Basic questions about air consumption

[Zeeman]

Here are some formulas that are handy:

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

Thanks for that, I haven't seen a lot of htose before

z...

 

[Zeeman]

surface air consumption. usually expressed in cubic feet per minute, this is the
rate at which you use gas at the surface. average is about .6 cfm (that is,
6/10ths of a cubic foot per minute)



easiest way is to do a dive with a computer than can track your average depth.

note how much gas you started with (say 3000 psi) and how much gas you ended
with (say 1400 psi). then note your average depth, say 40 feet.

so you used 1600 psi at an average depth of 40 feet.

your depth consumption rate (DCR) is 1600/40 or 40 psi/min

SAC = (DCR x 33) / (Depth + 33)

in this case (40 x 33) / (40 + 33) or 18.08 psi / min




excellent question! that's what comes next. you now need to convert your
SAC in psi/min to SAC in cubic foot/ minute (cfm)

you will need to know your tank volume at rated pressure. let's assume an
aluminum 80 with a rated volume of 72 cubic feet at 3,000 psi.

first, we find cubic feet per psi, or 72/3,000 = .024

using your SAC of 18.08 psi/min, let's convert to cfm by multiplying
the psi used by .024 to find out cfm = .43 cfm, a rather nice SAC.

that means that you are using .43 cfm at the surface



exactly... you're thinking along the right lines here

now that we know your surface air compsumtion, we can predict how much air
you will use at any depth by using the following formula:

SAC x ATM = gas needed in cubic feet per minute at that depth

ATM = (depth/water type) + 1, where fresh water = 34 and salt water = 33

so, to find ATM at 70 feet of salt water (fsw), you would do as follows: (70/33) + 1

or 3.12

and we plug that in so .43 x 3.12 = 1.34 cfm. thus, you will use 1.34 cfm at 70 feet.

a 72 cubic foot tank will last you 53 minutes at 70 feet.

now, i'll stop here with the SAC concept, before moving on to gas planning, which
is a whole new subject.

My only question is, can oyu just plug metric values in place of the imperial ones? Or are we looking at different formula (different constants)?

Z..

 

[H2Andy]

ack... well... it's actually easier in metric.. but... i don't recall the numbers,
and don't want to give you bad advice

 

[lamont]

I'm not certain how you guys do it in metric-land. Do you express SAC in L/min and use the gas volume in L to figure out how much time you've got?

If you've got a 15L (water volume tank) filled to 200 bar, that should be 3000L of gas? or if you're reserving 50 bar, you've got 150 bar usable, so 2250L with your consumption rate being SAC (L/min) * ATA -- divide 2250L / consumption rate and you get minutes it'll last you?

Looks like 0.75 cuft/min ~= 20 L / min, so in a 15L tank you'd be using 13.3 bar / 10 min / ata? So at 4 ata you get ~50 bar / 10 min so it'll last you 40 mins?

Metric make my poor little american brain hurt...

 

[grazie42]

Since I just read thru all your confusing imperial posts I´ll reply with the metric version:

Tanksizes are given in liters (as opposed to your cft). Pressure is in BAR. Depth in meter.

Volume of gas in a tank is: 15L x 200 = 3000 liters or about 3 m3.
200/233/300 are common here which is 3000/3500/4500psi.

SAC is expressed in "free liters/min" or L/min for short.

(I´m using H2Andy´s example but I couldn´t find any reference to divetime...)
H2Andy´s calculation looks like this in metric (notice how much easier this is):

Tank size: 15 L
Starting pressure: 200BAR. End pressure: 110BAR
Avg. depth: 12 Meters (40ft) Time: 45 minutes
Volume gas used: (200-110) x 15/2,2 = 614 liters, the "2,2" is the ATA of 12mtrs (1,2+1)
SAC = 614/45 = 13,6trs/min (which is pretty good)

For dive planning it is easiest to work in BAR (since that´s whats on your spg):
13,6 / 3000 x 200 = 0,9 BAR/minute or ~10BAR/10 minutes @ surface.
If you go to 12m/40ft that means you use:
10 (BAR) x 2,2 (ATA of 12m) = 22 BAR/ 10 minutes

The formulas are:
SAC (liters/min) = DCR/ATA or "consumtion, liters per minute at depth" / (Depth (meters)/10) +1
The metric equivalent to cft/min is m3/min but since our tanks are "sized" according to how many liquid liters they hold, that number has no practical value...
ATA = (Depth in meters / 10) +1

I THINK what I´ve typed above is correct, as people expect me to work they keep interrupting me, if I´ve made a mistake somewhere I´m sure someone can point it out...