How to calculate SAC rate?

[GoBlue!]

Some use the term SAC or SCR to define a term that ends up in the units psig/min (which of course will change depending on the tank you use). RMV (respiratory minute volume) is used when the value is describing cu ft (or liters) per min.

Of course, some report SAC or SCR in cu ft/min, in which case it's the same thing as RMV. The terminology is confusing, but if you understand the principles, you're fine. We can all tell what you're talking about when the units are given.

As for the 33 ft vs. 34 ft for fresh vs. saltwater. Remember that your depth gauge is NOT reporting a linear depth from the surface. Instead, it's converting the sensed ambient pressure into a depth reading. Since the physiological effects of diving are only affected by pressure, the actual linear depth is irrelevant. So, you can use 33 ft for fresh & saltwater, as 1 atm = "33 ft" by definition.

Suppose you were diving in some high-density medium such that 1 atm of pressure was felt on your body at a distance 10 ft from the surface (if you dropped a line w/ foot-marks). Your computer would read a depth of 33 ft, even though you're only in 10 ft. of this hypothetical substance!

(At least that's my understanding....feel free to correct if I'm mistaken.)

Jim

 

[mstudley]

Your computer doesn't know fresh v. salt

Mine does.... I have to set it before I get in the water (it can't autodetect salinity) but I can set it to fresh or salt water. Using 33 for sw and 34 for fw does make a difference when planning some dives especially on a rebreather.

 

[ndl]

good thread. I would like to add my 2 cents.

The best way to screw up a sac rate calc. is to start the time when you first enter the water. the reason for this is that it takes time for the gas in your tank to adjust to the temp of the water. by letting the gas adjust to water temp you gat a accurate sac rate calc.

 

[String]

SAC is one thing where the metric system certainly has advantages in terms of understanding and usefullness.

For starters it becomes irrelevent of tank size (whereas cu ft/min does vary with tank size)

Metric method

(i) Determine tank size (example 15l)
(ii) Determine how much gas used on the dive (Example 150 bar)

(iii) Multiply tank size by gas used to work out how many litres of gas were used (Example 15 x 150 = 2250)

(iv) Diving that by dive time ( Example 2250 / 50 minutes = 45 bar per minute)

Now factor in by your average depth (Example 20m which is 3atm pressure. 45 / 3 = 15 litres per minute)

That means corrected for the surface your air consumption is 15litres per minute regardless of tank size etc.

This can very easily be used for dive planning then.


As for water cooling effect, yes this can definately affect the air level, my tank in cold water can decrease by 10-15 bar just for the water. What i tend to do is look at my gauge mid descent and take that figure plus 2 or 3 bar (average per minute used).

 

[eponym]

SAC is one thing where the metric system certainly has advantages in terms of understanding and usefullness. For starters it becomes irrelevent of tank size (whereas cu ft/min does vary with tank size)

Hi, String. I have a question. Can you explain how a cu.ft. per minute consumption rate calculated with one size of tank will not apply to any size of tank? Or pehaps I misunderstand what you said?

It seems to me that if I breathe 30 cubic feet out of an eighty-cubic-foot tank in thirty minutes at 33 fsw, I'll also breathe 30 cubic feet out of a fifty-cubic-foot tank in thirty minutes at 33 fsw.

Thanks,
Bryan

 

[String]

Whoops i worded the thing wrong, what i was actually trying to convey was for different pressure tanks.

 

[eponym]

I get it. You meant expressing consumption in psi is tank-dependent.

I really like the way you label and refer to tanks by their interior volume at 1 bar. (Have I got that right?) I saw that in the seychelles, diving off a French boat. And you're right, it makes the calculations a lot easier and less error-prone. I imagine that, in metric countries, most divers skip a "bars per minute" step and work with liters/minute right from the start. Very cool.

Bryan

 

[String]

Correct, its known as "water capacity" which is literally that, the volume of water the space inside the tank can hold.

Simply multiplying that by the gas pressure gives the volume of gas it holds (ok assuming ideal laws etc etc).

Correct assumption about the rate calculation as well, everyone works in litres/minute corrected for surface as it can be applied to any tank and depth. The 10m == 1 atm means these calculations can be done fairly well in the head or very simply on a piece of paper. Some things about the metric system i dislike but the air consumption rate i think is sensible and simple.

FWIW we generally use 12.2l or 15l tanks.

Normal tank pressure is 232 bar (3410 psi) but 300 bar tanks exist (4410 psi). 300 bars arent that common as theyre (i) heavy and (ii) ideal gas laws break down above about 232 for air so you dont actually have as much gas as you think due to van der walls forces.

 

[mstudley]

Here's how I was taught to determine SAC (note that A need not mean 'air' and this is dealing with imperial units in salt water):

SAC = [((Pi - Pf) / Pw) * V] / [((D + 33) / 33) * T]

Pi = initial tank pressure
Pf = final tank pressure
Pw = tank working pressure rating
V = actual tank volume
D = average depth during dive
T = time in minutes

So for an example, diving with a steel 72cu that is rated at a working pressure of 2475psi with an initial pressure of 2100psi and a final pressure of 500psi on a dive of 35 minutes where the average depth was 54fsw your SAC would be 0.504.

 

[scubasean]

So my SAC rate is 23 (rounded up) PSI/minute

yes, but for that tank size only...Since you expressed it in psi/min, it will be different for different sized tanks.

cu.ft/min (or liter/min, I suppose) is the tank independant expression....