dsevern:
'Surface Air Consumption'
In the EU/UK/Metric world, its worked out in 'Litres per minute'. Its an indication of how much air you breathe at the surface, given an average depth. Its converted to the surface (1 ATA) so you can then work out what cylinder size would be required for a dive to a certain depth and time - No point taking at huge 15ltr cylinder if a 10ltr would do.
For example (sorry I'm working in Metric):
I have a 10ltr cylinder filled to 220Bar - I can work out that at the being of the dive I start with 2200ltrs of air (10 x 220)
The dive was a square profile down to 20 meters
The dive lasted exactly 60mins
At the end of the dive I had 100Bar left, meaning I had used 120bar of air, from this I can work out I used 1200ltrs of air (the 120bar times the 10ltr size)
So we have:
1. The amount of air I breathed
2. The duration of my dive
3. The depth
In 60mins I breathed 1200ltrs of air. This works out at 20ltrs per min.
But I did it at 20meters (3 ATAs) - where I used 3x as much as on the surface, so divide the 20ltrs by the depth in Atmospheres (3)
This gives you a SAC rate of 6.67ltrs per min!! YAY!
From there I can roughly answer the following:
I want to dive to 25meters, for 45mins - what size cylinder do I need?
To work this out we need:
A. Depth (25meters)
B. Time (45mins)
C. Fill Pressure (from above 220bar)
D. SAC Rate (6.67ltr/min)
Firstly we need to know how much air we would breathe for 45mins at the surface, to do this we times our dive time ('B') with our SAC Rate ('D') :
6.67 * 45 = 300.15 This gives us 300.15 litres
Obviously this is at the surface (1 ATA) and we are diving to 25meters, so we need to find our our ATA for the depth:
25 ('A') divided by 10 and then add 1 = 3.5 (ATA)
So times this (3.5 ATA) by our 300.15 from above gives us : 1050 ltrs used for 45mins at 25meters, great! However that doesn't give us a tank size...
From above, we know the tank will be filled to 220bar ('C') if we divide the 1050ltrs by the fill pressure ('C') we get: 1050 / 220 = 4.777 (Roughly a 5 ltr tank), however we need to add in 50bar for good measure and safety:
So make the fill pressure now 170bar (220 - 50) and of course the required cylinder size now goes up to: 6.179 (At this point I would pick a 7ltr!)
Hope that helps!
