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.
What is "Rock Bottom" or "Minimum Reserve Gas"?
It is the minimum amount of gas you think you will need after a catestrophic event at the deapest part of the dive. Think of it as "the worst case, but realistic scenario". You can get very precise about this or you can do "Battlefield Math" and use what is known as the CAT Calculation.Minimum Gas = C.onsumption * A.verage Depth * T.ime
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 for CAT, the 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. That is calculated by depth divided by 10ft per minute. So 100ft is 10min. But wait...we will add in some conservatism by assuming it takes 1 minutes to figure out whatever is going bad and call the dive. So it's Depth / 10ft + 1. At 100ft that becomes 11 min.
Putting it all Together
For a 100ft dive...Minimum Gas = C.onsumption * A.verage Depth * T.ime
1.5 * 2.5 * 11
= 41 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 using the "Tank Factor". If we assume an Aluminum 80 with a fill pressure of ~3000 PSI. The math is...
41 cuft / AL80 Tank Factor * 100
41 / 2.5 * 100
= 1640 PSI
However...it's damn near impossible to use your SPG and see anything less than 100 PSI increments...so we round UP (for conservatism) to the nearest 100. So 1640 becomes 1700 PSI.
The Minimum Gas (in PSI) we need for a 100ft dive using a single Aluminum 80 filled to ~3000 PSI is 1700 PSI
Which means...... We should begin our ascent to the surface NO LESS than when our SPG shows 1700 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...
