I’m very interested to hear your opinions about this topic. I’ve been a technical diving instructor for a long time. When I started, most deep diving activity involved using big, back-mount double scuba tanks and a backplate and wing as a harness/BCD.
The cylinders are paired using an isolator manifold allowing the use of redundant regulators. The isolator part is basically a cutoff valve between the two regulator “posts.” In the event of catastrophic valve failure, you can spin a knob and preserve whatever gas is in the non-leaky side.
Advantages of diving doubles with an isolator manifold are redundancy, massive gas supply and, as mentioned, the ability to preserve gas. If there is a regulator failure, you can simply shut off gas flow to the problem reg.
Advantages of Diving with Doubles
- Large volume of available breathing gas
- Simple operation and maintenance
- Redundant, backup regulators
- Ability to preserve gas in the event of a failure
- They don’t roll around in your truck bed
- They look cool
Doubles can simplify gas management
On the other hand, back-mount doubles are bulky and heavy. Smaller divers, or those with back or other problems often struggle with them. Getting in and out of the water, especially on a soft sand or gravel slope, can be outright dangerous.
Disadvantages of Diving with Doubles
- Double scuba tanks are big and heavy
- Doubles kill your knees and hurt your back too
- Less flexible divers often struggle with valve shutdowns
- You can’t see a problem that’s behind your head
- With that as background, I am having fewer and fewer technical diving students take courses in doubles. Consistent with that, I rare dive in doubles anymore either.
Technical Diving Gear Trends
I’ve been asked, what gear do technical divers prefer using these days? For starters, I’d say about 2/3 of my students prefer to learn technical diving in sidemount scuba. I’ll admit, when sidemount diving started to make it out of the caves, I didn’t see much point. Largely, this is because in deep, open water, diving in back mount doubles simplifies gas management. There’s also less clutter. Having all your bottom gas in your back, frees a ton of space for travel and deco gasses to be clipped off on your chest d-rings.
If I’m being honest, I find back mount doubles a little more comfortable in the water—especially if I need argon, plus a few deco bottles. In warm water, diving sidemount when using standard aluminum 80s, the tank bottoms get pretty floaty once the pressure drops to the 2,200psi range.
In Truk Lagoon recently, I did all the dives in sidemount. It makes wiggling through passageways pretty easy. I love that, even if I did have to turn sideways quite often. But, not too long into the dive, I’d need to reposition the bottom clips to compensate for the change in buoyancy. It’s not a problem, but along with the buoyancy change, your body position starts to matter quite a bit.
In my cold local water, I don’t dive with AL80s at all. Most often, I dive with Faber LP50s, and they are glorious in and out of the water. They’re light and skinny and stay negatively buoyant, even when empty. I carry them down to the water, along with any deco cylinders I might need and put it all on in when I am a little less than waist deep.
I warm water, sidemount makes technical diving possible just about anywhere. I’ve done hundreds of air-depth technical dives in places where doubles aren’t available at all. At Riding Rock in the Bahamas (go there!), it is so remote, they can’t get oxygen cylinders to the island at all due to cost. They tell me they will have nitrox at some point, which would be great for speeding up deco, but I can’t say I mind long-ish deco stops there anyway. It is warm and spectacular, even at 10 feet deep.
Advantages of sidemount scuba
- Sidemount scuba makes surface logistics more manageable
- Sidemount is easy on your back and other body parts
- Technical diving is possible in remote locations where doubles are unavailable
- Your valves are located upfront and within line of sight
- Gas shutdowns are a breeze
It’s not all roses with sidemount scuba. I never take sidemount on a recreational diving boat unless I am doing mostly deep dives on a given trip and that’s all I brought. On a crowded dive boat, you’ll be a bit of a space hog. It’s isn’t too bad, or at least doesn’t have to be.
I personally don’t like diving in sidemount with one one tank. No matter how “neutral” a cylinder is, its buoyancy will change throughout the dive. It’s lopsided and annoys me to pieces. There’s just no point. In back mount, you sit down, put your gear on, waddle to the swim deck and jump in.
Of course, there are always easier ways of doing thing. You can clip your sidemount cylinders on in the water. (As well as pass them up one at a time after the dive.)
Back to my initial question, “are doubles dead?” Without question, sidemount is making a major dent in the number of divers choosing back mount doubles.
Disadvantages of Sidemount Scuba
- Can be very crowded if you need to carry multiple deco gasses
- More complex gas management as you run out of places to clip tanks
- More complex and slower to don
- Buoyancy and trim tend to change throughout your dive
- If you use argon inflation, there never seems to be a good place to clip your drysuit inflation system
- You look like a dork on plain ol’ recreational dive boats (you also likely will annoy the dive staff)
- There is an arguably higher risk of switching to the wrong deco to travel gas
Why Choose Back-Mount Doubles?
