You’re thinking – and rightly so – that you really ought to have a redundant air source in case the shit hits the fan.
Those little funky Spare Air bottles look neat. Will they do?
Not by a long shot.
Let’s just do the math.
Suppose you’re going to dive the Niagara II in Tobermory. She’s a fun wreck that sits on the bottom at around 100 feet, although there’s lots to explore in the 50 to 70 foot range.
To understand how much backup air you need to carry in order to get yourself and/or your dive buddy out of trouble, you’d need to know exactly how much air you need. How long a tank of air lasts depends on three things:
- Your depth or absolute atmospheric pressure
- How long it will take you to ascend to the surface at a safe rate of ascent of no more than 30 feet per minute
- Your rate of breathing.
Absolute Atmospheric Pressure
You remember how to calculate this, don’t you? OK.
(Depth + 33) ÷ 33
Pretty straightforward. If you’re at 100 feet when Mr Murphy decides to visit, it should take you about 3 minutes to get to the surface if you maintain a nice controlled ascent rate of 30 feet per minute. Yes, it’s twice as slow as what you probably learned when you took your open water class, but for extra safety, 30 feet per minute is recommended.
Rate of Breathing or SAC Rate
We refer to our breathing rate as Surface Air Consumption rate or SAC rate. For us North Americans, SAC rate is measured in cubic feet per minute.
SAC rate is a useful way to work out how many cubic feet of air you need to take with you to make a safe dive, with lots to spare for emergency situations.
Most recreational divers aren’t trained or have been trained to measure their SAC rates, although this is a relatively straightforward thing to do. More on this in another article.
In general, a very comfortable and experienced diver can achieve a SAC rate of between 0.5 to 0.6 cubic feet per minute during the working part of a dive (that is, swimming around) and maybe half that during a safety or decompression stop. For most divers who get out once in a while during the summer or while on vacation, it’s not uncommon for that rate to double to 1 cubic foot per minute, especially in stressful situations such as a regulator free flow, an out-of-air, or even a low-on-air scenario.
For the simplicity of demonstration and worst case scenario, let’s assume that in a stressful state, your air consumption goes through the roof at 1 cubic foot per minute.
What’s in Your Tank?
Most divers use 80 cubic foot aluminum tanks. If you thought that means each of those tanks holds 80 cubic feet of air at 3,000 psi, you’d be wrong.
In fact, 80 cubic foot aluminum tanks hold slightly less than that. Catalina’s 80 cubic foot tanks, which you’d likely be familiar with, have a capacity of 77.4 cubic feet at 3,000 psi. This means that assuming your tanks are fully pressurized, each psi in your tank represents about 0.0258 cubic feet of air. The math looks like this:
Tank capacity in cubic feet ÷ psi or
77.4 ÷ 3000 = 0.0258 in our example.
So, you’re at 100 feet when you’re faced with the need to ascend with insufficient air in the 80 cubic foot tank on your back.
Since you’d be leaving the bottom, the pressure will be decreasing as you ascend, so you won’t be pulling on as much air as you make your way to your safety stop at 15 feet. So, instead of calculating your total gas requirements based on the pressure at 100 feet, we’ll take an average of the distance between your maximum depth of 100 and your safety stop at 15 feet.
Average Depth = (100 – 15) ÷ 2 = 42.5 feet
We’ll need to convert that depth into pressure:
Average Pressure = (42.5 + 33) ÷ 33 = 2.28 atmospheres absolute (ATA)
Next, we’ll need to account for how many minutes it will take you to travel from 100 feet to 15 feet, a distance of 85 feet. Assuming an ascent rate of 30 feet per minute, the math looks like this:
Time (in minutes) = 85 ÷ 30 = 2.83 minutes
Now, to find out how much air in cubic feet you’ll need to get yourself from 100 feet to 15 feet, here’s the math:
Time (in Minutes) x SAC Rate x ATA, or
2.83 x 1 x 2.28 = 6.46 cubic feet
That’s about 6.5 cubic feet.
Almost 7 cubic feet to get from 100 to 15 feet.
Remember, at 15 feet, you really ought to do a safety stop for 3 minutes. That will need a few cubic feet as well. Since you’re now within sight of the surface, we can assume you’ll relax a bit and not be sucking it back at 1 cubic foot per minute. Let’s say you’re relaxed enough to dial it back to 0.5 cubic feet per minute. For a three-minute safety stop, you’ll need another 2 cubic feet of air. Not a big deal: you can likely get that from the 80 cubic foot tank on your back now that you’re under only 1.45 ATA of pressure.
To re-cap, to get out of Dodge from 100 feet, you'll need 6.5 cubic feet plus 2 cubic feet for a total of 8.5 cubic feet. And that's just the bare minimum.
Will Spare Air cut it?
Let’s see. They offer two models:
- one with a capacity of 3 cubic feet
- one with a capacity of 1.7 cubic feet.
And that’s assuming they’re fully pressurized to 3,000 psi.
You need 8.5 cubic feet.