SAC Rates

[DA Aquamaster]

RMV vs. SAC - We don't really want to go there. SAC and RMV are terms that get confused. SAC started out as a reference to a unit of volume exrpessed (in the US) in cu ft per minute, but in nthe 70's and 80's was commonly done in PSI per minute. There is nothing wrong with that as long as the diver realizes it is dependent on tank size as a SAC in different sized tanks will have a different value.

In general though SAC has become the generic term and refers to volume and as such the SPG readings are converted to a volume based on the size of the tank. RMV is a relative late comer that has pretty much alwasy referred to units of volume rather than psi.

In either case, they produce a surface use rate that has to be multiplied by the atmospheres at a planned depth to provide you with a depth use rate.

 

[The Kraken]

DA,

I'd like a little clarification on this, please:

"a SAC in different sized tanks will have a different value"

How is the amount of gas that I breathe going to change during a dive predicated upon the size tank I'm diving.

I can see how the RMV may vary with respect to different sized tanks, but not the gas consumption rate.

the K

 

[DA Aquamaster]

Don't take it out of context (as is the sound byte in your quote) - if you figure SAC in units of psi per minute, then it is very much tank dependent.

For example, let's say I have an RMV of .65 cu ft per minute.

With an AL80 1 psi = .0258 cu ft, so 25 psi = .65 cu ft, giving a SAC in psi per minute of 25 psi per minute.

Now, if I dive an LP 95, the internal volume of the tank is larger so 1 psi = .0360 cu ft. and 18 psi = .65 cu ft. So in this case the same RMV has a different SAC as expressed in psi per minute (18 psi per minute, not 25 psi per minute.)

RMV has for the most part always been presented in cu ft/min or liters/minute, so it is not normally associated with psi per minute.

For a recreational diver, especially one who always dives the same size tank, it may be easier to work in psi per minute as the math is pretty simple. For example, a 25 psi/minute SAC would be multiplied by 4 at 4 ATM giving 100 psi per minute and that is easily envisoned by the diver in relation to the SPG reading and may have a lot more meaning for a recreational diver.

For a technical diver who engages in much more extensive gas planning and may have different sized tanks even on the same dive, working strictly in cu ft/minute or liters/minute makes a lot more sense.

 

[The Kraken]

I guess what it all gets down to is using a SAC rate defined in CuFt/Min is more suitable for planning the amount of gas needed and using a psi/min is more suitable for determining/planning how much time you have on the tank on the fly.

Macro and micro, if you would . . .

the K

 

[Teamcasa]

DA,

I'd like a little clarification on this, please:

"a SAC in different sized tanks will have a different value"

How is the amount of gas that I breathe going to change during a dive predicated upon the size tank I'm diving.

I can see how the RMV may vary with respect to different sized tanks, but not the gas consumption rate.

the K

When any Air Consumption Rate is converted to PSI per minute (vs cubic feet per minute), tank size does make a difference.

 

[geokr]

Just to go a little further with this, outside of these factors would SAC vary directly with body weight? Seems that it would but maybe percent body fat would temper that a bit. And, does age or fitness factor in overall consumption as well? If so, by how much? Enjoying the discussion and am just curious.

 

[The Kraken]

Weight can't be associated as a defining variable in the computation of sac rates.

While I'm sure one could determine certain relationships between displacement and weight but so many other variables come into the human equation, physiology, that the combinations and permutations would be staggering.

One can get into such arcane analyses as the temperature differential between the beginning and end of the dive and so forth.

By the way, for all of you to whom I sent the little Excel spread sheet,
please copy the following and paste it into cell E24 to correct an error in the calculation:

=(((C19-C20)/C19)*((C19/C21)*C22)/C23)/((C24/33)+1)

Thanx . . .

the K

 

[DA Aquamaster]

More body mass means more cells that need O2 and that would imply more O2 use and a higher SAC rate. But if you put a very relaxed 200 lb diver who is very proficient with buoyancy, proper trim, and streamlining in the water along with a 100 lb diver who is not relaxed or less proficient the odds are good the lighter diver will use a lot more gas.

 

[emoreira]

Interesting post.
As a new diver, I've been worried about my SAC figures.
I've heard a lot of times and everywhere that one's SAC figures will decrease with experience.
My SAC numbers are now 0,66 cuft/min as average and 0,62 as mean value.
One point I've seen when making my calculations (I have still no dive computer) is that I'm using the max. depth reached, measured with the depth meter in the console, and not the "time weighted average depth".
With this, and considering going down, going up and the 3 minutes, 5 meters security stop, my figures should be worst, as the time weighted average depth is always smaller than the max. depth.
A second point is how acurate are the manometers and depth meters used. I have no reg. of my own, so I'm always using rented regs and rented BCs, so each dive is measured with different meters.

 

[BobinNC]

OK, I am pretty sure I understand exactly how we get the SAC expressed in cf/min. My SSI book has the spot for SAC in psi/min. The equation given for cf/min includes volume so that rate should be the same in any given size tank, correct?

To figure for psi/min used I understand the equation to be

((Pb-Pe)/T)/((D/33)+1)=SAC in psi/min

But it is obvious that if you switch from an 80cf cylinder to a 120cf that your psi/min will change quite a bit. So good for comparing your dives if you use the same equipment every time and maybe easier for computing how much time at a certain depth you will have by plugging the SAC time back into this equation vs. the other one.

If anyone sees flaws in this please let me know. Personally I think I am going to stick with cf/min, unless someone can give me a better reason to use psi/min.