You state the SAC as a Surface Air Consumption, i.e. at 1 ATA.I'm not asserting that either one is more "correct" as there appears not to be a consensus, but I observe that Wibble's definition is not the same as SDI's definition -- I was using the one from SDI. It seems that experienced divers disagree on the exact method of gas planning -- e.g. berndo suggests determining that the buddy has 40-50 min of time with a 12l tank at roughly the same depth, from conversation. The assumption, then, is that the buddy's reply is based on his own past dives, so this is similar to estimating RMV from past data, except that it estimates the total consumption at depth, rather than the rate of consumption. It seems that scubadada has analyzed data from a large set of past dives.

Example 20 litres/min.

Then you multiply it by your depth in ATA:

e.g. 20m/66ft = 3ATA

Therefore you will consume 20 (litres/min) x 3 (ATA) = 60 litres per minute.

How much fuel in the tank? Simple, multiply your tank volume (litres) by the pressure (bar).

e.g. 11 litre (ali80) x 100 bar = 1100 litres.

How long will your gas last? Simple, divide your gas amount by your consumption at depth.

e.g. 1100 (litres) / 60 (litres/min) = 18 minutes before it's empty.

If you want to live, you might want to work out how much gas it takes to reach the surface WITH a safety stop from your depth. Worked example:

- 2 minutes to get your act together on the bottom (SMB, tell your buddy, get your A into G...), so that's 2 x 60 =
**120 litres** - 1 minute to ascend from 20m/66ft to 15m/50ft (no, it's not fast, it's sufficient): use the bottom amount for contingency:
**60 litres** - 1 minute from 15m/50ft to 10m/33ft. ALWAYS use the lowest amount 15m/50ft = 2.5 (ATA) x 20 (litres/min SAC) =
**50 litres** - 1min from 10m/33ft to 5m/15ft = 2 ATA x 20 =
**40 litres** - 3 minutes at 5m/15ft safety stop = 3 (mins) x 1.5 (ATA) x 20 (SAC) =
**90 litres** - 2 minutes from 5m to the surface (contingency, slowly slowly, maybe waity for buddee) = 2 (mins) x 1.5ATA x 20 (SAC) =
**60 litres** **Ascent total: 120 + 60 + 50 + 40 + 90 + 60 = 420 litres.**- THEN add your "must return contingency amount" which is 50 bar (or 500 PSI = 33 bar in the USofA) which equates to 50 (bar) x 11 litres (volume of your ali80) =
**550 litres**. **Grand total of gas needed from the bottom at 20m/66ft = 420 + 550 = 970 litres.**

Call it 1000 litres.

**Therefore your**

1000 (litres required) / 11 (litres for ali80) = 90 bar.

*end the dive*pressure in your ali80 tank is:1000 (litres required) / 11 (litres for ali80) = 90 bar.

Note: Even though our working SAC will be less than 20 for an experienced diver, you NEVER use the lowest rate as you must have a contingency for real problems; like that gas hog grabbing your alternate air supply or rescuing that person who's panicking, or having a big surface swim... etc.

For your homework, redo those sums for:

- An emergency, say using 25 litres/min SAC.
- Then try 30 litres/min SAC.
- Now do that for two divers sharing gas at 20 litres/min SAC EACH (that's 40 litres/min SAC)
- Then redo the sums for 30m/100ft.
- Consider the implications for those consumptions if using a small 3 litre (20 cubit ft) PONY