The First thing to know is the basics of the oxygen consumption. For example:
At 30 m/100 ft, divers consume gas four times faster than they do at the surface.
At 40 m/130 ft, they consume gas five times faster than at the surface.
Put another way, a cylinder that would last a diver 50 minutes at the surface may only last ten minutes at 40 m/130 ft. That’s not a lot of time when there is so much water over your head.
Because of this significant increase in gas consumption, deep divers need to be able to answer the following questions:
If everything goes right, will I have sufficient gas to accomplish my goals, plus be able to make a slow ascent and safety stop?
If things do not go right, will I have sufficient gas to deal with contingencies — plus make the required decompression stops should I accidentally exceed the no-stop limits?
This type of thinking and planning is an integral part of technical diving. Learning to “think like a tech diver” can be equally beneficial to recreational deep divers.
Use one third of your starting gas volume to enter the cave.
Use the second third of your gas supply to exit the cave.
Keep the final third of your starting gas volume in reserve in case the feces impact the rotary accelerator.
In technical deep diving, this manifests itself as the 150 Percent Reserve Rule. In other words, if you think you will need 50 units of gas (i.e., liters or cubic feet) to accomplish a task, make sure you have 75 units available to do it.
You may have already used this formula without realizing it. Dive operators who take customers to walls, wrecks and reefs in the 20 m/65 ft to 40 m/130 ft range frequently tell those divers to begin their ascent before hitting a pressure of 70 bar/1,000 psi. Assuming a starting pressure of 210 bar/3,000 psi, this represents a reserve of an additional 50 percent.
This approach generally provides a sufficient gas for a slow ascent and safety stop, providing everything goes as planned. However, if divers have difficulty finding their way back to the anchor or ascent line, or are forced to share air with others, it may not be enough.
This approach also works, in part, because when diving from boats, divers need only reach the top of the anchor line or ascent line. The boat will be there waiting for them, eliminating the need for long surface swims.
A different situation arises when diving in lakes or quarries, or a place like Bonaire, where divers enter from shore, but where the deepest water may be some distance away. If divers in these situations remain at depth until they hit one third of their starting pressure, then make a direct ascent to the surface, they may find themselves facing a lengthy surface swim. Not only is this not a lot of fun, such heavy exertion after in gassing so much nitrogen can substantially increase the risk of decompression sickness.
Under these circumstances, it makes more sense to begin in deeper water, but then move to progressively shallower water in the direction of the exit. Doing so will not only reduce (or eliminate) any post-dive surface swims, it will allow divers to take advantage of the multi-level capability of their dive computers and greatly increase their bottom time. The catch is, you need to have sufficient gas to do this. This is where “thinking like a tech diver” comes in.
At first glance, the task of calculating how much gas may be consumed on every segment of a multi-level dive may appear daunting. In reality, it is not all that different from something with which you are already familiar.
You most likely know what kind of mileage your car, truck or SUV gets. In fact, you probably have at least two different mileage values you use — one for around town and one for out on the highway. If you tow a trailer, you may even have a third, substantially lower mileage value that you use.
Knowing what sort of mileage you can expect can be helpful in a variety of ways.
If you know, for example, how many liters or gallons remain in your gas tank, you can calculate whether you have sufficient fuel to make it to the next state, province or county, where gasoline prices are substantially lower.
If you notice a significant drop in the mileage your vehicle gets over time, it may indicate the need for service.
Determining your vehicle’s gas mileage is not that difficult.
After filling your tank, you note your mileage.
The next time you fill your tank, you note how far you have gone and divide this distance by the liters or gallons purchased to determine the kilometers-per-liter or miles-per-gallon you achieved between fill ups.
Of course, this sort of information is only valuable if you did one type of driving (i.e., city or highway) between fill ups. Also, if you calculate your mileage several times, you will notice that it is not always the same. Averaging the number you get for similar types of driving will give you a more accurate overall figure.
The diver’s equivalent of gas mileage is air consumption rate (although you may be using a gas mixture other than air, we will use the terms air and gas interchangeably). As with gas mileage, air consumption is affected by work load. For example, it is not unusual to use two thirds to one half the gas resting at safety stop depth as you would if swimming at the same depth.
More significant than the impact of work load on air consumption, however, is depth. Imagine if driving at altitude caused your vehicle’s fuel consumption to double. Well, swimming at a depth of 30 m/100 ft will cause your air consumption to quadruple.
Because air consumption varies significantly by depth, we need a baseline from which to work. For this, we use sea level; relating air consumption at depth to its equivalent volume at the surface. Thus, you will hear technical divers talk about their SAC or Surface Air Consumption rate.
Determining SAC Rate
Determining your own individual SAC rate is not all that different from figuring out your vehicle’s gas mileage.
You begin by swimming at a fixed depth for a specific number of minutes (typically five or ten), noting your starting and ending tank pressures.
You then divide the amount of air used by the number of minutes swam. This will give you a gas consumption rate in either bar-per-minute or psi-per-minute at depth.
Finally, you divide your air consumption rate at depth by depth in atmospheres absolute to determine the equivalent Surface Air Consumption rate.
If you always dive the same size cylinder, you can (theoretically) leave your SAC rate expressed in bar-per-minute or psi-per-minute. However, to be able to use this value with cylinders of different sizes, you will need to convert your SAC rate into liters-per-minute or cubic-feet per minute.
Metric Example: Peter swims for ten minutes at a depth of ten meters (i.e., two atmospheres). During that time he uses 30 bar. Dividing that number by ten minutes, he sees that his air consumption at depth was three bar per minute. To determine his SAC rate in bar, he simply divides that number by his depth in atmospheres (in this case two), arriving at a Surface Air Consumption rate of 1.5 bar/minute.
Peter is diving a ten-liter steel cylinder. Simply multiplying this volume by 1.5 bar, Peter determines that his SAC rate in liters is 15.
Imperial Example: Joe swims for ten minutes at a depth of 33 feet (i.e., two atmospheres). During that time he uses 400 psi. Dividing that number by ten, he sees that his air consumption at depth was 40 psi per minute. To determine his SAC rate in psi, he simply divides that number by his depth in atmospheres (in this case two), arriving at a Surface Air Consumption rate of 20 psi/minute.
Joe is diving an 11-liter/80-cubic-foot aluminum cylinder (actual capacity 78 cubic feet at 3,000 psi). Dividing 78 by 3,000, Joe determines that this works out to 0.026 cubic feet per psi. Multiplying this value by his psi-per-minute SAC value, Joe determines that his SAC rate is 0.52 cubic feet per minute.
When determining SAC rate, it is easiest to use depths that represent an even number of atmospheres (i.e., 10 m/33 ft equals two atmospheres; 20 m/66 feet equals three atmospheres, etc.). If this is not possible, you will need to be able to convert your actual depth to depth in atmospheres.
If measuring depth in meters, this is pretty easy. You simply add ten meters to your actual depth, then divide by ten. So, if swimming at 14 meters, you add ten meters to get 24, then divide by ten to get a depth in atmospheres of 2.4 meters.
If measuring depth in feet, you use basically the same formula; however, the math is not quite as easy to do in your head.
For example, let’s say you need to do your gas consumption check at a constant depth of 47 feet. By adding 33 to 47 you get 80; divide this number by 33 and you get a value of roughly 2.4 atmospheres absolute.
In addition to determining your “working” SAC rate, you will also need to determine a “resting” SAC rate to account for gas breathed during safety stops. You will find it best to do this at a depth of 5.0 m/16.5 ft, as this is precisely 1.5 atmospheres.
Accompanying this article are SAC Rate spreadsheets in metric and imperial versions. Each shows one of the examples given above. By substituting your own values for depth, time, starting and ending pressures, and cylinder capacity, you can use the spreadsheets to determine your own SAC rate (without having to do any actual math).
Additionally, some air-integrated dive computer download and logging software can calculate a SAC rate for an entire dive. All you have to do is provide information on cylinder capacity. This tends to be very accurate, thanks to the computer’s ability to constantly monitor precise depth and cylinder pressure.
Using SAC Rate to Plan Multi-Level Deep Dives
So now that you have determined your personal “fuel consumption” while resting and swimming, how do you use these values to plan a multi-level deep dive? In so far as gas consumption will vary, depending on depth, the trick is to break the intended dive down into its component segments, then calculate your gas needs for each segment.
When maintaining a constant depth, this is relatively easy. Let’s say you will be spending ten minutes on a wreck in 30 m/100 ft of water. Hopefully, by now, you recognize this as a depth of four atmospheres absolute — meaning that you will go through your gas four times faster than you do at the surface.
For example, if your SAC rate, in cubic feet, is 0.5 cubic feet per minute, you will consume four times this, or two cubic feet, for each minute you spend at depth. Over ten minutes this will be 20 cubic feet. Allowing for our 150 percent safety margin, you will need to set aside 30 cubic feet for this portion of the dive.
What about ascents and descents? During these, your depth is constantly changing. Fortunately, if your ascent or descent rate remains relatively constant, you can use the average depth of the ascent or descent to plan your gas needs.
In other words, if descending to 40 meters, your average depth during the descent will be 20 meters, or three atmospheres absolute. If your SAC rate is 15 liters per minute, you will consume an average of three times this, or 45 liters, during each minute of descent. For a two-minute descent, you will need 90 liters. This means that, for the descent, you should set aside 135 liters to allow for your 150 percent safety margin.
Here is an example of how this all comes together on a multi-level recreational deep dive. (Just so you will know, there are some slight differences in depth between the metric and imperial examples given here. We’re just trying to keep the math simple.)
This dive will take place at your local rock quarry. There is an old cabin cruiser at 40 m/130 ft; unfortunately, it is some distance from your preferred exit point. By walking part way around the quarry, you can enter from a dock that is only a two-minute swim from the descent line to the cabin cruiser. You are willing to make this swim before descending; however, you want to get out of the water as close to your preferred exit point as possible, to avoid exertion after a deep dive.
To accomplish this, instead of ascending directly to the surface, you plan to ascend to a dump truck in 20 m/60 ft of water. After checking this out, you will continue to the base of a dock in 5 m/15 ft of water, and complete your safety stop there. So how much time can you spend and how much gas will you need?
Metric Example: Here is how you might approach this problem using metric measurements.
Start by assuming that your descent will take two minutes, just as previously outlined. Using the values from our earlier example, you plan to use 90 liters during descent — but are setting aside 135 liters “just in case.”
Once at depth, you plan to spend five minutes checking out the cabin cruiser (hey, it’s cold down there). As you are at a depth of five atmospheres, your 15-liter-per-minute SAC rate translates into 75 liters per minute at depth, meaning you will go through 375 liters in five minutes. Allowing for contingencies, you set aside 532.5 liters for this segment of the dive.
As it is a little bit of a swim to reach the dump truck, you set aside five minutes for the next segment. This will represent a fairly continuous ascent as you work your way up the quarry’s sloping walls, making your average depth 30 meters or four atmospheres. Applying our basic formula, you should consume around 300 liters during this segment — but set aside 450 liters for contingencies.
The dump truck lies in much warmer water, and there is a lot to see. Therefore, you allow yourself ten minutes to look around. As the dump truck is at a depth of three atmospheres, you will go through a little less than 450 liters at this depth, and set aside 675 liters for Murphy.
The next ten-minute segment will take you from 20 meters at the dump truck to five meters at your preferred safety stop location. Your average depth during this segment will be 12.5 meters or 2.25 atmospheres absolute. Multiplying this value times your SAC rate and ten minutes, you plan on consuming 337.5 liters, with a total of 1.5 times this much, or 506.25 liters, set aside for the unforeseen.
Finally, there is your three-minute safety stop at 5 meters or 1.5 atmospheres. Remember that here you will be resting, so you can use your resting SAC rate of ten liters per minute. Over the course of the three-minute stop, this works out to a projected gas consumption of 45 liters, with 67.5 set aside for contingencies.
In theory, we should allow something for that final five-meter ascent to the surface but — guess what? — it is such a small number, we’re not going to worry about it.
The following chart summarizes all of the math we just did. In the Totals row, you will see that we will need a little less than 1,600 liters if things go as planned — but need to take at least 2,400 liters of gas with us in case they do not. A 13-liter cylinder filled to 200 bar (the equivalent of 2,600 liters at the surface) should do it.
Imperial Example: Here is how you could approach this problem using imperial measurements.
Start by assuming that your descent will take two minutes. Your average depth during the descent will be 65 feet — just a hair short of three atmospheres absolute. Assuming a SAC rate of 0.52 cubic feet per minute, you will need roughly 1.5 cubic feet for each minute of descent, or 3.09 cubic feet total. However, because “feces occur,” you will set aside 1.5 times this much, or 4.63 cubic feet for your descent.
