What's a good SAC

[MargaritaMike]

Now that I have a dive computer that talks to my laptop computer, I have been looking at different data on my dive history. One of the pieces that I have been looking at is my SAC. I know what mine is, but I'm curious as to what other people have for a SAC. Is 8 psi/min good? Is 12 psi/min good? Is 16 psi/min good? Obviously, 8 is better than 16, but how does that compare to what other divers have for their SAC?

The question is threefold: 1) What is a good SAC?, 2) What is your average SAC?, and 3) When does your SAC increase or decrease during a dive?

I'm asking SAC. I guess RMV would be a good statistic to track also.

Cheers -

Edit: Added "psi/min" and sentence "I'm asking SAC. I guess RMV …." M²

 

[tmassey]

Mine, in a cold-water quarry while swimming squares is 0.68 cu ft/min. I read somewhere but cannot find or verify that BSAC found average SAC was 0.7. So I’m basically dead average.

Wish I was better: most of my buddies are... But it is what it is: you breathe what you need. Just bring enough gas for it.

Though with >1000 dives, you clearly don’t need to hear that from me.....

Edit to add: For gas planning I use 0.8 for working, 1.0 for contingency (x 2, rock bottom style) and 0.6 for deco. Knowing I have more than I need and I’m not cutting margins close helps me to relax, making the margins even bigger!

 

[Snoweman]

Mine, in a cold-water quarry while swimming squares is 0.68 cu ft/min. I read somewhere but cannot find or verify that BSAC found average SAC was 0.7. So I’m basically dead average.

Wish I was better: most of my buddies are... But it is what it is: you breathe what you need. Just bring enough gas for it.

Though with >1000 dives, you clearly don’t need to hear that from me.....

Edit to add: For gas planning I use 0.8 for working, 1.0 for contingency (x 2, rock bottom style) and 0.6 for deco. Knowing I have more than I need and I’m not cutting margins close helps me to relax, making the margins even bigger!

Does your computer calculate your SAC in ft3/min? I assume you have to put in your tank size for the computer to calculate it for you. I can only see psi/min on my Perdix. Am I missing something?

 

[Bob DBF]

I know what mine is, but I'm curious as to what other people have for a SAC. Is 8 psi/min good? Is 12 psi/min good? Is 16 psi/min good? Obviously, 8 is better than 16, but how does that compare to what other divers have for their SAC?

I guess it would depend on the tank you are using.

RMV works better as it is not tank dependent, so you can leave off the tank size without confusion. The terms are used interchangeably by many, so stating SAC in cuft/min is not abnormal.



Bob

 

[scubadada]

Hi @MargaritaMike

SAC is reported as pressure/time (psi/min) and is cylinder dependent. RMV is reported as volume/time (cf/min) and is cylinder independent. I would suggest converting your SAC to RMV for easier comparisons. I'm assuming you have a Perdix AI or a Teric, that gives you only SAC. Here is the RMV calculation from the Perdix AI manual, p16

Here are the results of 2 SB polls, one from 2009 and one from 2016 asking for average RMV. There were 339 respondents.

The median RMV is 0.5-0.59 cf/min while the mode is 0.4-0.49 cf/min. Of course these polls have all the caveats of being posted on SB, but gives you an idea about the distribution.

Good diving, Craig

 

[tridacna]

Hi @MargaritaMike

SAC is reported as pressure/time (psi/min) and is cylinder dependent. RMV is reported as volume/time (cf/min) and is cylinder independent. I would suggest converting your SAC to RMV for easier comparisons. I'm assuming you have a Perdix AI or a Teric, that gives you only SAC. Here is the RMV calculation from the Perdix AI manual, p16
View attachment 508670
Here are the results of 2 SB polls, one from 2009 and one from 2016 asking for average RMV. There were 339 respondents.
View attachment 508676
The median RMV is 0.5-0.59 cf/min while the mode is 0.4-0.49 cf/min. Of course these polls have all the caveats of being posted on SB, but gives you an idea about the distribution.

Good diving, Craig

Minor nitpicking: True volume on al80 is 77.4 cu. ft. not 80.

 

[Basking Ridge Diver]

The question is threefold: 1) What is a good SAC?, 2) What is your average SAC?, and 3) When does your SAC increase or decrease during a dive?

1) An average SAC / RMV - is around 7 cu ft per min.
2) Mine is .72 for the last 300 dives - the lowest was .377 and the highest was 1.9.
3) It varies based on your exertion and anxiety levels - hunting or new dives sites may influence my rate. I have found the shorter the dive and when shore diving - my SAC goes high. For me several reasons - if I have to inflate my wing and find others that may have been lost - as well as fighting the waves to get out and under the waves is energy that is using air. For me the shallower the dive and the longer it goes the lower the SAC / RMV evens out. If I am not hitting the surface much and I can relax - usually on a solo dive - my SAC is much lower because I am not following / catching up / or concerned about others - only myself. I find I relax a lot more because I go at my pace and I am doing my own thing.
YMMV - just go with it and your SAC / RMV does not need to be low for every dive. There are times if you are diving different environments and for different activities your breathing will change. Don't fight it.

 

[MargaritaMike]

SAC is reported as pressure/time (psi/min) and is cylinder dependent. RMV is reported as volume/time (cf/min) and is cylinder independent. I would suggest converting your SAC to RMV for easier comparisons. I'm assuming you have a Perdix AI or a Teric, that gives you only SAC. Here is the RMV calculation from the Perdix AI manual, p16

I would think that if my Perdix AI says that I am breathing 16 psi/min, it doesn't care what size tank I have. It sees what the tank pressure is at time zero and then at time 1 and subtracts one from the other. Why would it be tank size dependent?
Also, from the PerdixAI manual p. 16 is the following:
SAC has the great property of not requiring any setup, making it the simplest and most reliable choice.The drawback is that it is not transferable between tanks of different sizes.

SAC Formula
The SAC is calculated as follows:
SAC = ((Ptank(t1) − Ptank(t2))/t2 − t1)/Pamb,ATA

Ptank(tn) = Tank pressure at time t [PSI] or [BAR]
t =Time [minutes ]
Pamb,ATA = Ambient pressure [ATA ]
The time samples are taken 2 minutes apart, and Pamb,ATA is the average ambient pressure (i.e.depth) over this time frame.


Since the Perdix AI displays and logs SAC, the formula for calculating RMV from SAC is useful. Knowing your RMV can help with planning dives using tanks of various sizes.

Cheers -

 

[BlueTrin]

Even though I try to track RMV, I convert it in SAC for the tank I use because I can then track my usage underwater, you still have to adjust by the pressure.

(I wish my computer displayed the RMV over the last x minutes)

 

[RainPilot]

My very best sustained RMV was 0.35 cf/min, super chilled watching a cuttlefish for 30 min.

My normal, fun dive RMV is around 0.4

When I am diving with new OW students i get around 0.5

Tech dives with simulated failures etc (high workload) I generally get the same 0.5

I plan on 0.7 and 1.0 for tech dives (normal and contingency) and 0.5 for deco.