Emergency air supply planning

[rkinder]

How large of a pony bottle or emergency air supply should Public Safety Divers carry with them? We often get caught following rules of thumb, like minimum pony bottle size should be 19 Cubic Feet. But is this the proper way to look at the sizing of an emergency air supply?

I hope to open a line of discussion by first explaining the tests my team ran several years ago. Please understand that all divers where on surface supplied air, and the emergencies where simulated. At no time was the diver with out primary air, along with a 40 cubic foot bail out bottle on a gas block, full 4 wire communications. As part of the tests we took 4 foot visibility and reduced it to 2 to 6 inch visibility. Depth was 65 feet liner run for rescue diver was 110 feet from the time they entered the water. An additional safety diver was within touch contact of the diver having an emergency. Along with having 4 cutting tools appropriate for cutting any material used in our tests. All divers where asked if there was any reason they could not participate in the dive prior to entering the scenario.

To begin the Primary diver and Safety entered the water upon reaching the testing location the primary diver was tangled with #18 nylon braided twine around the tank valve to a pipe and then one of their legs to the pipe between the knee and fin. The rescue diver was at 90 percent prior to the diver declaring an emergency. Once the diver declared an emergency the safety diver splashed, What we found was a surprise the safety followed the tangled divers umbilical of course this was thru an area with several obstacles on average it took 3 to 4 minutes to reach the tangled diver, another 2 to 4 to cut the diver free and not things that should not be cut, and finally 3 to 4 to return to the point of entry. As a conclusion best case requires 8 minutes of emergency air and worst case 12 minutes. Time would have been less had we allowed a direct accent to the surface, and panic levels may have caused increased air consumption in a real emergency so we thought this balanced out real world against simulated conditions. In addition all divers would be considered by most to be highly experienced.

So for planning purposes presuming a diver uses 1 cubic foot per minute at the surface then at 65 feet the diver should be consuming 3 cubic feet per minute. And if we add just 3 minutes worth of additional emergency air supply planning then we should provide the diver with no less than 15 minutes of emergency air. This requires a tank size of no less than 45 cubic feet based on this depth. Of course if the depth is shallower we can use smaller bottles, based on this test my team settled on more than double the recommended 19 cubic foot bottle as a minimum. So here is the final question is there any reason we are not planning emergency air supply based on time at depth, rather than just a general rule?

I look forward to your responses.

Sincerely,

Bob Kinder

 

[tbone1004]

you did it the way it's supposed to be done. calculated the actual air amount that you're going to need. Why don't more teams do it? Most PSD dive teams suck to begin with, don't dive enough to get better, and rely on an incredible amount of useless gear to "save them" because "more is more". The reality is that with FFM's you will likely need to factor in more than 1cfm for real world, especially if they are positive pressure masks with the controlled freeflow.

Basically what you're saying is you need 12 minutes to get out, 1 cfm, so you need 12cf per atmosphere of depth. Depending on the max limit you are willing/allowed to dive, I'd say 100ft is a fair assumption, so 4x12=48cf. If they are backmounting the bottles, I would get Faber LP50's or AL80's, if slinging, I'd just use an al80 and call it a day. No harm in having some extra gas and they aren't that unwieldy.

 

[rkinder]

Tbone1004,

Thank you for your response, you are correct many teams are comprised of divers with greater than 1CFM Surface Air Consumption rate. The one part of dive planning that should be preached into each and every diver is knowing your SAC rate. This becomes even more important for PSD teams that only use line signals.

I would prefer to change the term most PSD teams SUCK, to most PSD teams consist of divers that have not been properly trained or experienced for the job they are performing. Unfortunately most have only received recreational training and many do not have the resources to dive often enough or in varied conditions to fully develop the skills they have learned.

 

[tbone1004]

the suck comment was certainly directed at inferior training qualities, but also a mentality issue that most dive teams I've encountered have where the divers don't think they need to train regularly and it shows when you get in the water with them. It's a very sad situation, but the reality is that while in technical diving we use 1cfm for easy math as an unrealistically high sac rate for planning, most of these guys are probably breathing 1cfm in their FFM's during regular diving. They also use ridiculous gear for what they're doing with massive wings, huge BC's, and there's been no thought in what they are buying and why from an equipment standpoint which makes them much more likely to get hung up and in a situation that you proposed above, or do something more stupid like try to pick something up off of the bottom with their overly large wing and then shoot up like a cork.

If I were running a team like that, I wouldn't let them in the water with anything less than an al80 on their back for bailout. 2cfm, 4ata max, gives them 4 minutes at the bottom to figure their stuff out and 8 minutes to get up to the surface including a precautionary stop. Anything less than that is asking for a bad situation to become worse. If they were properly trained, and trained regularly, then I would say an AL40 would suffice, but at the current standing, anything less than an AL80 for bailout is asking for trouble.

 

[Akimbo]

… Please understand that all divers where on surface supplied air, and the emergencies where simulated. At no time was the diver with out primary air, along with a 40 cubic foot bail out bottle on a gas block, full 4 wire communications…

For commercial diving, the bailout is sized so the diver can surface, get back to the bell, or be reached by the standby diver in case of entanglement. Lots of assumptions; you do your best. When in doubt, carry more gas until the physical burden becomes a safety or productivity problem.

In practice, I see no reason to ever use less than a 19 Ft³ bailout since the physical burden is so small. Also in practice, it is extremely rare for a properly rigged umbilical to fail. Lots of backup gas banks on deck is imperative in your environment due significant entanglement risks.

---------- Post added December 28th, 2015 at 09:48 AM ----------

… It's a very sad situation, but the reality is that while in technical diving we use 1cfm for easy math as an unrealistically high sac rate for planning, most of these guys are probably breathing 1cfm in their FFM's during regular diving…

We have always used 1.5 ambient CFM in commercial diving for gas planning, but we are working harder than technical divers. However, public safety divers can work just as hard fighting currents. One CFM would be light in my view, maybe not for the entire dive but definitely for emergency bailout situations.

 

[rkinder]

Dear Akimbo,

I have no problem with using a minimum 1.5 cfm, sac rate for calculating air consumption, time is the real killer. As a fellow commercial diver we do our best to plan for an entire emergency rescue evolution that includes getting the diver back to the bell or to the surface. The true point to my post is that PSD's need to plan emergency air sizing by time and depth rather than just size of tank presuming a rapid accent to the surface. In addition by properly logging dives including air consumption team leaders will be better able to ensure sufficient emergency air to include allowances for the divers actual air consumption rates. Bottom line is I never heard any one complaining about too much air at the end of a dive.

The idea that SSA should be used still is not widely accepted, I have been on a scene with a full SSA spread and had my decision to deploy with SSA questioned, and this for a submerged car discovered by a fisherman. Unknown time in water unknown what may have been in the car. I was told 99 percent of teams would have just used SCUBA with wet suits, and not SSA with KMB-37 and attached dry suit.

So please help pass on good knowledge thank you

 

[Akimbo]

... The true point to my post is that PSD's need to plan emergency air sizing by time and depth rather than just size of tank presuming a rapid accent to the surface...

I'm not a PSD, but isn't calculating ambient gas consumption part of the most basic gas planning for recreational Scuba divers? I'm very surprised that it is an issue anywhere.

... The idea that SSA should be used still is not widely accepted, I have been on a scene with a full SSA spread and had my decision to deploy with SSA questioned, and this for a submerged car discovered by a fisherman. Unknown time in water unknown what may have been in the car. I was told 99 percent of teams would have just used SCUBA with wet suits, and not SSA with KMB-37 and attached dry suit...

I wouldn't think that surface supplied would be a tough sell between voice communications, the ability of a second or standby diver to follow the umbilical to deliver tools or assistance, and unlimited air supply due to the ability of the supervisor to swap banks and compressors. Just showing someone an air control panel should make the advantages obvious. However, I can see a reluctance to use a full hat due to the weight and nature of the work. Unlike most commercial divers, PSDs spend a lot of time on the surface so the weight of a hat can become a physical strain. Although I'm not a fan of FFMs used by recreational divers, they do have their place in many types of PSD work, just as they do in welding habitats.

Having to climb on barely submerged vehicles for rescue and rig for lifting, going over logs and branches in lakes and rivers, traversing sand bars, and searching for objects like a hand gun in very shallow water and zero vis come to mind. FFMs are also less intimidating to recreationally trained Scuba divers, who are often the volunteer pool for PSDs, than a Kirby Morgan or other lightweight hat that weighs 25-35 Lbs. The ideal would be to have both rigs available and divers trained to use both, at least in areas with water deeper than a typical river or pond.

As for chemically resistant drysuits used by PSDs even in warmer water, I have one thing to say: floating gasoline and caustic cargos. Duh!

I hope these comments are helpful to your objective.

---------- Post added December 29th, 2015 at 10:38 AM ----------

I just thought of two other features of SSA, which I am sure you are aware of, but I would think would help "sell" the idea to recreational trained Scuba divers.

Force Regulator Freeflow
A diving supervisor can force a hat or mask to freeflow if they have an air control panel with an adjustable HP pressure reducing regulator. All Scuba divers know that a demand regulator will freeflow if the IP pressure creeps too high, typically about 140 PSI over bottom pressure. A supervisor can crank up the umbilical supply pressure if they suspect that the diver has lost consciousness and force ventilation of the mask or hat. Here are links to air control panels:

Diving Air Control Systems | Amron Amcommand

KMACS-5 With Communications | Kirby Morgan

Pneumo Hose Backup
Even if the main umbilical air supply hose fails (blown fitting or hose), it is highly unlikely that the pneumofathometer hose will also fail. A pneumo hose is a small hose (3/16" – 1/4") bundled in the umbilical that is normally used to accurately monitor the diver's depth at the control panel. The diving supervisor can open the pneumo valve on the control panel and the diver can stuff the hose in their mask or hat before resorting to their bailout bottle.

This technique is also very useful in a rescue. A rescue diver can stuff their pneumo hose in the disabled diver's mask or hat. For PSDs, a QD without a stopper can also be installed so a second stage can be snapped in for the very rare submerged rescue (rather than body recovery).

 

[rkinder]

Dear Akimbo,

You have just reminded me to add Forced Regulator Free Flow to my SSA classes have not tried it on the KMB-57. 77 or EXO-26 BR. 'Have used it on KMB"s equipped with a Super flow but not on newer balanced regulators, but will try it out, I also need to look into the Viking Jet Stream regulator and AGA as to how well they will respond to this. Will first see if Mike Ward has the information at dive lab if not. Looks like time for another test, if you want to talk feel free to call 864-444-2524.

Thank you Have a Happy New Year

 

[Akimbo]

Another selling point for using drysuits... remind people what happens a person's digestive system when they die and the contaminants produced by decaying bodies. If that doesn't make someone give up their wetsuit nothing will.

Top 10 Occurrences Before and After Death - Listverse

 

[muddiver]

I would think the Viking Jetstream with the inline pressure relief valve in the LP hose would not free flow from a raise in IP. Just a guess.

I am not surprised that teams would prefer open circuit scuba over SSA. Unfamiliar with the equipment because they don't use it, lots of additional equipment to setup and tear down, who wants to slog a 300' umbilical around if you can throw on a 40 lb scuba unit, OC Scuba is perceived as faster to deploy (like that matters to retrieve a car), who wants to stick their head in that icky hat that never gets cleaned because everyone's too lazy to do so, etc.

But back to the OP, bail-out should be sized based on the recommendations of the equipment manufacturer. Kirby has printed information in the manual for every hat indicating the minimum bail-out flow is for the hat. I think it assumes that worse case scenario is a diver opening the free-flow valve on the hat and needing to return to the surface. For the KM-17K that is 4.6 cfm, not 1 cfm. 1 cfm is probably ok for a regular demand flow OC Scuba regulator because the diver can't really make it free-flow like the bail-out on a diving helmet. So an SSA diver should carry no less than a 40 cf bail-out cylinder for shallower dives.

OC Scuba divers might be limited by the additional weight and entanglement hazard of a larger cylinder. I doubt that any PSD diver would consider a slung cylinder because it would just be in the way during the search work. It would difficult to mount anything larger than a 19 or 30 cf cylinder on the divers back. Just my observation.