I’ve already listed some advantages and drawbacks, but the one place where I believe doubles win is extremely deep trimix dives. If you need a lot of bottom gas, it is a near certainty that you also need a lot of deco gas.
On a 100 meter (330 feet) dive, you’ll likely be carrying your bottom mix, 3 deco mixes and possibly a separate travel gas. Most of your deco bottles will need to be AL80s in order to meet your reserve requirements. Imagine carrying all that in your armpits and up front. It’s doable, but a handful.
Back mount doubles clear a lot of that up. Moving your highest volume scuba tanks (bottom gas) to the back frees a huge amount of space on your chest.
The Price of Helium (Trimix)!
In some locations, I’ve seen the helium price rise above $6 per cubic foot! Even in my relatively tame local area, we’re paying over $2 per cubic foot and it is almost guaranteed to go up significantly in the coming months.
Do the math… Let’s say you need 140 cubic feet of Tx12/65 (12% oxygen, 65% helium) to conduct a dive. With a reserve, you’ll need to carry 210 cubic feet of trimix. That works out to 136.5 cubic feet of helium. Even at $2 (unlikely), that’s $273 just fore the helium!
But, you’ll also need helium in one of your deco mixes. Let’s say you need to fill an AL80 with Tx19/20. There’s another $40. Add to that, the cost of Oxygen, OCA and blending and you’ll be paying at least $350 to do a dive that’s spent mostly hanging on a reel. A more realistic price for helium and the gas price can easily double or even triple.
Very few divers are motivated to spend that on a single dive. I’m certainly not—especially given that the same dive on a closed circuit rebreather would cost $30-$40, including scrubber.
Rebreathers Make Deep Trimix Very Unattractive on Open Circuit
I won’t get much into how rebreathers work here. In simple terms, a rebreather allows you to add oxygen at exactly the rate that your body consumes it. To keep the oxygen partial pressure constant (and survivable), you carry a second gas to dilute the oxygen. That’s where helium would be if you are doing deep dives.
This is a little in the weeds, but I have 13, 15 and 23 cubic foot rebreather bottles. In round numbers, a similar dive to 330’ using Tx12/65 (again, that’s 12% oxygen, 65% helium) requires 15 feet of helium if I fill the largest pair of cylinders I have—the 23s. That adds up to $30-$35 cost of helium compared to at least $300+ for open circuit diving using doubles.
The key point is, on a rebreather dive, you only use enough helium to maintain your PO2 and loop volume. Chances are, you’ll only use a couple cubic feet of helium for your entire dive. Your CO2 scrubber will actually cost more than the helium will. That doesn’t account for bailout and a couple other things that may or may not cost a bit more, but the math speaks for itself. Rebreathers start looking pretty cheap considering the high cost of helium.
So, Are Back-Mount Double Scuba Cylinders Obsolete?
Well, to take a line from one of the best movies ever, I’d say doubles only mostly dead. For more extreme dives, I can make a solid case for back mount. For example, the dive guides on my recent Truk Lagoon trip used doubles. It simplified logistics and made sharing gas easy if it became necessary. (On a side note, I have a lot to say about how many of these dives are conducted, so stay tuned, and read my old article about diving the S.S. President Coolidge in Vanuatu.)
Also, for air dives with long deco, doubles are probably a less cluttered configuration. Personally, my SAC rate is quite low, so I can do some pretty big dives using small-ish tanks, while still maintaining my reserves.
Aside from special cases like these, I have a hard time rationalizing doubles. Two years ago, I had 4 sets. Now I have one (LP85s). They just sit there, out of hydro and full of trimix I don’t dare dump. Eventually, I’ll boost it into a CCR diluent bottle and probably sell that last set to someone that will split them and use them for sidemount. Or I’ll do it.
For air depths, sidemount is more manageable on land, and you can use normal cylinders. Mine are fancy with right and left hand valves, but they don’t have to be. On air, you don’t need so many deco mixes, so sidemount is a great option.
Once helium comes into play, the cost of open circuit scuba is prohibitive, so most deep technical divers are switching to rebreathers.
And, that’s it. Not surprisingly, most open circuit cave divers are switching to sidemount. It makes sense. Sidemount provides a more compact profile for diving in tight spaces. So, if not dead, doubles are suffering a huge decline. I’d love to hear your thoughts.