SAC Rates - Comparing

[Zeke XA3]

Ok so iv signed up to do about 5 TDI courses with a freind and group of friends.

Today we did a stress test to work out our max SAC rate. Full gear, surface swim , as fat as we can maintain for 10 mins.

I came out with a SAC of around 74l/pm

Is this a usual way to find your SAC, is this what you use to plan and how does it compare to other peoples?

When i planned wih BSAC we took our normal SAC rate rahter than stress, but can see benifits of both now!

Id like to hear some other opinions....

[maxguru]

In DSAT courses, we do it at depth, resting, working, and REALLY working. Does it take the same effort to surface swim as it does to swim at depth? Probably not. I think it's easier to move through the water at depth, but that's just me. Your method may give a worst-case result, which would cause you to calculate a larger gas supply for a given dive. Safer that way, but those worst-case numbers will cost you some bottom time.

Max
DSAT Tec Deep Instructor
198186

[DA Aquamaster]

I have a dive computer that figures average depth, so I track my SAC on most dives by noting the start and end pressures of the back gas and deco gas. It gives me my normal real world SAC on a regular basis and it stays very consistent.

In the past I have periodically done a calculation at depth using a timed swim. I will swim at a fast but steady and sustainable cruise speed - the speed I would use exiting a cave where time was important - for about 5 minutes to ensure the O2 and CO2 levels are fairly steady, then note the time, the depth and the SPG reading. I will then continue at that rate at constant depth for exactly 5 or 10 minutes (5 minutes if I am below 60 ft., 10 minutes if I am above 60 ft.) then note the end pressure. After that ypou just crunch the numbers for the depth use rate, figure the surface use rate.

I am not a big believer in doing some of the odd "at rest" or "kicking in the pool" SAC rates excercises and then taking the number you get times some constant.

[Zeke XA3]

In DSAT courses, we do it at depth, resting, working, and REALLY working. Does it take the same effort to surface swim as it does to swim at depth? Probably not. I think it's easier to move through the water at depth, but that's just me. Your method may give a worst-case result, which would cause you to calculate a larger gas supply for a given dive. Safer that way, but those worst-case numbers will cost you some bottom time.

Max
DSAT Tec Deep Instructor
198186

Yeah this seems to be the planning using our worst SAC rate.
Just glad i came out at less than the smoker and the same as one of the smaller girls, damn the other girl and her small lungs though!

[Kevrumbo]

I do my pre-dive gas planning based on my nominal SAC rate of 22 lpm at the particular depth of interest; for emergency/contingency reserves, I pre-plan using a stress rate of 28 lpm. During the actual dive itself, if I find I'm working hard, swimming against a current for example, I'll add 30% of my nominal rate of 22 lpm and anticipate abbreviating/shortening the bottom time at depth. Any exertion of 50% or more, or if I feel I'm starting to overbreathe my regulator --I'll just abort the dive and start the profile ascent to the deep stops/deco schedule appropriate for that shortened time at max depth. . .

[Zeke XA3]

Kevrumbo

Using your 22 and 28 rates, how much air do you come out with compared to your planning?

We already know we are going to be back on the boat with wayyyy more than the 1/3 planned! But for beginners i think this isnt so bad!

[Kevrumbo]

Kevrumbo

Using your 22 and 28 rates, how much air do you come out with compared to your planning?

We already know we are going to be back on the boat with wayyyy more than the 1/3 planned! But for beginners i think this isnt so bad!
For beginners, it's a good exercise on paper to contingency plan with your stress level of 74 lpm, but are you willing to take with you an extra stage bottle to cover your gas requirements? If you're using double AL80's for backgas (twin 11 litres), you will have to bring an extra single stage 11 L bottle of bottom mix (in addition to any deco gases you're carrying) to cover your Minimum Gas Reserve/Rock Bottom Planning with a emergency stress SAC of 74 lpm (and even with that, it's still not quite enough).

