Proper gas planning & management is critical to safe diving since humans cannot breathe underwater without life support equipment.

Unfortunately, many divers don’t understand what gas planning & management is, and simply use the standard “Back on the boat with 500psi” or the rule of “thirds”.  While those types of plans may work in certain circumstances, they can also be very dangerous, since, in the first example, the diver is planning his gas so he arrives at the surface with a set pressure in his tank.  He’s not planning his gas for what would be needed should an emergency occur underwater, which could be the difference between life and death.

It’s important to remember that an Out Of Gas (OOG) emergency is unacceptable.  Proper gas management and gear maintenance is all that is required to avoid this type of situation.  However, when planning our dive, we plan according to worst case scenarios.

What is Rock Bottom

Rock Bottom is, quite simply, planning for the worst case scenario…the instance where a diver in a 2 person buddy team experiences a catastrophic and complete loss of gas at the deepest part of the dive.  It’s the minimum amount of gas needed for 2 divers to ascend, completing all required stops, sharing air on a single cylinder.

Review

In your open water class you learned about Boyle's Law.  Simply put, Boyle’s Law states that at a given temperature as pressure increases, volume decreases. 

In salt water, every 33’ is an atmosphere.  At the surface, we are under 1 ata (atmosphere absolute) of pressure.  Therefore, at 33’, we’re under 2 ata’s of pressure (the single atmosphere at the surface, and the 33’ of water above us).  At 66’, we’re under 3 ata’s of pressure.  99’ is 4 ata’s, and so on.

Since every 33’ of saltwater is an atmosphere, to determine the pressure, in ata’s, of any given depth, we simply divide the depth by 33, and add 1 (for the pressure at the surface).  In other words;

Ata = Depth / 33 + 1

Likewise, to determine the depth of any given pressure, we simply subtract 1 (for the pressure at the surface) from the pressure, then multiply by 33.  Also shown as;

Depth = ata – 1 x 33

Consumption

It’s important that every diver know his/her own consumption rate.  If you don’t know your consumption rate, and would like to learn more, please Contact Us to schedule a FREE Gas Planning & Management Workshop.

At the recreational level, we typically plan for a stressed diver to breathe approximately 1cuft per minute (Respiratory Minute Volume, or RMV), at the surface.  Applying Boyle’s Law, from above, if a diver breathes approximately 1cuft per minute at the surface, stressed or working, he’ll breathe 3 cuft per minute at 66’, due to the fact that your 2nd stage regulator must deliver air to you at slightly above the ambient pressure;

1 (RMV) x 3 (ata of 66’) = 3

At 99’ the diver will breathe 4 cuft of gas, when stressed or working;

1 (RMV) x 4 (ata of 99’) = 4

You can certainly increase your planned stressed RMV upwards, for conservatism, if desired or if you know your working RMV to be higher than 1.

Calculating Recreational Rock Bottom

When calculating Rock Bottom, there are certain rules and assumptions that we must apply and consider;

• Use working RMV (1.0 each if not known, as previously stated)
• :01 to donate the regulator, assess the situation and initiate the ascent
• Ascend at 30fpm to first stop (50%)
• Ascend at 10fpm from 50% to surface

One final rule; never use a Rock Bottom of below 500psi.  If your calculated Rock Bottom is below 500psi, use 500psi.  This is due to the inaccuracy of the spg when reading pressures below 500psi.

When applying the rules and assumptions above, its easiest to visualize the situation in 2 segments; the bottom segment (donation, assessment and initiation of ascent) and the ascent.

Additionally, there are only 3 pieces of information needed to calculate Rock Bottom;

• How deep (depth)
• How long (time)
• Breathing rate (combined RMV for TWO divers, since they will be breathing off a single tank)

Depth and time are both variable.  The deeper the dive, the more that is needed/used in the bottom segment, due to the increased depth.  Likewise, the deeper the dive, the longer the ascent time.

However, since we work with pressure, and not volume, while underwater, we must convert the volume (40cuft) to pressure, or psi.

To do that, we’ll simply run a quick calculation using tank factors

Tank Factors

Tank factors are simply the amount of cubic feet per 100 psi, for a given tank.  Using tank factors is a very easy way of converting between volume (cuft) and pressure (psi).

To calculate a tank’s tank factor, you simply divide the tank’s rated pressure by the tank’s rated volume, then multiply by 100;

Tank Factor = rated volume / rated pressure x 100

For an al80 (al80’s actually contain 77cuft, not 80), the tank factor would be:

77 / 3000 x 100 = 2.66

This means on an al80, there are 2.66 cuft per 100psi.

To make life simple, we’ll just call it a tank factor of 2.5.

It’s important to remember that you must use the rated pressure and volume.

Now that we know the tank factor of an aluminum 80, we can convert the volume (40cuft) to psi in the above example, using a simple formula.  The diagram below is actually a formula in itself.  If you want to know the volume, you’d simply multiply pressure by the tank factor.  If you want to know the pressure (psi), you’d simply divide the volume by the tank factor.

So, using the formula above, to determine how many psi 40cuft of gas in an al80 is, we simply divide the volume (40) by the tank factor (2.5);

40 / 2.5 = 16 or 1600

Applying this to the above example of a Rock Bottom Ascent, we’d get this:

1600psi is needed for the 2 divers to ascend from 100’, sharing air on a single al80, completing all required stops.  Since this is based on assumptions, if you’d like to add a buffer of 200 or 300psi, making the Rock Bottom 1800 or 1900, that is perfectly acceptable.  However, you should never lower the Rock Bottom number.

There are other gas plans as well, as Rock Bottom doesn’t work in every scenario (overheads, such as caves or wrecks, where additional gas is needed to get to the ascent point).  But for most everyday recreational dives, Rock Bottom gives you the knowledge that you need to safely execute a dive.

It’s important to understand that Rock Bottom is for a specific depth, and not a specific dive.  In other words, 100’ has a different Rock Bottom than 90’.  For instance, if you hit your Rock Bottom at 100’, and ascend up to 80’, you must ensure that you don’t exceed your Rock Bottom for 80’.

It’s a good idea to make note of your Rock Bottom in 10’ increments, from 60’ to the maximum depth you dive to.  An easy way of doing this is to keep them in your wetnotes.  If you know the volume, you need only use the tank factor to adjust it for the tank you are using.

If you’re interested in learning more about Gas Planning & Management, please Contact Us to schedule a free Gas Planning & Management Workshop.