An important skill for advanced diving is planning your dive and knowing how much gas you will need to carry with you to complete that plan.
The Rule of Thirds:
In simple terms, for penetration or overhead environment dives, a good gas plan uses the rule of thirds – which means 1/3 of your gas is used for the way in, 1/3 of your gas is used for the return trip out and 1/3 of your gas is saved as a reserve for an unplanned event in your dive – maybe yours, maybe your buddy’s. Some divers go further and use 1/4’s or an even more conservative fraction, but the bottom line is you should be saving a sufficient reserve to deal with any problem and get out of the water safely.
Many of us carry a pony bottle as a source of both reserve and emergency gas, which is great because not only is it a guaranteed reserve, it is redundant – meaning that if something goes seriously wrong with your primary breathing equipment you can switch to a separate system and abort the dive. Pony bottles will be discussing in the last part of this workshop.
The rule of thirds is not as straight forward when you are doing the typical wreck dives that we do in the club. Dives are not necessarily linear and don’t have an exact turn location, but we still want to plan for appropriate reserve gas. The easiest way to do this when your dive is not a straight forward out-and-back plan, is to plan for a specified bottom time and a turn pressure. This considers both your NDL and your available gas for the dive. This is also a fundamental skill you will need to master if you should ever choose to explore the path to technical diving, but it has its place in recreational diving too.
To complete a dive plan that includes an accurate gas plan, we need to know how much gas we use – this is often referred to as your SAC (Surface Air Consumption) or more commonly, your RMV (Respiratory Minute Volume).
What is RMV?
RMV is the term used for the volume of gas that you breathe in a typical non-working minute of a dive at a given depth. It is also the critical number used for planning your gas requirements for any dive. If you have an RMV of 1, you will consume 1 cuft of gas at the surface (1 ATA) and you can easily extrapolate from that, that given the same conditions, you will consume 2 cuft of gas per minute at a depth of 33 feet (2 ATA) – so knowing your RMV and how to use it is critical when you start thinking about a gas plan.
How to calculate your RMV:
To work out your RMV, you need to complete a series of dives where you measure your consumption at a uniform depth over a specific amount of time.
Start by finding a flat dive spot at a reasonable depth. Between 20 and 30 feet works well, but you can do this at ANY depth providing you maintain it for the duration. For this example, I am going to simulate a dive at a constant 20-foot depth. Once you’re in your location, you can begin:
Step One: At depth, on a slate, mark the time, your pressure and the depth (eg, 1:15, 2700 PSI, 20 feet)
Step Two: Maintain that depth for 10 minutes, casually swimming around the entire time
Step Three: At the 10-minute mark, record your pressure and your time (eg, 1:25, 2300 PSI)
In 10 minutes, we used 400 PSI of gas at 20 feet.
Now we have our baseline for our first calculations.
Calculating your Cylinder Volume
To find our RMV, we will need to calculate the volume of your cylinder in more precise numbers than merely volume @ working pressure.
If you have an AL80, your have 78 cuft at a 3000 PSI working pressure.
How many PSI per cuft is that?
How many cuft is 100 PSI?
For an AL80:
3000/78 = 38.5 PSI per cuft and 100 PSI is 2.6 cuft or .026 cuft per PSI
For a LP108:
2400/108 = 22.2 PSI per cuft and 100 PSI is 4.5 cuft or .045 cuft per PSI
For a HP100:
3442/100 = 34.42 PSI per cuft and 100 PSI is 2.9 cuft or .029 cuft per PSI
Putting it Together
Let’s assume our timed swim was done with an AL80 and we know that we consumed 400 PSI in 10 minutes.
That adds up to 40 PSI per minute at depth. If one PSI is 0.026 cuft, then we consumed .026 * 40 in one minute.
Our RMV is 1.04 cuft @ a depth of 20 feet
To take that to the surface:
Depth / 1 ATA) + 1 ATA for surface pressure … or ((20/33)+1) = ((0.61) + 1) == 1.61 ATA at 20 feet
Your RMV is: 1.04 cuft / 1.61 ATA = .65 (at the surface)
What is your RMV at 90 feet?
((90/33)+1) = 3.72 ATA 3.72 * 0.65 = 2.42 cuft per minute
What is your RMV at 50 feet?
((50/33)+1) = 2.52 ATA 2.52 * 0.65 = 1.64 cuft per minute
What is your RMV at 140 feet?
((140/33)+1) = 5.24 ATA 5.24 * 0.65 = 3.41 cuft per minute
Ignoring the NDL’s for a minute, how much gas do you need to spend 20 minutes at a depth:
Of 90 feet?
20 * 2.42 = 48.4 cuft
Of 50 feet?
20 * 1.64 = 32.8 cuft
Of 140 feet?
20 * 3.41 = 68.2 cuft
Now plan your dive and include a decent, a bottom time, an ascent and a stop.
Can you do this while keeping 1/3 of your gas in reserve?
Decent Rate: 60 feet per minute
Ascent Rate: 20 feet per minute
Stop(s): 3-minute safety stop, deco stops?
Building your Gas Plan
A Dive to 100 feet for 15 minutes with an RMV of 0.7
Runtime Time Depth Cuft Gas Required Phase
0 0 0 0 start
2 2 100 5.6 descent
17 15 100 42 bottom
21 4 20 11.2 ascent
24 3 20 3.3 safety stop
25 1 0 1.1 surface
25 63.2 cuft Total Gas Required
What cylinder volume is required to complete this dive following the Rule of Thirds?
95 cuft or more
You have two primary factors that will end your dive, what are they?
1. Dive time reaches 17 minutes.
2. Gas pressure equals your turn pressure.
What is Turn Pressure?
Turn pressure is the volume of gas remaining in your cylinder at which you must abort the dive regardless of your bottom time if you wish to follow your gas plan.
In the above example, we know we need 32 cuft for our reserve pressure and we also need 16 cuft for our ascent and stop. The dive should be aborted if the cylinder volume reaches 48 cuft remaining. If we assume a 100 cuft HP steel cylinder with a working pressure of 3442 PSI, the turn pressure for the above dive would be 1655 PSI. If you reach this volume before your 17-minute dive time you turn the dive and begin your normal, safe and slow ascent.
In recreational, no-stop diving, the converse can also be true.
If all members of a dive team reach their turn time and all have both NDL time left as well as more than their turn pressure, it is possible to extend the bottom time until a member of the team reaches either their NDL or turn pressure, at which time the team should immediately begin the ascent.
In technical diving, run-times and turn-pressures are absolutes that must be followed to the letter. Overstaying your planned time or gas plan will change a decompression schedule and may result in an OOA situation.