Once at depth, you plan to spend five minutes checking out the cabin cruiser (hey, it’s cold down there). As you are at a depth of nearly five atmospheres, your 0.52-cubic-feet-per-minute SAC rate translates into a little more than 2.5 cubic feet per minute at depth, meaning you will go through 12.84 cubic feet in five minutes. Allowing for contingencies, you set aside 19.26 cubic feet for this segment of the dive.
As it is a little bit of a swim to reach the dump truck, you set aside ten minutes for the next segment. This will represent a fairly continuous ascent as you work your way up the quarry’s sloping walls, making your average depth 95 feet or 3.88 atmospheres. Applying our basic formula, you should consume around 10.08 cubic feet during this segment — but set aside 15.13 cubic feet for contingencies.
The dump truck lies in much warmer water, and there is a lot to see. Therefore, you allow yourself ten minutes to look around. As the dump truck is at a depth of 2.82 atmospheres, you will go through 14.65 cubic feet at this depth, and set aside 21.98 cubic feet for Murphy.
The next ten-minute segment will take you from 60 feet at the dump truck to 15 feet at your preferred safety stop location. Your average depth during this segment will be 37.5 feet or 2.14 atmospheres absolute. Multiplying this value times your SAC rate and ten minutes, you plan on consuming 11.11 cubic feet, with a total of 1.5 times this much, or 16.66 cubic feet set aside for the unforeseen.
Finally, there is your three-minute safety stop at 15 feet or 1.45 atmospheres. Remember that here you will be resting, so you can use your resting SAC rate of 0.4 cubic feet per minute. Over the course of the three-minute stop, this works out to a projected gas consumption of 1.75 cubic feet , with 2.62 set aside for contingencies.
In theory, we should allow something for that final five-meter ascent to the surface but — guess what? — it is such a small number, we’re not going to worry about it.
The following chart summarizes all of the math we just did. In the Totals row, you will see that we will need 53.52 cubic feet if things go as planned — but need to take 80.25 cubic feet with us in case they do not. An aluminum 80 will work for this dive — barely.
Isn’t This a Lot of Math for One Dive? Yes it is — and, fortunately, most recreational deep dives do not require this level of planning. Still, for those dives in which you can benefit from this type of planning, the pay off may come in the form of longer bottom times, shorter surface swims and less overall risk.
Nevertheless, you needn’t have a degree in rocket science to know, that when dive planning gets this complicated, a simple mathematical error can throw off your entire plan. Fortunately, we have made it easy for you.
Included with this article are dive planning spreadsheets in both imperial and metric formats. To use them, all you need do is plug in the values for your working and resting SAC rates, then, for each segment of your dive, plug in:
The starting and ending depths for each segment.
The projected duration, in minutes, for each segment.
The spreadsheets will do the rest, including calculate gas volume needed for each segment, plus a contingency volume based on the 150 percent reserve principle.
The spreadsheets have space to list up to nine separate dive segments, plus a safety stop at the end. (Note that the safety stop always needs to be Segment 10, as this row is programmed to use the resting SAC rate, not the working one.)
When you first open them, you will see that the spreadsheets have the values for the examples described earlier. Looking at the underlying formulas for each cell, you will be able to learn more about the math involved.
Additionally, you will see a column labeled Run Time. This will tell you how many minutes into the dive you should be at the end of each segment. It should correspond to the time shown on your timer or computer. The Run Time concept is an integral part of technical diving.
What Gas Planning Does Not Address
Hopefully, you have already noticed this. Just because you have sufficient gas to make a particular dive, it does not mean that you will also have sufficient bottom time — especially if the dive you are planning is a repetitive dive, or you are using a Nitrox mixture with a higher concentration of nitrogen.
You still need to determine whether multilevel dives, such as those discussed here, can be modeled as no-decompression dives using dive planning software or the PADI Wheel. During the dives themselves, you will need to carefully monitor your dive computer to make certain you do not accidentally overstay the no-stop limits.
The Benefits of Thinking Like a Tech Diver
You may never want to go past the recreational dive limits or deal with the added expense or complexity of technical diving. Nevertheless, as you have seen, “thinking like a tech diver” can have practical applications for recreational deep dives — especially when exceeding depths of 30 m/100 feet. If nothing else, what you have learned will give you a better understanding of diving and, hopefully, a greater appreciation for the challenges technical divers face.