SAC versus depth

[mikerault]

When I look at a plot of SAC verses depth for a profile of 85 dives for myself I get a decrease in SAC with depth. Does this make sense? Other divers are telling me that SAC for a given exertion rate should be a constant regardless of depth, but is that really true?

 

[vondo]

I think my SAC increases slightly at depth because I am more nervous and breath faster. I think I saw this in another thread and it wasn't clear if you were using avg. depth to calcuate SAC or maximum depth. You should use average.

 

[mikerault]

I did it with both and saw the same trend with average reporting higher SAC. I wish the dang attachments worked I would post a PDF of the spreadsheet showing the results.

Mike

 

[Bobbin-along]

The change in SAC is not a function of depth, but rather a function of you and all your wonderful variables. If you are excited at the beginning of a dive your SAC with show it in the samplings near the beginning. If you get totally into the zone while at depth it usually lowers, if you are swimming out to a location and back then expect your SAC to be higher on the descents and ascents.

For me, I am usually a little spun up at the beginning of a dive (moreso depending on who I'm diving with and my mental state at the beginning of the dive). Then remember you are adding air to your BC and drysuit (if applicable). If I'm diving with the boys it's my worst period of consumption as they swim like their fins are on fire to get out to the location we plan to go.

Once I get out to where we are going exploring it can go up or down depending on my exertion or how excited I get when finding something and wanting to share what I found with others. Six gill, GPO, or antique bottle sightings? It goes up as I'm like a 4 year old in Toys R Us. The return is ALWAYS my lowest SAC period, my breathing has slowed down dramatically, I'm usually just drifting with the current floating along never exerting myself, and very relaxed. All my fretting just melts away.

 

[mikerault]

If I were basing this on just one dive or two dives maybe I would agree, but this is data from 78 dives spanning 2 years of time so those types of variables should be evened out. I would be interested to see folks go back over their logs and plot their SAC verses MAX and Average depths according to depth. If anyone is interested I will provide a spreadsheet just PM me.

Mike

 

[rjack321]

On shallow dives you use proportionally more gas for bouyancy. Thats where the bigger corrections are for depth changes. That drives up your gas usage and consequently your SAC calculations.

On deeper dives once you have bouyancy established you don't have to fix it much, eg. a 5 ft change is easy to not bother with.

 

[spectrum]

mikerault

Personally I have not noticed a big difference. My wife whose SAC could be considered a little high has had some of her nicest numbers on deeper dives. Maybe she's a little narked and that relaxes her, I duno.

Pete

 

[bperrybap]

I did it with both and saw the same trend with average reporting higher SAC. I wish the dang attachments worked I would post a PDF of the spreadsheet showing the results.

Mike

The SAC calculation definitely uses Average depth.
You can use this link to check your calculations: SAC Calculator

This is what I've seen for my SAC rates:
I went from about 10 dives to nearly 100 this year.

The trends relating to SAC rate that I've noticed:
- My average SAC rate is going down with more experience.
(started around 0.6 now averaging about 0.45)

- My SAC tends be lower when the water is warmer than 80 F.

--- bill

 

[BKP]

Six gill, GPO, or antique bottle sightings? It goes up as I'm like a 4 year old in Toys R Us. The return is ALWAYS my lowest SAC period, my breathing has slowed down dramatically, I'm usually just drifting with the current floating along never exerting myself, and very relaxed. All my fretting just melts away.

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...

I can see how that would skew your SAC...

 

[rakkis]

When I indicated my take on it in the other thread, I said "all things being equal". Or at least I should have said that

Like mentioned above, keeping neutrally buoyant typically wastes a bit of air and takes more work closer to the surface where pressure ratios are higher. I would think that a majority of the discrepancy in your data comes from that.