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Grateful Diver’s
School of Diving

Bob (Grateful Diver) Bailey

NAUI Instructor 41751


Understanding Gas Management

(continued)

Rock bottom

Rock bottom is defined as the absolute minimum amount of gas you need to get both yourself and your buddy safely to the surface from a specified depth while you are both breathing off of one cylinder. It assumes a worst-case … that one of you had an out-of-air emergency, and that you will be sharing air during the ascent.

There are some “rules and assumptions” that you should take into account when calculating rock bottom pressure. These are …

Calculating Rock Bottom

Calculating Rock Bottom is simple if you envision the actions that must be taken after the emergency occurs in segments. In other words, first you’ll do the regulator exchange and assess the problem, then you’ll ascend to your first stop, perform your first stop (if dive was deeper than 80fsw), ascend to 15’ for the safety stop, perform the safety stop, then ascend to the surface. All you need to do is determine how much air you need for each segment. To calculate RB simply follow these steps...

Step 1-OOA Regulator Exchange & Problem Assessment at Depth
time at depth (in minutes) X depth (ata) X RMV of 2 divers (working RMV) =
Step 2-Ascent from Depth to 1st Stop
time to ascend (in minutes) X avg depth (max depth to stop depth, in ata) X RMV =
Step 3- *Stop (1 minute deep stop or 3 minute safety stop)
stop time X stop depth (ata) X RMV =
Step 4-*Ascent from 1st Stop to Safety Stop (max depth deeper than 80fsw)
time to ascend X avg depth (stop depth to safety stop depth) X RMV =
Step 5-Safety Stop
time at stop X stop depth (ata) X RMV =
Step 6-Ascent from Safety Stop
time to ascend X avg depth (stop depth to surface, ata) X RMV =
Step 7-Total Gas Used
Add steps 1-6
Step 8-Convert to PSI
CuFt of gas needed / rated volume of tank X rated pressure of tank =
Step 9-Add 200psi for Reserve
Add 200 to step 8 for total Rock Bottom

Let’s look at an example....

You and your buddy are both diving AL80 cylinders. Your working RMV rate is 1.82 cubic feet per minute and your buddy’s is 1.3 for a total RMV of 3.12 CF per minute. You are planning a dive to 66fsw. Let’s follow the steps to determine your Rock Bottom...

Step 1-OOA Regulator Exchange & Problem Assessment at Depth
1 (minute) X 2.97 ata of 65’) X 3.12 (combined RMV) = 9.27 CuFt
Step 2-Ascent from Depth to 1st Stop
Max Depth was not deeper than 80fsw, bypass this step
Step 3- *Stop (1 minute deep stop or 3 minute safety stop)
Max Depth was not deeper than 80fsw, bypass this step
Step 4-*Ascent from 1st Stop to Safety Stop (max depth deeper than 80fsw)
1.66 (1:40, ascent time, 65’ to 15’) X 2.21 (avg depth (40’), in ata )X 3.12 = 11.44 CuFt
Step 5-Safety Stop
3 (minutes) X 1.45 (ata of 15’) X 3.12 = 13.57 CuFt
Step 6-Ascent from Safety Stop
.5 (30 seconds, 15’ to surface) X 1.22 (avg depth (7.5’), in ata) X 3.12 = 1.91 CuFt
Step 7-Total Gas Used
9.27 + 11.44 + 13.57 + 1.91 = 36.19 CuFt Total
Step 8-Convert to PSI
36.19 / 77.4 X 3000 = 1403
Step 9-Add 200psi for Reserve
1403 + 200 = 1603 psi Total Rock Bottom

For this dive, you want to start your ascent no later than when there is 1,600 psi in your cylinder or your buddy’s cylinder.

Figure 1 illustrates some relationships between depth, amount of gas required, common cylinder sizes, and rock bottom pressures for a new/inexperienced diver. Rock Bottom is based on a working RMV of 2.0 or a working SAC rate of about 70 psi per minute on an AL80 cylinder. Note the diver isn’t able to safely dive to 100fsw unless he uses a cylinder with 98CuFt capacity. Additionally, using an AL80, the diver can would probably hit his Rock Bottom for 90fsw (2913) before even reaching 90fsw. A common rule of thumb for some divers use is to never dive deeper than the volume of gas your cylinder holds. In other words, if you’re using an 80CuFt cylinder, you’d limit your maximum depth to 80fsw, 100fsw if you’re using a 100CuFt cylinder, etc.

Note that while this is the RMV of an actual diver, consumption rates could be higher for divers with less experience, or lower for divers with more experience.

Figure 1. Rock Bottom Pressure & Volume for Common Cylinders-
Inexperienced Diver (w/ 200psi Reserve)

Rock Bottom Graph

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