Alternate Air Sources | Scuba Diving

Alternate Air Sources


January 1999
By Jon Hardy

Unfortunately, the answer involves one of the most confusing categories of dive equipment: the alternate air source. Diverse products, confusing terminology, conflicting procedures and a lack of standardization all make this piece of life-support equipment one of the hardest to get a grip on.

Today's alternate air source can be:

  • An octopus or octo regulator. The octopus is sometimes called a secondary regulator or by the hopeful misnomer "safe second." In reality, it can be any regulator second stage intended for use as an octopus.

  • An alternate inflation regulator. This device combines the octopus with the power inflator. They can be integrated into one unit or with the octopus reg as an add-on simply placed in line with the power inflator.

  • A redundant air supply. It uses an entirely separate cylinder and regulator. A redundant air supply can be a small, independent unit with an integrated regulator or a larger pony bottle using a conventional first and second stage regulator.

Selection: The Bottom Lines

Regardless of which type of alternate air source you select, remember that its performance will depend on the ability of the regulator first stage to deliver air, particularly when it is supplying two second stages and a power inflator. The bottom lines: Given the type of diving you plan to do, how will it work for you in a worst-case scenario? How much risk are you willing to accept? If you dive solo, under demanding conditions or in overhead environments, the greater your need for a completely independent air source and a high-performance regulator as your alternate air source.

Torture Tests

For this comparative evaluation we ran the usual breathing simulator tests, based on U.S. Navy procedures with one difference: Whenever possible, we used the same first stage to supply the second stages being tested in order to focus on the relative performance of the second stage. Next we conducted our usual ocean tests with multiple experienced evaluators scoring the ergonomics of:

  • Ease of breathing

  • Ease of clearing

  • Comfort

  • Differing positions

  • Bubble interference

  • Wet/dry breathing

  • Ease of air sharing

  • Use of adjustments or inflators if so equipped

Does Yours Measure Up?

If you're an active diver, you most likely have an alternate air source that you consider sufficient for your needs. Big question: Is it really? Are you willing to bet your life and your buddy's on it?

Since our last evaluation of alternate air sources in January 1995, there have been a few improvements, but no significant breakthroughs such as those we've seen in primary regulators. At the same time, alternate air sources still receive mainly lip service. Sure, they're required during training, and by most dive operators. But to what end? Sharing air is often not practiced, and the alternate air source is likely to be a regulator of lower performance that has not been well cared for. So what happens at the time air-sharing is needed? Chances are, neither technique nor equipment is up to the task.

Based on the tests carried out for this article, here are what we consider to be the fundamental aspects you should keep in mind for your alternate air source, its configuration and use:

  • It should not leak or free flow when not in use; detuning, user adjustments and keeping the mouthpiece turned down all help with this.

  • It should be securely attached so it does not swing free and get damaged by or cause damage to the environment, yet it should detach easily when needed.

  • It should be easy to use.

  • It should be able to be used in a right-side-up position for either diver: the donor or the needer.

  • It should be completely separate or have a hose that is long enough so that divers can share air without being so close that their movements are inhibited.

  • If the donor is passing a second stage to the needer, that reg should have a longer hose, regardless of which one it is, the primary or the secondary. We suggest at least 32- to 39-inch hoses, not the usual 28-inch hoses.

  • It is often best to use an alternate air source and primary reg from the same manufacturer. This is important because the primary first and second stage's intermediate-pressure and low-pressure port sizes, position and intended use vary. It is even more important if you select an alternate inflation regulator because it must match your BC's low-pressure hose and ambient corrugated hose, while still allowing the remote exhaust to work. Caution: If you mix and match products from different manufacturers, it is best to have your dive store technician set up your reg for you.

  • The performance of the first stage has a significant impact on the second stage's performance, particularly with two second stages and possibly an inflator all in use; therefore, it is important to use a high-performance first stage.

Knowledge is Golden: 10 Tips for Using Your Alternate Air Source

Someday--maybe not tomorrow or next month or even next year--but someday you will have to share air. Your preparation for this moment begins with equipping yourself properly: thus, this comparative evaluation of alternate air sources. But equally as important as what to share is how to share. Here's what you need to know when the inevitable finally happens.

1) Know this: It's almost impossible to be out of air. In most "out of air" emergencies, the diver is low on air, not out of air. After most dive accidents, the victim still has air in the cylinder and a regulator that still functions. What happens is that divers make human errors and breathe their air supply down so low that the regulator can no longer provide air at the effort level required by the diver.

According to textbooks, this occurs when the ambient or surrounding pressure is equal to tank pressure. Therefore at 100 feet this would occur at about 60 psi. Theory, however, rarely matches reality. Regulator studies and diver experience have shown that due to variations in the mechanics of regulators, diver breathing demands, regulator maintenance, and the inaccuracies of submersible pressure gauges, a diver will feel out of air at a cylinder pressure much higher than ambient pressure, and this disparity will increase as depth increases. These are among the reasons why the current practice is to return to the surface with 500 to 750 psi remaining in your tank.

2) Know this: Equipment can fail but rarely does. With a regulator, failure usually takes the form of an air leak, a water leak or a free flow. If the regulator causes a problem, it almost always fails safe (still provides air) and creates an inconvenience rather than a serious situation. Despite what textbooks and out-of-date instructors say, we do not learn to make emergency ascents because of equipment failures, but primarily because of human errors.

3) Know this: Power inflators continue to work even when you can no longer breathe from your second stage. A poorly known fact about power inflators is that they will continue to operate at a lower tank pressure than a diver can comfortably continue to breathe. At low pressures and greater depths, the first stage's flow rate is slower but the power inflator still works. On the other hand, the second stage uses a demand valve that the diver must suck on to activate. The regulator attempts to deliver air from the cylinder but cannot do so at low pressures. Despite the consequent "out of air" feeling, the power inflator will still work.

4) Know this: Sometimes the surface is a better bet than your buddy. Before you dash off to share air in a low-on-air situation, consider which is closer: your buddy or the surface. Remember that to share air you must not only get to your buddy, but must also communicate your needs, then take or receive another regulator to breathe from. This process could take more time than going to the surface using an independent emergency ascent. Of course, if there is an obstruction to surfacing, including a decompression ceiling, going to your buddy may be the best move. Regardless, this situation requires a judgment call only you can make.

5) Know this: Successful sharing requires rehearsal and practice. No matter the alternate air source you use or which reg you pass, you should agree with your buddy before the dive how each of you will handle an air sharing situation. You can rehearse on the surface before the dive and also practice in the water, though few divers ever do. You can also practice using your own alternate air source (regardless of type) during safety or deco stops. Yet another way to give you and your buddy practice at air sharing is to swim side-by-side during some easy part of the dive, letting whoever has less air breathe from the other's air supply. This also works to equalize available air supplies and to extend the dive.

6) Know this: The "Golden Triangle" can make it safer. An outstanding concept for how alternate air sources and primary regulators should be rigged is the Golden Triangle, originally brought forward by Glen Egstrom, a professor at UCLA. With this method of rigging gear, any regulator to be shared with another diver should be located in plain view on the front of the diver in the area between the right and left hip and the diver's mouth, forming the three points of a triangle. In this way, the backup is available to both divers.

7) Know this: Breathing alternately can prevent overbreathing. One of the significant concerns regarding air sharing is the possibility of overbreathing the regulator first stage when two second stages and possibly a BC or dry suit inflator are also in use. Overbreathing is also more likely at greater depths and lower tank pressures. One way to avoid overbreathing a reg is to breathe alternately: when your buddy exhales, you inhale. Using the inflator only in small, short spurts also helps.

8) Know this: Sipping can let you breathe. Because you most likely will be low on air and not out of air, breathing in a very slow relaxed manner--sipping air--can give you more time to get to your buddy and establish air sharing. Also remember that once you are sharing air, your power inflator will still work for buoyancy control, so there is no need to inflate your BC orally or to swim against negative buoyancy to the surface.

9) Know this: Your hose must be long enough. Whichever reg you pass to share air--the primary or the alternate air source--that reg needs a long enough hose (32 to 39 inches). Many divers have learned that using longer, smaller-diameter, more flexible hoses without hose protectors on their primary reg improves fit and comfort, regardless of how air sharing is to be done.

10) Know this: Buddy breathing is not a good idea. The obsolete skill of buddy breathing--sharing one regulator second stage--makes no sense in today's diving world. At its best, buddy breathing was a difficult skill to learn and retain. At its worst, it often failed in real emergencies with disastrous consequences. With the current standard being that all divers should be equipped with alternate air sources, there should be no reason for properly equipped divers to buddy breathe.

What's the Best Set-Up?

What's the best alternate air source configuration? Textbooks differ. Instructors argue. Newsgroups flame. And the manufacturers don't help: some alternate air sources are meant to be used in only one way. Others can be used in several. Result: There is no standardization in how alternate air sources are manufactured or rigged, nor is there a standard technique to sharing, including the fundamental issue of which regulator should be passed from donor to needer. Result: You must make choices based upon a clear understanding of what is right for you, and what is not.

Option: Donor passes the alternate air source to needer.

Advantage 1: Less risk to donor because donor's life support is not interrupted.

Advantage 2: With longer hoses, donor can mount alternate air source on either side (so long as the alternate air source passes to the needer in the correct position).

Advantage 3: Freedom of arrangement allows more use of color codings or markings on the alternate air source to improve its accessibility.

Advantage 4: Needer can take the alternate air source without involvement of the donor.

Option: Donor passes primary to the needer.

Advantage 1: Donor is in more control.

Advantage 2: Only the donor needs to know location and operation of the alternate air source.

Advantage 3: The primary is easier for needer to find and use since it's already in the donor's mouth and operational.

Advantage 4: Needer cannot take the reg without the donor knowing it.

Advantage 5: The primary is usually the better breathing reg. Alternate air source may have to be adjusted and cleaned before being used under water.

Advantage 6: Secondary alternate air source can have a shorter hose, be positioned closer to the donor's body and be pre-positioned to go correctly into donor's mouth.

Advantage 7: Most often the donor will be the more experienced diver, perhaps even a dive professional, and therefore better able to deal with the switching while maintaining control over the situation.

Accident reports indicate that regulator confusion is a significant contributor to the failure of air sharing, along with overbreathing of the donor's regulator, exhausting the air supply and lack of buoyancy control. Even though sharing air with an alternate air source is vastly superior and far safer than buddy breathing, it still has inherent difficulties and requires careful pre-planning and rehearsal for best results.

Performance by Group

All regulators in Groups B, C, D and E were submitted for testing during this or prior tests. These and other manufacturers also have other alternate air sources that were not made available for testing. All scoring is based on the suitability of these devices as alternate air sources, not as primary regulators. Tests were conducted to 198 feet, but comparisons were done at 132 feet. Scoring Guide: 1 = outstanding | 2 = very good | 3 = good | 4 = fair | 5 = poor

Group A: Primary regulators used as octopus regs

The Scoop: This is the route to high performance--and high price. You take the highest performing second stage you can find and afford and use it as an octopus. Case closed. Our lab and ocean tests clearly show that these regs have the best scores. All are primary regulators that have been Tester's Choices in the recent past. But remember: you're still sharing from the same air source and you still need to decide which reg will be passed. Diver adjustments on these models provide the advantage of being able to tune out possible leaks or free flows.

|||||| |---|---|---|---|---| | Alternate Air Sources Group A | Number of Adjustments| Tester's Choice| Best Buy| | Aqua Lung| Impulse| 2| Yes| No| | Aqua Lung| Micra Adjustable| 2| Yes| No| | Aqua Lung| Micra +| 1| Yes| No| | Aqua Lung| Sea 4| None| Yes| No| | Atomic Aquatics| Ti 2| 1| Yes| No| | International Divers Inc.| Osprey| None| Yes| No| | Mares| Abyss| None| Yes| No| | Mares| Ruby| None| Yes| No| | Mares| Voltrex| None| Yes| No| | Mares| XTR| None| Yes| No| | Oceanic| Alpha IV| None| Yes| Yes| | Oceanic| Delta II| 1| Yes| Yes| | Scubapro| G 250| 2| Yes| No| | Scubapro| G 500| 2| Yes| No| | Scubapro| R 190| 1| Yes| Yes| | Sea Quest| XR 2| 2| Yes| Yes| | Zeagle| Tech| 2| Yes| No| | Zeagle| Z| 1| Yes| Yes| | Prices are not reported here because these second stages are normally packaged as a unit with their first stages. | {mospagebreak}

Group B: Second stages designed and dedicated to be octopus regs

| | Pictured left to right: Oceanic Alpha IV, International Divers Inc. Osprey, Oceanic Omega, Oceanic Gamma, Sherwood Minimus (front), Aqua Lung XLC Octo, Aqua Lung Micra +, Sherwood Standard Octo, Oceanic Slimline, Oceanic Delta II, Zeagle Sentinel| The Scoop: These regulators include many from group A with yellow covers and longer hoses. Many of these group A models also became the Tester's Choices of this group. The others in this category are less costly, somewhat lower performance second stages that still serve well as alternate air sources.

The Breakdown: Of special note in this group:

  • The Osprey by IDI received the best overall scores for a dedicated octopus at a budget price. The Oceanic Alpha IV, Scubapro R 190 and Sea Quest XR 2 Octopus were very close behind.

  • The Oceanic Omega and Zeagle Sentinel are side-exhaust regulators, giving them the distinct advantage of no up/down or right/left. Consequently, they can be mounted on either side of the diver.

  • The Zeagle Sentinel also has an interesting feature: an optional on/off switch (isolator valve) that completely prevents leaks or free flows when not in use. Our test divers were concerned that if this reg were passed in the off position during an emergency, it could make a bad situation worse. Also, the switch functions counterintuitively: "on" is away and "off" is toward the regulator. Breathing machine tests revealed that the switch decreases performance under heavy demand. The Sentinel also comes with an extra long hose (48 inches) and a sleeve for storage.

  • Both the Oceanic Slimline and the Sherwood Minimus come standard as either right- or left-hand versions. Other regs on this list can also be special ordered for southpaws. Remember: the alternate air source still needs to be right-side-up for either diver.

  • The lowest scores in general for the dedicated octopus regs came in bubble interference and differing positions. Hopefully, both of these would be of limited concern for the short time they would be used. Ease of breathing and comfort were the next lowest scores, which is logical since these regs are not intended for continuous use throughout a dive.

  • Any adjustment on an alternate air source provides you the advantage of being able to detune the reg to prevent free flows or leaks. Six of these regs provide this feature.

Alternate Air Sources Group B Number of Adjustments Breathing Machine Ergonomics Tester's Choice Best Buy
Aqua Lung Micra + 1 2 1 No No
Aqua Lung XLC Octo 1 1 2 No No
Atomic Aquatics Ti 2 1 1 1 Yes No
International Divers Inc. Osprey None 1 1 Yes Yes
Mares Akros None 1 2 No No
Mares Nikos None 1 2 No No
Mares Voltrex None 1 2 Yes No
Oceanic Alpha IV None 1 1 Yes Yes
Oceanic Delta II 1 1 1 Yes Yes
Oceanic Gamma None 1 2 No No
Oceanic Omega None 1 1 No No
Oceanic Sumline None 2 2 No No
Scubapro R 190 1 1 1 Yes Yes
Sea Quest XR 2 2 1 1 Yes Yes
Sherwood Minimus None 2 2 No No
Sherwood Standard Octo None 2 1 No No
Zeagle Sentinel None 2 2 No No
Prices of octopus regulators vary according to packages offered by different retailers.

Group C: Alternate inflation regulators as add-ons

| | Sherwood Shadow+| The Scoop: These alternate inflation regulators were created by putting one of the dedicated octopus regs in line with the power inflator. As a result, breathing machine performance for these regs are nearly the same as above. In-water ergonomic scores, however, differ since the regulator and inflator are now in line together, whereas before the reg was used separately. You now need to pass your primary and switch to your alternate, if you are the donor. Often cited advantages: you eliminate one low-pressure hose; and since they are used often for buoyancy control, they are easy to find.

The Breakdown: The forerunner of this group and our Tester's Choice is the Sherwood Shadow +, simply a Minimus with an inflator; it is closely followed by the very similar Oceanic Inline Slimline. The Aqua Lung Air Mic is somewhat more difficult to use, as the inflator and reg get in each other's way, making the use of the diver adjustment and inflate/deflate valve awkward.

Alternate Air Sources Group C Number of Adjustments Breathing Machine Ergonomics Tester's Choice Price
Aqua Lung Air Mic 1 2 2 No $199.00
Oceanic Inline Sumline None 2 1 Yes $149.95
Sherwood Shadow+ None 1 1 Yes $185.00

Group D: Alternate inflation regulators as integrated units

| | Zeagle Octo+ MKII| The Scoop: This is the innovative group of alternate inflation regulators, pioneered by Scubapro with the Air 2, the first and namesake of this type of alternate air source. Other manufacturers have been challenging Scubapro in this field and will likely step up their efforts because Scubapro's patent has recently expired. At the same time, Scubapro plans an even better Air 2 for release in 1999.

In the past, this type of regulator has suffered from air leaks, less ability to swivel into position, a bulky profile and greater breathing resistance, yet it has provided simple integrated units that stay right where they're supposed to for buoyancy control, and eliminate one hose for less drag and snagging problems. With these alternate inflation regulators and those in group C, you not only know where your alternate air source is; you also know it works when you need it.

The Breakdown: In this group, the Scubapro Air 2 and the TUSA DUO-AIR are ergonomically superior with inflate/deflate buttons that can be easily used with one hand and don't require the diver to change hand position. The Oceanic Air XS offers the lowest work of breathing, but the inflate/deflate buttons are awkward to use. The Sea Quest Air Source has easy-to-use buttons and a comfortable mouthpiece, but also higher breathing resistance. The Air Source mouthpiece is of a different type and cannot be easily replaced with a generic mouthpiece, so buy an extra one to keep in your save-a-dive kit. Both the Air XS and Air Source have a special fitting midway up the corrugated ambient BC hose so the remote exhaust can be used more easily.

One little known difficulty of these integrated alternate inflation regulators is that if you exhaust your BC through the oral deflate when the BC is full, or nearly so, and inhale at the same time, you will actually be breathing from the BC at a much higher breathing resistance. To prevent this, use one of the BC's remote exhausts, keep the BC less full or do not inhale at the same time. The Zeagle Octo+ MK II routes the airflow differently, and thus is the only alternate air source in this group that does not have this problem. Nonetheless, the Zeagle model has more awkward buttons, yet you can operate them by sliding your thumb from one to the other.

As a group, these alternate air sources are best at ease of clearing and dry breathing and at their worst in comfort and bubble interference.

Alternate Air Sources Group D Number of Adjustments Breathing Machine Ergonomics Tester's Choice Price
Oceanic Air XS None 1 2 No $229.95
Scubapro Air 2 None 2 1 Yes $199.95
Sea Quest Air Source None 3 2 No $230
TUSA DUO-AIR None 3 1 No $210
Zeagle Octo+ MK II None 2 2 No $193

Group E: Redundant air supplies with small cylinders and integrated regs

| | Submersible Systems Inc. Spare Air| The Scoop: These small redundant independent air supplies are the most controversial of the alternate air sources, receiving both significant praise and criticism, especially for the limited amount of air they offer. One brand, the Spare Air, is a source of continuing controversy on one internet newsgroup, with opponents providing complex calculations to try to prove that an in-trouble diver could not, in theory, surface safely with such a small capacity of air.

On the other side are defenders who forgo the math lessons and point to real world use: By providing a separate supply of air, these units work regardless of what goes wrong--you are low or out of air, your buddy is low or out of air, the both of you are low or out of air, a regulator fails for either of you. Adding their voice are the test divers at ScubaLab who have taken these units to 130 feet and deeper, then made controlled ascents with them, no problems, no math.

The truth? Like any system, these units have their advantages and disadvantages, best uses and dangerous ones. In this case, the units are best suited for general open-water diving or limited penetrations into caverns, under heavy kelp or areas of boat traffic. If you need to use one of these units, move promptly to shallow water (10 to 20 feet) and breathe the rest of your redundant and primary cylinder's air as a safety stop. This way the limited air will last longer and your out-gassing will be more effective. Those who do extended-range diving and especially diving in overhead environments that require horizontal exits should not rely on these units as their bail-out.

The Breakdown: Submersible Systems Inc., with its Spare Air, was the first manufacturer to produce these alternate air sources and has had the most success and greatest following over the last decade. They have also continued to make improvements, offering both 1.7- and 2.7-cubic-foot models and now three different ways to fill them. The basic filling method is to equalize pressure directly with your primary cylinder; in addition, a fitting is now available so you can do this through a high-pressure port on your regulator's first stage. Of even greater value is an easy-to-use fitting that allows the Spare Air to be filled directly from a fill station or compressor. This method assures a complete fill without sacrificing any air from your primary cylinder. The Spare Air has a simple single-stage wholly integrated regulator that is always on, making it the essence of simplicity in an air-sharing situation.

The Micra Air System (MAS) by Aqua Lung uses a 2.5-cubic-foot tank with a simplified, but more conventional first and second stage regulator joined by the usual low-pressure hose, thus providing greater flexibility. Although it comes with an attachment for filling from your primary cylinder, an adapter for use at a fill station or compressor is available. The MAS turns on by rotating the entire cylinder to the correct position. This is intended to be done prior to starting the dive; the MAS is difficult to turn on when under water if this step is forgotten. As one of its best features, the MAS has an easy-to-read pressure gauge. The MAS also comes in a smaller 1.7-cubic-foot military version: the HEED.

Both of these units are best at comfort, dryness and ease of clearing, but weakest at bubble interference and breathing resistance.

Almost in a category by itself is the X-tra by Aquavit. It comes in four sizes of bottles (6, 13, 19 or 30 cubic feet) with a first-stage regulator, high-pressure transfer hose, an excellent mounting bracket and very thorough instructions. It is intended to be mounted beside your primary cylinder in such a way that the high-pressure hose feeds air from your primary first stage to the redundant regulator's first stage. This line has a non-return valve so the air can go only in one direction (from your primary cylinder to the smaller redundant cylinder). The high-pressure connection is screwed on or off by hand, using left-handed threads. The instructions recommend that you fit your existing alternate air source, BC inflator, dry suit inflator and submersible pressure gauge to the X-tra first stage. Care needs to be taken as to which high-pressure port is used for the pressure gauge, so you will know which air pressure is being read: the primary or redundant cylinder, as they will be different. For our tests we used the six-cubic-foot model with hoses mounted as recommended and a high-performance alternate air source. We also used three pressure gauges; one on the primary and two on the X-tra, so we could monitor each cylinder's pressure and the pressure differential.

The reason given for mounting the hoses through the X-tra first stage was so you would be assured that the redundant system was working. The instructions also stated that buoyancy control would use less than one cubic foot of air during a typical dive. Our tests show that with multilevel wetsuit or dry suit diving, the use of air for buoyancy control is greater than one cubic foot; and because the pressure drop from breathing is greater in the primary cylinder than the pressure drop due to buoyancy control in the redundant cylinder, the redundant cylinder is not replenished. Because the feed from your primary cylinder to the redundant cylinder is through your regulator's first stage high-pressure port, with its extremely small orifice, the transfer rate is so slow that the X-tra system can easily be overbreathed. Both the breathing simulator and the human testers repeatedly overbreathed the system due to depth, supply pressure or breathing rate. Even when tested on the simulator with an unlimited stable air supply, the X-tra first stage failed to perform adequately beyond 132 feet. Human testers were able to easily overbreathe the system at 15 to 60 feet with supply pressures of 550 to 750 psi.

If this system is to be used, you should select one of the larger cylinders, keep your inflator hose on your primary regulator and maximize the air fill in the X-tra cylinder. Given all this, it will cost less and you will get higher performance by using a conventional pony bottle that is filled to capacity and with a better performing first and second stage regulator.

Alternate Air Sources Group E Number of Adjustments Breathing Machine Ergonomics Price
Aquavit X-Tra NA 4 2 $395
Aqua Lung Micra Air System None 4 2 $575 w/cylinder $450 w/o
Submersible Systems Inc. Spare Air None 4 2 $233
See text for limitations on performance.

Group F: Redundant air supplies with pony bottles and separate first and second stage regulators

The Scoop: For years, divers have assembled pony or bail-out bottles for diving situations that require redundancy and significant air capacity. This is an open field for mixing and matching components. Many different sizes of cylinders are available, as well as regulators--from the best high performers to simple less costly regs made just for these systems. The disadvantages of pony bottle and reg continue to be the bulk, out-of-water weight, rigging, drag and cost. But if your diving requires true redundancy, this is the way to go. The primary manufacturers who supply these types of products are Aqua Lung, Oceanic, Sherwood and Zeagle.