Check my math? (SAC)

[superspeed]

or buy a nice air integrated dive computer and let it do the math for you....but that was is a lot more expensive.

 

[Charlie99]

When in the water, I find that expressing the SAC in psi/min/ata is even easier than tank factor.

The 2.5 cu ft/100psi that TS&M mentioned is 40psi/cu ft when flipped upside down. Your measured SAC of 0.56cfm can also be described as (with AL80 only) as 22 psi/min/ata. The 22 is from multiplying 40 * 0.56. The more accurate calculation is 0.56cu ft/ min * 3000 psi full pressure of AL80 / 77.4 cu ft of AL 80

At 0' depth you will use about 22psi per minute. At 33' you will use 44psi per minute, at 100' 88psi per minute. Another good depth to remember is that 50' is 2.5 ata., and a 15' safety stop is about 1.5ata.

SAC varies according to how hard you are working, how nervous you are, and lots of other factors. So I use convenient round numbers such as 25psi/min/ata or 250psi/ 10 minutes/ ata.

 

[farsidefan1]

I second the air integrated computer method

 

[aquaregia]

I second the air integrated computer method

Why? Serious question.

 

[Kevrumbo]

So I started reading threads about SAC and since I had no idea what mine was I thought I'd try and calculate it, and I'm just not sure if I did it right, though the number I got seems to be about right. Here is what I used[this was done while sitting on my couch]:

I breathed from an AL 80 for 30 minutes. Initial pressure: 1500 psig. Final Pressure: 850 psig. Total consumed: 650 psig.

I calculated from those numbers that I used 22.67 cu. ft. of air in 30 minutes or a resting SAC of 0.76 cu. ft. per minute.

I'm bouncing back between metric and imperial lately for work, so while I am pretty sure I got my maths correct if anyone is bored and once to double check, thanks!

Michael

Stay with metric . . .it's much easier and makes more sense!

Keep your 0.76 cf/min for a convenient reference SAC rate --in metric, this is approximately equivalent to 22 litres/min.

An AL80 holds 11 litres volume at the surface standard of 1 ATA (or 1.01 bar), for a metric rating of 11 litres/bar.

22 [-]litres[/-]/min divided-by 11 [-]litres[/-]/bar equals 2bar/min SAC rate in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units.

Your depth in meters, which converts easily to ATA (divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SAC rate.

Example: 30m depth is 4 ATA; your 2bar/min SAC rate at depth now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .

 

[superspeed]

Why? Serious question.

Air Intergrated Computers keep track of PSI used per min and with most of them when you upload them to your computer and put the tank you used it will tell you SAC for the dive...and some of them at any given point of the dive.

 

[divermike1011]

Kev, thanks for metric conversions, I'm slowly trying to get all of my tables in meters, it's a bit schizophrenic jumping back and fourth for the various PADI exams, everything I can do in meters I do, but my RDPs are all in feet, and of course my tanks are cf / psi.

Michael

 

[Kevrumbo]

Look at this thought exercise as well --using double AL80's if and when you get into tech diving:

Double AL80's metric total tank rating is 22 litres/bar (i.e. two manifolded 11 litre/bar tanks is 22 litres/bar total).

Your SAC rate or reference RMV is 22 litres/min.

22 [-]litres[/-]/min divided-by 22 [-]litres[/-]/bar equals 1bar/min SAC rate in pressure units.

Therefore all you need to know is your time at a particular depth, and the depth in ATA (i.e. again it's easy to figure that out from depth in meters: divide-by-10 and add 1 gives you depth in ATA) --and you'll know how much gas you've consumed in bar even before looking at your SPG.

Example: You're at 60m depth in double AL80's. 60 meters is 7 ATA. Therefore in 10 minutes of nominal workload, you will expect to consume 70bar of gas.

 

[paddler3d]

I'm too lazy to do some math on this to test my thought...

Does it make that much of a difference to calculate your SAC using the tanks actual volume, ie 74cuft for an AL80 vs. the advertised tanks volume?

It seems to me that also long as you do the math the same, the error is the same, and you'll end up with the same answer in the end?

Are tanks that different? Or would the difference really come in the material they are made from.

 

[aquaregia]

Air Intergrated Computers keep track of PSI used per min and with most of them when you upload them to your computer and put the tank you used it will tell you SAC for the dive...and some of them at any given point of the dive.

So you find that the tiresome/difficult portion of the activity is the calculation of the SAC, rather than the dive planning that you do after you have your SAC?