Alternate Air

By: Mark Phillips

It’s 3 am when the phone rings and wakes you from a sound sleep. The call is from your dispatch center, and they inform you that a car is in the river and there are victims. You tell them you are on your way and to activate the dive team. 

The drive is quick. At 3 am there is little traffic and the moon is full. On the way, you call to see which dive team members dispatch was able to reach. The team members responding are all top-notch. The dive truck is on the way from Headquarters and someone has been sent to get the dive boat to launch it and bring it to the dive site.

On scene, you gather info and assess the dive site. Water temp is 73 degrees. Witnesses are able to pinpoint where the car went into the water and saw where it went down after floating downstream a while.

From past experience, you know the depth is about 60 feet and the bottom is littered with junk and old weathered trees. The team members responding can handle that. 

Once everyone has arrived and has been given assignments, you have the opportunity to watch them prepare for the dive at hand. Will this be a scuba dive or a surface supplied air dive?  Surface supplied for sure….but….  After the last training day, the airbox was in need of servicing and was to be sent out. Did we get it back? Why don’t you know this? 

The airbox is on the truck but still has a red tag on it. It was never sent for servicing. If it was red-tagged, can we take the chance it is OK? After all, it just needed servicing. What’s the worst thing that could happen? Your divers all carry additional air, right? 

3:45 am and the dive boat has arrived. 15 minutes later it has been loaded and the divers are preparing to start their first boat dive. A weather front has moved in and has completely blocked the moonlight, causing long and dark shadows to play on the water. What light there is comes from streetlights on the bridge and spotlights on the responding vehicles.

Question 1: When your divers enter the water will they be on surface supplied air or will they be configured for scuba?

Question 2: However they are configured, can you identify the air sources each diver is wearing?

Question 3: However they are configured, can you identify the air supplies that are available to each diver?

Let’s focus on these questions 

The airbox was red-tagged. If you use it and a diver gets in trouble or worse, how do you think it will go when you are asked WHY you used gear that was tagged for servicing?  Why it was red-tagged and why it was not sent out for servicing and instead put back into service is another matter. It should not be used if the reason it was red-tagged was not rectified. 

For this dive, scuba it is

What do you identify as an air source? In recreational scuba, an additional air source is usually your dive partner’s extra second-stage dive regulator – the octopus. If you are sourcing air, you are using a device to access air.

What do you identify as an air supply? 

Do you use a separate air cylinder as a backup to your primary air supply?  Most of us use pony cylinders or in some instances, an 80 cuft cylinder. These contained cylinders of air are alternate air supplies. Without a means to access the air supply, the diver cannot source that air.

The air the cylinders hold is accessed by a first stage and a second stage regulator. Combined, the pony cylinder and the regulators are used to complete an alternate air system.

So if your divers are diving with surface supplied air as their primary air supply, you use a gas switch block attached by a hose to your alternate air supply (your pony cylinder), and your primary air flows through the gas switch block to the second stage regulator on your full face mask, you carry an alternate air supply -NOT an alternate air source nor an independent alternate air system.

To have a true alternate air system, you will need to add another air source to the pony cylinder regulator first stage. You basically add an independent second stage that directly sources the air supplied from your pony bottle and bypasses the entire primary system. This addition gives you a true alternate air system that should be carried by each diver and is immediately available to that diver in an emergency.

Bailout systems 

A bailout system is often interchanged with the alternate air system previously described. While similar, the intent and function are a bit different. A bailout system requires the diver to have a primary source of air and supply as well as a separate alternate air system that can be used in an emergency. 

In some instances, a bailout system – designed to be used once the primary system has failed – may allow the diver to shed the primary air system to self-rescue. The bailout system allows a diver to get to safety. That could even mean having enough air to do a decompression stop or simply reach the surface from the diving depth. A typical attached pony system may not be as easily shed.

What about scuba teams? 

If your team is diving all scuba and uses 80 cuft cylinders as the primary air supply and a smaller pony cylinder, you may still run the system through a gas switch block. To make the pony bottle air supply independent of the primary, an additional second stage must be utilized as well. 

The size of the pony cylinder is important. If you carry an alternate air system for potential emergencies, how much air do you want? Of course, the more the better, but the size and weight become a factor. The basic rule of thumb is the deeper the dive, the larger the cylinder.

For public safety diving, particularly in zero visibility, shedding your primary air system is not a common need. Some agencies will teach that you can leave your rig with a diver who may be trapped underwater. This assumes the diver in trouble has had an out-of-air emergency and needs the extra air to survive while a rescue is attempted. If this is the case, then a bailout system is indeed needed. But consider the implications.

If the diver is using surface supplied air and an 80 cuft cylinder is included in an alternate air system and is in the predicament above, why do we need to leave an additional 80 cuft cylinder? So the first rescue diver in can leave? Entangled or trapped is far different than an out-of-air emergency.  If the divers are properly trained and prepared, there should be a very low probability of experiencing an out-of-air emergency, but it could happen.

It is that very low probability emergency we should be prepared for and train for.

In zero visibility, the diver in need, if entangled, may need air eventually, but needs help getting free first. If the backup diver did indeed leave their alternate air system with the primary diver and returned to the shore, what exactly is your next step? What is your team prepared to do if one of your own is in trouble underwater?

How does your team outfit the divers? Is your system designed to leave an alternate air system with a diver in trouble?  When exactly was the last time you practiced and prepared for this type of emergency?

What if your diving configuration is not a shedable bailout system, but an alternate air system that is not as easily shed? Who would be more impacted by the difference, the primary diver or the backup diver attempting to aid the primary? Keep in mind, if you are following the teachings, all the divers are outfitted the same. The gear brands may differ but placement is the same. If the primary was having a true out-of-air emergency, will you now be forced to commit two divers to a single air supply? If it cannot be shed, the rescue diver cannot leave unless he ascends on a breath hold! 

What is your plan? What are your procedures for such an event? How often has your team trained for such an eventuality? 

So your team doesn’t use surface-supplied air. Your team is scuba only. You pay for and provide your own equipment and only one diver on the whole team has a pony bottle to dive with. You read about it and saw the pictures; you have a contingency cylinder that can be delivered to a diver in an emergency, so that is what you do. 

When was the last time you practiced the procedure that gets that air to the diver underwater? How long did it take? At what depth and distance? How long SHOULD it take? How long can you hold your breath when you are trying and perhaps failing to keep from panicking? 

A contingency cylinder is usable, but most teams who rely on them rarely practice the actual process of delivery to a diver on the end of a search line. Even well-practiced teams will have difficulty reaching a diver who is experiencing an out-of-air emergency in time. Contingency cylinders are the least desirable of all the redundant air systems.

An air supply that is not independent of the primary system may not be accessible from a single air source. An alternate air system that depends on another diver to deliver it is a choice, but not necessarily the best one. 

In a diver emergency underwater, we should survive if we have air. But a true alternate air system is required and that is an independent air supply connected to an alternate air source that is immediately available to the diver.

Exactly how important is this?

How long can you hold your breath?

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