Study the attached spreadsheet with planning for bare minimum gas to get you to the surface from a 30 meter deep dive, with 1 minute stops every 3 meters to the surface, using 28 lpm versus 74 lpm. The important numbers to look for in the spreadsheet is MGR Ltrs (Minimum Gas Reserve in litres): 1540 L is your Rock Bottom bare minimum reserve using a SAC of 28 lpm, while 4070 L is your Rock Bottom reserve using 74 lpm.

[Zeke XA3]

ohh thanks, you make this up yourself? Ill have a go at understanding later when i have time. Just got cut off the internet so in a net cafe!

Although maybe this could be one of our lessons next! That we plan with this and see that we cant take that many tanks and as such learn you cant plan for every type of contingancy???

[roturner]

I got 28 on the stress test. My typical is between 10-12 in my normal diving conditions. In the tropics it's 8 or so but that's pretty easy diving all things considered. The difference might be that I run so my body is conditioned to high levels of effort over prolonged periods. Also, 74 seems improbably high to me. Did you double check your calculations?

In terms of technique, I did the stress test at 10m and calculated it for the surface. Surface conditions can obviously affect the result. In terms of planning, I don't use 28, I use 18 because I've haven't booked a dive hgher than 18 for many years.

R..

[Zeke XA3]

I got 28 on the stress test. My typical is between 10-12 in my normal diving conditions. In the tropics it's 8 or so but that's pretty easy diving all things considered. The difference might be that I run so my body is conditioned to high levels of effort over prolonged periods. Also, 74 seems improbably high to me. Did you double check your calculations?

In terms of technique, I did the stress test at 10m and calculated it for the surface. Surface conditions can obviously affect the result. In terms of planning, I don't use 28, I use 18 because I've haven't booked a dive hgher than 18 for many years.

R..

I used 60 Bar in 10 minutes at the surface with a 12l tank.

60 x 12 = 720 litres of air
720 / 10 = 72 l / pml

When i first learned SAC i was taught the average surface resting rate is 25 l /pm (UKDivers.net - Air calculations (http://www.ukdivers.net/science/aircalc.htm) or Supplied air - capacity (http://www.sea.com.au/suppliedair/capacity.htm)), so under heavy workload, such as a surface swim in full gear as fast as you can for 10m, a 3 times increase in breathing rate seems fine!

[HowardE]

For gas planning; I use 20 for bottom segment and 17 for deco, even though my actual average is 14.5 on the bottom and 10 on deco.

I calculate my SAC (in CFM not L usually) from one of my dive computers. I have a wireless air integrated computer with transmitters on my back and one on my deco bottle.

(I keep a hardwired standard SPG on my back gas as well)

[Slamfire]

We did the exercises as part of our TDI courses. They are very informative but I felt that using SAC rates based on only one dive is not representative enough of your true real-world SAC.

So, sometime before taking the TDI courses, I started measuring SAC rates for all my dives and have never stopped doing so. I do not have an air integrated computer nor does my computer calculate average depth for me. I have resorted to downloading all the depth readings of the computer to excel and then calculating an average depth. I use that in conjunction with the total gas consumption (initial psi minus final psi) and then calculate a real world SAC rate. Based on this I use .7 cu ft / min for bottom and .6 cu ft / min for deco for planning. I feel it's an accurate and slightly conservative real world breathing rate for me.

I am attaching a file with 2 spreadsheets that includes samples of my calculations. Hope it helps.

[Zeke XA3]

Thanks bud. I have some old SAC rates from when i was BSAC but that was drysuit in 7C waters so a bit different to what im planning for.

I agree SAC rates on one dive are very generic. But at the moment it is hard for me to work with any other figures. Of my 500 dives, the overall average depth for all of them is only 8m. And a 40-50 min dive at that average i only use around 1000psi but because i can be up and down with students sometimes making many ascents and its all drift diving so not using much air its hard to use those dives as a comparison.

Thanks for the spreadsheet, i can just modify my own dive log to show this though i work in BAR and liters which i find a lot easy to think in!!! But i know have some more number to compare with ;-)

[Kevrumbo]

Here's Slamfire's spreadsheet with some metric equivalent values appended to his data: