minor buoyancy adjustments, either at depth or shallow, are made without using the BC at all-unless you're using a rebreather.

Sure, but the definition of minor changes with depth. At 10ft on a free ascent in doubles, minor is <6inches. At 100ft on the bottom minor is like 10ft. The former uses gas, the latter does not. The use of gas for bouyancy drives up your calculated SAC.

You can lay off boasting of your vast knowledge and experience.

 

[Bobbin-along]

Let's all be nice, and take a deep breath. We aren't at a Big 8 summit to solve world hunger and poverty in the next 12 months, we talking about diving. We've all got a good excuse, it's the holidays, the one time of year where even commuters are nice to each other.

There are a number of points everyone is agreeing on, just variation in how important of a variable it is to them in their equation.

 

[Kendall Raine]

Sure, but the definition of minor changes with depth. At 10ft on a free ascent in doubles, minor is <6inches. At 100ft on the bottom minor is like 10ft. The former uses gas, the latter does not. The use of gas for bouyancy drives up your calculated SAC.

You can lay off boasting of your vast knowledge and experience.

You don't get it at all. Not only is the buoyancy issue a red herring, but your analysis is backwards. The amount of gas needed to offset wetsuit compression, combined with the gas density multiplier effect from pressure means achieving neutrality at depth uses far more gas deep than shallow. In either case it's a rounding error. Minor buoyancy adjustments are done with the lungs, not the BC. Try it sometime, if you can find time away from the keyboard to go diving.

As for my boasting, I don't know what you're talking about. That's no surprise since it's obvious neither do you.

Last word is yours. I'm done with you.