Critical Air Definitions Explained

Critical air: The term, seems to have originated with the Cave Divers and it means: the minimum amount of air required from ‘dives end to surfacing’, including enough air for stops. Also included is the Take-to-Surface air.

Take to Surface Air: The amount of air that every dive should plan to still have in their tank after exiting the water. Dan recommend 375 litres (in a 10lt tank that is 37 bar). This is emergency air and if needed, use it.

Ferry a Buddy to Surface: SCUBA divers are trained in how to ferry a buddy who is out of air, to the surface. This could be by buddy breathing or by the use of the octopus. It is deliberate that the calculator does NOT factor this event into its calculations. The reason for this is twofold.

1. It is an extremely rare event that a diver must ferry his buddy to the surface, but it must ALWAYS be catered for. How do we do that? Read on….
2. If you have to ferry your buddy to the surface, it might be wise to forgo the recommended safety stop. In doing so you will save about 90lt of air, this extra 90lts of air becomes available for your buddy to breath and if extra is need (depth or stress or both), this extra air could come from the Take-to-Surface air, with the divers exiting the water with approximately 30bar still in the tank. A perfectly satisfactory end result, particularly as you have just ferried your buddy to the surface.

Bottom Time is the time from when you first descend into the water until you begin your ascent.
When should we begin the ascent? Certainly not when we are out of air. This calculator, calculates all the variables and produces the answer in bar. Every diver should keep a constant watch on his/her tank pressure gauge. When the gauge reaches the bar level that the calculator gives, that
is the time to begin the ascent. If your tank gauge expresses pressure in psi, and you wish to convert it to bar, simply divide the psi by 14.7.

Dive Depth is the maximum depth of the dive in meters Almost all dives vary in depth. However, in planning a dive we should always plan using the deepest expected depth of the dive. All other fine
tuning is done by the divers PC.

RMV is your Respiratory Minute Volume. The average amount of gas you breath, per minute, while diving (calculated at surface).

• 20 litres per minute at surface is the upper side of the average amount of air that divers use during a dive.
• As we descend, the ‘air usage’ will of course increase (Boyle’s law).
• So, you might ask, what if my RMV (assuming I don’t know what it is), or my buddies RMV is higher than 20? The calculator has two safety factors built into it. If the RMV’s are higher, it is not a problem.

Those two safety factors are:
1. Ferrying your buddy to surface. Although we are trained how to do this in case of an out of air emergency, almost no divers have had to
actually do it in real life – thus, for almost all of your dives, you will never use this extra air that is allowed for in your ascent.
2. OK, so you might again ask, what if I do need to ferry my buddy to the surface – what then?
3. That brings us to the second safety factor, which is that we have also calculated an amount of ‘bring to surface air’, air that we do not intend to use. Why do we do that? It is emergency air and if it is needed, use it. There is no rule that says we must surface with any given amount of air in our tank (of course, we should never suck our tank dry), but wise pre-dive planning is to ‘plan’ to bring spare air to the surface. DAN recommend this amount to be 375 litres (that’s 37.5 bar on a 10 litre tank). If you RMV is higher than 20, you have more than enough air for this. We recommend that you read the information which is built into the app under the blue information
icon, where you will find more detail and explanation on this and other subject.

Ascent Speed is the speed you ascend at. This is normally 10 metres per minute, or the rate of your rice grain size bubbles (about 8m per min). If you use your bubbles to measure ascent speed, remember to always choose a new one from each exhalation. Bubbles expand in volume as (Boyles Law) and thus
their ascent speed up as they ascend.

Stop 1 – Metres is the depth of the first stop during the ascent. This stop could be a deep stop, safety stop or a deco stop – you choose. While we strongly
recommend a safety stop, the calculator will simply ignore this field if you leave it blank.

Stop 1 – Minutes is the length of stop expressed in minutes. If you do not plan a stop, then simply leave this field blank.

Tank Size – Litres is the internal size of the tank, expressed in litres. If you are not sure, you will find this stamped into the upper neck of the tank.

Tank Pressure – Bar is the amount of air currently in the tank, expressed in bar. (To convert bar to psi, multiply bar by 14.7 = psi).

BSA is ‘Bring to Surface Air’. It is the amount of air you plan to bring to the surface at dives end. DAN recommend 375 litres.

Stop 2 – Metres is the depth of the second stop during the ascent. If you have used Stop 1 (i.e. for a deep stop) this second stop could be a safety stop etc.While we strongly recommend a safety stop, the calculator will simply ignore this field if you leave it blank.

Stop 2 – Minutes is the length of stop expressed in minutes. If you do not plan a stop, then simply leave this field blank.