Gas Planning: It’s Not Just for Tech Divers

“Plan your dive and dive your plan.”

Gas Planning: It’s Not Just for Tech Divers

For most open-water divers today this phrase means checking your no decompression limits and perhaps reviewing communications and a route with your buddy. Unfortunately, one of the key steps for safe diving —planning for an adequate gas supply -- has been forgotten. A few simple calculations will tell you exactly how much breathing gas you’ll need on any dive and whether your cylinder is up to the task. The techniques described here are borrowed from the world of technical diving today, but they originally come from the more robust rec classes of yesteryear and they still work just as well in open-water applications.

Think about it: How many group dives are concluded early because one diver ran low on gas? How many divers surface ahead of the group, ending a dive for both buddies in the team because of insufficient gas supply? How many divers fail to reach the objective of a recreational dive – like reaching the bow of a shipwreck – for the same reason?

If you know your consumption rate, the planned depth and the duration of the dive, you can avoid all of these inconveniences by simply selecting a cylinder with adequate volume. Even if you are not the gas hog, this knowledge might help you deal with a potential dive buddy who is.

The process of planning your gas supply for any dive is very simple. It only takes about 10 minutes of dive time and three minutes with a calculator to complete.

STEP 1: KNOW YOUR SCR

The first step is to measure the amount of gas you actually use in a typical dive scenario, by measuring your surface consumption rate (SCR). On your next dive, drop to depth of 10 meters seawater (msw) or 33 feet seawater (fsw). Swim at an average pace for 10 minutes while maintaining that depth. As you begin your swim, carefully check your SPG and write down your start pressure. As soon as you end the swim, check your SPG again and record the end pressure. Subtract the end pressure from the start pressure to get the total gas used in psi/bar.

Example:

Diver A starts with 2,700 psi/183 bar and finishes his swim with 2,100 psi/142 bar in an aluminum 80 cu.ft./11-liter cylinder rated for 3,000 psi/200 bar.

183-142 = 41 bar used (metric)

 2,700–2,100 = 600 psi used (imperial)

The diver measured his gas use at 33 fsw/10 msw so he must now find the equivalent usage at the surface. First, convert the depth to pressure in atmospheres absolute (ATA) if you are using imperial measurements or to bars of pressure if you are using metric measurements.

33/33+1 = 2 ATA (imperial)

 10/10+1 = 2 bars (metric)

Then divide the total gas used by the pressure of the dive in ATA.

600/2 = 300 psi

 41/2 = 20.5 bar

Now divide by the number of minutes in your swim to find your SCR.

20.5/10 minutes = 2.05 bar per minute SCR (metric)

 300/10 minutes = 30 psi per minute SCR. (imperial)

Step 2: Using SCR to Plan Your Dive

If you are using the same size cylinder on all of your dives you now have enough information to plan any dive. Simply find the pressure in ATA/bar of your maximum dive depth and multiply it by your SCR -- 30 psi or 2.05 bar in this case -- to find out how much gas per minute you will use on your planned dive.

So, what is the maximum time this diver can spend on a dive to 100 fsw/30 msw based on his gas supply?

First, find out how much gas we need per minute at depth.

(100 fsw / 33) + 1 = 4.03 ≈ 4 ATA;  4 ATA  x 30 SCR = 120 psi per minute at depth

 (30 msw / 10) +1 = 4.0;  4 x 2.05 SCR = 8.2 bar per minute at depth

For deeper dives, you should plan to use the rule of thirds to manage your gas supply. So you would use one-third of your gas supply to swim away from the entry point or anchor line, use one-third to get back to the anchor line and keep one-third in reserve for any delays, your ascent, a safety stop and to breathe on the surface until you can exit.

With a 3,000 psi/ 200 bar cylinder, the diver would use 1,000 psi/66 bar out and 1,000 psi/66 bar back to the exit point. That gives the diver 2,000 psi/132 bar of gas to use. So to calculate the available time we have on this cylinder at this depth, we take the pressure available and divide it by the consumption per minute at depth:

2,000/120 = 16.6 minutes (imperial)

 132/8.2 bar = 16.1 minutes (metric)

The diver in this scenario has about 16 minutes of planned bottom time based on gas availability.

STEP 3: CHOOSING THE RIGHT CYLINDER

For divers who choose to select the proper equipment for their dive, instead of limiting a dive for the equipment they have, this information is not quite complete. Divers using cylinders of different sizes need to add one more step to the process and be careful not to fall into the trap of confusing pressure with volume. I frequently hear from divers who ask, “How can my buddy stay down so much longer when his tank only contains 2,400 psi/165 bar full and mine has 3,000 psi/200 bar?”

The problem is that the 2400 psi/165 Bar cylinder holds between 85 cu. ft./2,405 liters to 120 cu. ft./3,396 liters of gas and the 3,000 psi/200 bar cylinder only contains 80 cu. ft./2,264 liters. Adding to the confusion is the fact that the outside dimensions of the two cylinders may be very similar. Steel cylinders typically have thinner walls making the inside volume of the cylinder larger than a similar size aluminum cylinder but limiting the pressure they are rated to hold. So even though a steel 85 cu. ft./13 liter cylinder is almost identical in outward appearance to an aluminum 80 cu. ft./11 liter cylinder, it holds nearly 6 cu. ft./170 liters more gas with around 400 psi /27 bar less pressure.

To adjust your dive planning to any cylinder size, you simply need to convert your surface consumption rate from psi/bar to a volume measurement, commonly called a respiratory minute (RMV). For divers using metric tanks and the metric system this process is much easier as there is no need to convert! Metric tanks are rated in water volume from the outset (the liters of non-compressible water the tank will hold). A 13-liter cylinder holds 13 liters when it is empty. To find out how much it will hold full, simply multiply the 13 liters by the rated bars of pressure:

13 liters x 175 bar = 2,275 liters of useable gas.

To get the diver’s RMV, take the bars used per minute and multiply by the water volume of the cylinder. In our example above we used an 11-liter cylinder and the diver has an SCR of 2.05 bar.

11 x 2.05 = 22.55 liter per minute or an RMV of 22.6 liters.

Going back to our sample dive and using the rule of thirds, we need to determine how much gas the diver has to use.

2,275 liter/3 = 758 liters

The diver can use about 758 liters out and 758 liters back to the anchor line, or 1,516 liters prior to ascent.

How much time does the sample cylinder give the diver with an RMV of 22.6 liters on the sample dive?

22.6 X 4.03 = 91 liter per minute at depth. And 1,516 / 91 = 16.6 minutes of bottom time.

Now for the imperial measurement system in use by the U.S. and that other world powerhouse, Liberia.

Since most tanks in the U.S. are rated in cubic feet that is the unit of measurement we will use. To make this conversion we first need to determine how much gas, in cubic feet, is used every time the SPG drops 1 psi in the cylinder you used for your SCR calculation. We accomplish this by dividing the “rated” pressure of the tank into the “rated” volume of the tank.

For an aluminum 80 at 3,000 psi:

80/3000 = 0.0267 cu. ft. in every psi. We call this the conversion factor or CF.

Note: Remember to use the rated pressures and volumes NOT the pressure you read on your gauge for this calculation. These numbers can generally be found stamped into the metal near the top of the cylinder.

Now multiply the cylinder’s CF by the SCR from above:

30 x 0.0267 = 0.80 cubic feet per minute or an RMV of 0.8 cu. ft.

Going back to our sample dive and using the rule of thirds, we need to determine how much gas the diver has to use.

80 cu. ft./ 3 = 26.6 cu. ft.

The diver can use about 26 cubic feet out and 26 cubic feet back to the anchor line, or 52 cubic feet prior to ascent. How much time does the sample cylinder give the diver with an RMV of 0.8 on the sample dive?

0.8 x 4.03 = 3.22 at depth. 52/3.22 = 16 minutes of bottom time.

If this diver were doing this dive with a buddy who routinely dives to the U.S. Navy NDL of 25 minutes at 100 feet, he can simply reverse the math to determine what size cylinder he needs to complete a dive of similar duration.

Imperial: An RMV of 3.22 at depth x 25 minutes = 80.5 cu. ft. of gas. If the divers are properly using the rule of thirds, that 80 cubic feet equates to two-thirds of the total gas supply. This diver would need another 40 cubic feet of gas, or a 120 cu. ft. cylinder to take full advantage of the NDL.

Metric: An RMV of 91 liters at depth x 25 minutes = 2,275 liters of gas. If the divers are properly using the rule of thirds, that 2,275 liters equates to two-thirds of the total gas supply, so this diver would need another 1,138 liters of gas or a 19-liter cylinder to make the dive. (2,275 + 1,138 = 3,413;  3,413/175 bar = 19)

To be safe these divers would also need to match their gas consumptions in case they were forced to share gas at depth. That will be the subject of a future advanced training article.