THIS DISCUSSION/EXAMPLE IS BY A RECREATIONAL DIVER FOR RECREATIONAL DIVING WITHIN RECREATIONAL LIMITS.
DO YOUR OWN CALCULATIONS USING YOUR OWN RMV VALUES AND YOUR OWN SAFETY ESTIMATES.
PLAN YOUR DIVE. DIVE YOUR PLAN.
Rock Bottom Gas - More Realistic than basic CAT
RBG = C.onsumption * A.verage Depth * (T.ime to 50% of depth + T.ime to surface from 50% of depth)
Let's deal with Consumption first:
Consumption is the amount of gas you AND a buddy consume per minute. If you want a precise number you can calculate your own or you can go with the industry accepted standard of 0.75 cuft/min per person. So consumption is 0.75 * 2 = 1.5 cuft/min
Next comes Average Depth:
What is the deepest point on your dive and divide that by 2. Simple. So 100ft dive will be 50ft average. Then convert that to Atmosphere. Because this is battle field math we will use rounding to make the math simple.
| max feet | avg feet | precise ATA | Battlefield ATA |
|---|---|---|---|
| 30ft | 15ft | 1.45 | 1.5 |
| 40ft | 20ft | 1.6 | 1.5 |
| 50ft | 25ft | 1.75 | 2.0 |
| 60ft | 30ft | 1.9 | 2.0 |
| 70ft | 35ft | 2.06 | 2.0 |
| 80ft | 40ft | 2.21 | 2.0 |
| 90ft | 45ft | 2.36 | 2.5 |
| 100ft | 50ft | 2.51 | 2.5 |
Now finally Time:
Time is how long it would take for you and a buddy to get to the surface from the maximum depth of your dive. This is the primary difference with the simple to use CAT calculation. For Rock Bottom Gas we travel at 30 ft per minute to half of the depth and then 10 ft per minute for the rest of the depth. However, we still do rounding to make things easier to calculate. Just like CAT, we also add in the 1 minute of bottom time to figure out whatever is going bad and call the dive.
So our Time from 100 ft would look like this:
2 minutes to go from 100 ft to 50 ft at 30 ft per min
5 minutes to go from 50 ft to surface at 10 ft per min
1 minute for bottom 'figure it out' time
8 minutes total
Putting it all Together
For a 100ft dive...Minimum Gas = C.onsumption * A.verage Depth * T.ime
1.5 * 2.5 * 8
= 30 cuft of minimum gas needed
Minimum Gas in cuft is great and all...but we don't have any devices with us on the scuba dive that will tell us cubic feet. So we need to convert that into PSI. To do this, you need to know the tank(s) you will be diving and their fill pressure so you can do the conversion from cuft to PSI. If we assume an Aluminum 80 with a fill pressure of ~3000 PSI then the math is...
( Volume of Tank / Fill Pressure of Tank ) = "baseline" [some folks will call this the Tank Factor]
( 80 / 3000 ) = 0.0266666
Battlefield Math = 0.025 for an Aluminum 80 tank
So putting it all together we have...
30 cuft / 0.025 = 1200 PSI is the minimum gas we need
Which means...... We should begin our ascent to the surface NO LESS than when our SPG shows 1200 PSI
Note: If our calculation shows anything less than 500 PSI we always round up to 500PSI.
Because these are numbers that do not change, they are great for putting into a grid in a wetnote like so...
