Calculating air consumption SAC / SCR / RMV

[Charlie99]

ppilot once bubbled...

SAC = ((Air used psi/time in min) * 33) / (depth + 33)

It seems to me that SAC is good only for a given tank size. Is there a way to relate it to a different one? Or am I totally missing something?

The equation above results in an SAC expressed in psi/minute. As you have noted, that only is useful if you always use the same tank.

A more useful version of SAC is to express it in cubic feet / minute.

Simply take the psi number you have and multiply by rate cu ft of the tank, divided by the rated fill pressure. For an AL80 that is really 77.4 cu ft at 3000 psi, the formula would be:

SAC = (psi used/ time in min) * 33)/(depth+33) * 77.4/3000

That SAC is now in cubic feet per minute. Of course, to be useful, you will have to convert that back to psi on whatever tank you are using, but that's left as exercise for the reader

 

[ppilot]

Thanks Charlie...but now I'm really confused.

Example: I have a 74ft tank @2400psi. I do a 60 foot dive for 30 minutes and burn 1600psi out of my tank.

((1600/30) * 33)/93)) * (74/2400) = ~ .58 cu ft/ min

So if I'm breathing .58 ft/min, in 30 minutes I should use 17.5 ft. of air...less than a quarter of its volume, but I've breathed 2/3 of its pressure?

 

[roakey]

Do a search on SAC in the title field only. You'll find the answers to all your questions.

Roak

 

[CheeseWhiz]

The 0.58 cfm is the SAC (Surface air consumption) rate. Since you're at 60 feet or approx. 3 atmospheres, your air consumption rate at depth will be 3 x 0.58 cfm = 1.74 cfm.

Over 30 minutes, you will have consumed 1.74 cfm x 30 min. = 52.2 cf.

As a sanity check, 52.2 divided by 77.4 = 0.6744, which is close enough to 2/3 of the tank capacity.

Capisce?

 

[Spectre]

If you have a 74 cuft @ 2400. then:

1600/2400 * 74 = 49 1/3 cu ft.

Depth in ATA is: 93/33 = 2.818

So you used 49 1/3 cuft in 30 minutes at a pressure 2.818 times the surface pressure.

So you did 49 1/3 cuft in 84.545 "surface minutes" or: 49.333/84.545 = .58 SAC rate.

So yes... 30 minutes at .58 ft/min = 17.5 feet _at the surface_.

At 66 feet, that's .58*3 * 30 = 52.2 cu ft.

Along those lines, what if you wanted to know "How much time should I suspect to get at 99 feet with 1/2 my tank".

1/2 tank = 37 cuft. SAC rate is .6 . .6 SAC @ 99 feet = .6 * 4 ATA = 2.4 cuft/min. 37/2.4 = 15 minutes.

Now the one thing to be really careful about if you're using SAC rates to plan out gas management. Remember to calculate your SAC rate off your _Average_ depth, not your max depth.

For example. A dive to 86 feet for 70 minutes with an HP steel 120, surfacing w/ 900 PSI would be:

86+33/33 = 3.6 ATA
2600/3500 * 120 = 80 cu ft used.

80 / (3.6 * 70) = .32 SAC rate

However the average depth of the dive was 50 feet, which brings the rate up to .47. So if you used .32 to plan... you're screwed.

 

[ppilot]

Thanks guys, I figured out my error after I got out of the office and took a walk and thought about it.

Note to self: think THEN post

Thanks for all your help!

 

[DA Aquamaster]

I assume you have a SAC-RATE Surface Air Consumption Rate Calculator. It is essentially a rotary slide rule and does the math for you. It was designed to be user freindly and idiot proof and consequently uses psi rather than cu ft/min. I have not seen one around for a long time (other than mine) which is too bad, it's a neat planning tool.

It was more common in the past to figure air consumption using psi per minute and then just stay there and do dive planning using psi/min rather than cu ft/min. In that regard the SAC Rate calculator made perfect sense and it prevented any potential errors when converting psi/min to cu ft/min.

There is still nothing wrong with it using psi rather than cu ft/min as you have to do the same thing anyway manually in the course of figuring your air consumption in cu ft/min. Withthe SAC rate Calculator the only math you have left is the conversion to cu ft/min and it's a pretty simple process.

If I dive for 10 minutes at 65 ft and use a total of 550 psi from a steel 72, it will give me my SAC rate of 18.5 psi/min after I input the depth, time and total psi used. To convert this result to cu ft/min you need to know how much your tank holds at what psi.

A steel 72 holds 71.2 cu ft at 2475 psi, so:
(18.5 psi X 71.2) / 2475 = .53 cu ft.

If you want to know how many PSI per minute you could expect to use with an aluminum 80 you can work the ratio the other way:

An AL 80 holds 77.4 cu ft at 3000 psi so:
(.53 cu ft X 3000psi) / 77.4 cu ft = 20.5 PSI/min

 

[Atomic_Diver]

my Sac rate falls between 15-17psi per min at the surface....

 

[coldsmoke]

Wow - I wonder what the record is for the oldest resurrected thread.

Hunter

 

[captain]

Knowing your PSI per minute is not terribly useful and it's not a true air consumption rate. A true air consumption rate is useful.

Air consumption is a personal aspect of diving. In other words, it varies from diver to diver. Even more, it varies for the same diver from dive to dive. Factors which will affect your air consumption rate are: work load, physical fitness, recent illnesses, water temperature, depth and comfort level. A useful method of calculating your personal air consumption rate involves finding your Respiratory Minute Volume (RMV). This is more useful than other methods because you can use it with any size tank. To find your RMV, descend to a comfortable depth. Relax, note your tank's pressure and sit quietly for 10 minutes. Again note your tank's pressure. Next, repeat the process while swimming normally for 10 minutes. At this point you know how many PSI/10 minutes you use at a specific depth for a specific tank both swimming and at rest. This is easily converted to cubic feet. Divide the total volume of the tank by the working pressure of the tank. This results in a tank factor expressed as cu ft/1 PSI for this particular tank. Multiply the tank factor by the PSI used then divide by 10 for each of the figures, PSI used resting and PSI used swimming. This is your RMV for that depth. Now convert it to a surface figure. Take your RMV at depth and divide by the ATMA of your depth. (D+33)/33=ATMA. RMV at depth/ATMA = RMV. Do this for both resting and swimming. You now have two baseline figures. To figure your air consumption at any depth, merely take your surface RMV and multiply by the ATMA of the depth you are planning.

Pounds per square inch (PSI)
Depth (D)
Atmosphere absolute (ATMA) ATMA = (D+33)/33
Tank factor is different for each type of tank (F)
Total tank volume (V)
Working pressure of the tank (W)
PSI used (P)
Respiratory minute volume (RMV)

V/W=F
(F*P)/10 = RMV at depth
RMV at depth/ATMA = RMV


WWW™

You really don't even need to actually dive to do this. Sit and breath from a tank for 10 minutes and then walk around wearing the tank for 10 minutes. It simplifies the math since you don't have to correct for depth. Just wear a mask so your nose doesn't cheat on you.