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Air Consumption..

Discussion in 'General Scuba Diving' started by Al1x, May 22, 2008.

  1. Al1x

    Al1x New Member

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    How do you work out your sac?

    sstickley said how to work it out in this thread but I'm still not really sure. :confused:

    I'll post what I worked out and see if you think it looks right!
     
  2. Al1x

    Al1x New Member

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    I dived with a 12, started with 220 bar and ended with 130

    12 x 220 = 2640
    12 x 130 = 1560

    2640 – 1560 = 1080 litres used

    Bottom time was 21 minutes

    Depth 25.4 metres (Average 18.7m)

    1080 / 21 = 51.43 litres per minute


    Is that right? It seams a lot doesn't it?
     
  3. Ian

    Ian New Member

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    To work out your surface air consumption you need to know

    1) How many litres of air you used on the dive. You can calculate this from (start pressure - end pressure) * cylinder capacity in litres

    2) Duration of dive in minutes

    3) Average depth of dive in metres, from your computer or from a square profile dive where the average is going to be pretty much the same as max depth.

    Then you use the following formula

    SAC = ( litres gas used / minutes ) * ( 10 / ( av_depth + 10 ) )

    An example, from I dive I did on the Thesis a month ago.

    (1) Air used = (200 bar - 90 bar) * 15 litres = 1650 litres

    (2) Duration = 40 minutes

    (3) Average depth = 17.5m

    SAC = ( 1650/40) * ( 10 / ( 17.5+10 ) )
    = 15 l/min

    Suunto Dive manager gives a slightly lower figure of 14.1 l/min, but it is using some exact values where I have had to round which I think explains the discrepancy...

    http://www.ukdivers.net/science/aircalc.htm has calculators for some functions, but not actually calculating your SAC.
     
  4. Ian

    Ian New Member

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    You've not factored in the depth. So on average you used 51.43 litres of air per minute (average ACTUAL consumption), but on the deeper sections of the dive you will have used a lot more, and a lot less on the shallower ones.

    Favtor in the average depth and you get a SAC (average SURFACE EQUIVALENT AIR CONSUMPTION) of around 18l/min

    BSAC use a baseline of 25l/min in calculations, I usually get between 14-19l/min SAC depending on the dive so that sounds pretty reasonable to me...
     
  5. bottlefish

    bottlefish Super dooper member
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    As Ian said, you need to factor in your depth - remember that gases compress as you get deeper into the water column, that's why a tank of air will last much longer at the surface then at, say, 30 metres.

    You have an average depth of 18.7 metres, which equates to 2.87 Atmospheres Absolute (ATA). This is calculated by dividing the depth by 10 to give you the atmoshpheres of water pressure, and then adding 1 to allow for atmospheric pressure at the surface.

    18.7 / 10 = 1.87
    1.87 + 1 = 2.87

    You can now divide the 51.43 litres per minute total air consumed by this conversion factor to find out your actual average SAC during the dive:

    51.43 / 2.87 = 17.9 litres per minute

    You will find that SAC rates can change quite considerably, in response to workload, stress, conditions, temperature etc etc, so well worth while adding in a fudge factor, or monitoring your SAC across a number of dives and then getting a feel for the highest you can expect in normal conditions.

    When diving twins, my SAC rate is around 15 litres per minute on the bottom phase of the dive, however I use an elevated SAC of between 17 and 19 for gas planning calculations, depending upon the dive I am taking on and the conservancy I want to include.
     
  6. Al1x

    Al1x New Member

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    Thanks for that.. 18ish litres per minute sounds much better :)
     
  7. Stix

    Stix New Member

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    Al1x - sorry I confused you; just realised I missed the all important depth bit... STOOOOOOPID boy! Slaps head in disgust...
     

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