Nothing is more critical to a diver than having gas to breathe.
Having a gas plan, and managing your gas, is critical to safe diving. And, with a few minutes playing with numbers, calculating your Rock Bottom (Min Gas) becomes a quick and easy exercise.
Understanding how much gas you consume over a given period of time, and how depth, conditions and your own health (mental and physical) can affect your gas consumption allows you to better manage your gas plan, as well as make informed and educated decisions on which equipment (such as cylinders) are most appropriate for a given dive.
It’s important to understand that several factors can affect your consumption.
Conditions, such as strong current, will affect how hard you have to work during your dive. If you’re kicking into the current, you’ll be working harder, and thus breathing heavier, than if you were simply drifting with the current. This is no different than varying conditions here on land. Your breathing rate right now, as you read this, is different than if you were outside walking up to the store, and significantly different than if you were running or cycling.
Just as on the surface, your health directly affects your consumption rate. Someone in very good physical shape who is kicking against a stiff current will not exert as much energy as someone in poor physical health.
Your depth on a dive has a very significant and direct effect on your consumption. In order for a diver easily breathe underwater, their regulator needs to deliver gas properly. The 1st stage regulates the high pressure from the cylinder down to intermediate pressure (usually ~145psi), the pressure the gas is delivered to the 2nd stage. The 2nd stage then reduces the pressure down to just above ambient pressure. If it were less than ambient pressure, the diver would have to work very hard to suck air.
If the 2nd stage delivers delivers gas at just above ambient pressure, at 100fsw (4 ata’s), the regulator is delivering gas at just over 4ata’s. In other words, it’s provided gas at over 4 times the pressure than that at the surface (1ata). So, you can expect to go through your gas at 100fsw 4 times faster than the same amount of gas at the surface (1ata), all other factors being equal.
“All other factors” are rarely equal, however. Another consideration of depth is the role narcosis plays. At 100fsw, the diver is narc’d, to varying degrees. Narcosis has a significant impact on consumption, and once stress or task loading enters the equation, consumption will increase very rapidly.
Stress psychologically triggers humans to breathe faster. It’s no different underwater. A stressed diver is going to breathe heavier (any many instances dramatically heavier), than a non-stressed diver.
Compounding / Multiple Factors
All individual factors above can significantly impact a diver’s consumption rate. Often times, there isn’t one single factor in play, however. And building an awareness underwater, to recognize when, and why, your consumption is higher than normal or expected, is very important to managing the gas plan. Understanding why allows the diver to make adjustments (decrease depth to decrease narcosis, change direction to prevent swimming directly into current, etc).
SAC vs RMV
There are two ways to work with consumption….pressure and volume. Typically, when discussing consumption on land, or when planning a dive, volume is used, as it’s easier to work with (explained in the next paragraph). However, when underwater, we use pressure, as that is the information our SPG’s provide us.
Working with volume is often easier than working with pressure because our volume is consistent, where the pressure will vary, depending on the cylinder. In short, if your average consumption volume is .75cuft per minute, it’s .75cuft/minute, regardless of the gas source. However, that .75cuft/minute will be a different pressure when breathed from an al80, than when breathed from a hp130.
Because of this, the most common and easiest method is to work with volume when developing a gas plan, then use Tank Factors to convert to pressure specific to the tanks being used.
SAC, or Surface Air Consumption (also called SCR or Surface Consumption Rate), is simply the amount of gas you breathe, in pressure (psi), in one minute, at the surface (1 ata).
Consumption by volume is called Respiratory Minute Volume, or RMV, and is simply the volume (in cubic feet) breathed in one minute at the surface (1 ata).
Before moving further, it’s important to note that “SAC” is the slang term often applied when discussing consumption rates. However, the RMV number (volume) is what is typically being discussed….ie “My typical SAC rate is .7”, meaning the diver is saying he breathes .7cuft/minute at the surface (his RMV).
When calculating consumption rates, we typically start with SAC, since we’ll be logging the information while underwater. We then convert it to RMV once on the surface. As you gain more comfort through practice and repetition, you’ll be able to easily work with SAC and RMV in your head.
To calculate your SAC rate, you need 3 pieces of information:
For a refresher, when converting depth in feet to ATA:
Pressure (ATA) = depth / 33 + 1
For example, 50fsw, converted to ATA is:
50 / 33 + 1 = 2.515ATA
Rounding to single decimal, or 2.5, is acceptable, and makes it easier to work with. At 50fsw, your regulator is delivering gas to you at slighter more than 2.5 times what it would at the surface.
Now that we’ve converted depth to pressure, we can calculate our SAC rate from a dive.
SAC = Gas Consumed (psi) / Time (minutes) / Depth (ATA)
For example, on a dive averaging 50fsw, for 20 minutes, a diver consumed 1500psi:
1500 / 20 / 2.5 = 30
The SAC rate for that diver, on that dive, was 30 psi per minute.
A couple of points, before moving on…
First, it’s important to use average depth, as that provides an accurate number for your depth for the entire dive. Using max depth will provide a unrealistically high SAC number, and using a shallower depth will yield an unrealistically low SAC number.
“On that dive” is important. As mentioned above, many factors affect your SAC rate. It’s important to determine your SAC rate over several dives, in varying conditions, to gain a better understanding of your average SAC rate.
Also, that SAC rate is specific to the tank used on that dive. If another diver on that team breathed the exact same amount of gas (in psi), but using a different size tank, he would have a different SAC.
Now that we’ve calculated our SAC rate, we can now convert it to RMV, which is a more useful number, as it is not specific to any tank.
To convert SAC to RMV, we need to account for the tank that was used. To do that, we calculate the tank’s “baseline”, which is similar to its tank factor, without multiplying by 100.
Baseline = cylinder’s rated volume (cuft) / cylinder’s working pressure (psi)
It’s critical to use the cylinder’s rated volume. That is, how many cubic feet the cylinder holds when it is filled to its working pressure. Additionally, it’s equally important to use the cylinder’s working pressure. That is, what pressure the cylinder is rated to be filled to, not what it was actually filled to for the dive.
For this example, let’s use an aluminum 80:
77.4 / 3000 = .026
The baseline for an al80 is .026. That number represents how many cubic feet of gas are in 1psi in that cylinder. If you want to determine its tank factor, you simply multiply that number by 100, which is 2.6. If you were using an hp100, you’d simply divide 100 by 3500, getting .029.
If you know the cylinder’s tank factor, you can simply divide that by 100.
Note that an al80 actually has a rated volume of only 77.4cuft, and not 80cuft.
Now that we know our SAC rate from that dive (30 psi/min), and the baseline of the tank used (.026), we can convert to RMV:
RMV = SAC x Baseline
30 x .026 = .78cuft
In this case, the diver breathed the equivalent of .78cuft per minute, at the surface.
Now that we know both the SAC and RMV, we can make some calculations to help get a better idea of our consumption while at depth. For instance, if the diver breathes ~.78cuft / minute at the surface, what would he breathe at 100fsw (4 ata)? You’d simply multiply the RMV by the depth, in ATA:
DCR (Depth Consumption Rate) = RMV x Depth (ata)
.78 x 4 = 3.12 cuft / minute
If the diver is using an al80 again, we can use his SAC number (30psi/minute), multiplied by depth in ata:
30 x 4 = 120psi/minute
We can also determine if the tank we want to use has enough gas for us to safely execute a dive. For instance, the diver with a SAC rate of 30psi on an al80, RMV of .78, wants to do a dive for 20 minutes at 100fsw. How much gas can he expect to use. For SAC:
30 x 4 x 20 = 2400psi
.78 x 4 x 20 = 62.4cuft
While 62cuft is certainly less than 77.4cuft, it would appear an al80 is adequate for this dive. However, we still have Min Gas / Rock Bottom to consider, in which case the diver does NOT have enough gas to spend 20 minutes at depth, and still have enough for Rock Bottom requirements.
Average Consumption Rates
Consumption rates are a range, and will vary not only dive to dive, but will vary throughout a single dive as well.
For consistency and simplicity, we use the following consumption rates:
Resting (deco, ascent) - .5
Bottom (some finning/kicking, light work) - .75
Working & stressed – 1.00
Some considerations with these average consumption rates…
First, if you know your consumption rates are higher than the average, use your true consumption rates in your gas plan calculations.
Second, if you know your consumption rates are higher than the average, work to get them down to the average. There are many factors that can impact your consumption rate, including kicks, buoyancy control, trim, cardiovascular health and experience, to name a few.
Putting it Together
You’ve probably noticed this isn’t an exact science. Our underwater adventures are very dynamic, with lots of variables that change often…environmental conditions, physical health, mental health, dive profile, task level, the unexpected big life encounter, etc. Those variables are the reason that calculating your consumption rate often is important. By doing so, you get an accurate portrait of your consumption in a variety of conditions and situations, allowing you to make educated decisions when formulating your gas plan, as well as managing the plan.
Copyright © 2005-2017 Frog Kick Diving. All Rights Reserved