Unfortunately, this rule of thumb is completely useless when it comes to dive planning. The trouble is that it doesn?t answer the question, ?At what tank pressure do I need to leave the bottom??Think about it for a second. Why do I care about how much gas I have left on the surface? There’s an unlimited supply of it up there! What I really care about is how much gas I have underwater and how much I need to make it back to that unlimited supply without getting injured or drowning!

Consider this realistic emergency situation:

You are happily diving along at 100 ft when a team-mate comes up to you, panicked and completely out of gas. Being a good team-mate, you donate your regulator to him and sort out the situation. Both of you are diving typical single aluminum 80s. You look at your pressure gauge and see that you have 800 psi remaining. Is that enough gas to get you and your buddy safely to the surface? How do you know? What are your options? Do you have enough gas to start swimming for the anchor line, should you begin a safe, controlled ascent now, or do you need to bolt for the surface as fast as you can?

To answer this question, before entering the water we must calculate something called Rock Bottom. Rock Bottom is the minimum amount of gas required to get two divers safely to the surface from the maximum depth including a controlled ascent (maximum of 30 ft/min ascent rate) and safety/decompression stops. With proper planning, just because one diver is out of gas is no reason to blow to the surface or eliminate safety stops!

Rock bottom must never be exceeded! It is the absolute minimum gas reserve that is required to safely ascend with an out of gas team-mate. If a team member reaches rock bottom, the dive is over then and there, without question or debate. Remember, the gas on your back isn’t just your gas, it is also your team?s gas and conversely, the gas on your team-mate’s back, is also your gas.

Just a disclaimer here: you might die if you do this my way, and my description of Rock Bottom may or may not bear any resemblance to GUE’s version of Rock Bottom or Minimum Gas Reserve.

Calculating Rock Bottom

Calculating Rock Bottom for any given dive is really not that difficult, but there are a few pieces of information that must be collected before beginning.

1. Actual: capacity of our tank in cubic feet
2. Working: pressure of our tank in psi
3. Planned: maximum depth of the dive
4. Stressed: SCR (surface consumption rate) for each diver

In this example, we?ll be using a typical aluminum 80 tank which has an actual capacity of 77.4 ft³ and a working pressure of 3000 psi (it holds 77.4 ft³ of gas when pressurized to 3000 psi). For the sake of easier math, we?ll round the actually capacity to 80 ft³. In real life scenarios, you probably shouldn’t round up like that. We?ll also assume a dive with a maximum depth of 100 ft. If you don?t know your actual stressed SCR, 1 ft³/min per person is a reasonably conservative number for most divers and helps assist and encourage planning by making the math easier.Exact numbers are not necessary here; just close, but conservative approximations.

The first step is to break up the ascent into sections.For a 100 ft dive, given an ascent rate of 30 ft/min plus a 3 minute safety stop at 15 ft, this dive will have 4 sections:

1. 1 minute to handle the emergency at 100 ft
2. 3 minutes to ascend from 100 ft — 15 ft at 30 ft/min
3. 3 minutes at 15 ft
4. 1 minute from 15 ft to the surface

So, it will take 8 minutes to reach the surface.

Next, convert the depth ranges to depth averages in ATA (absolute atmospheres), where one ATA is equal to 33 fsw (feet of sea water).

1. 100 fsw/33 msw = 4 ATA
2. 100 fsw to 15 fsw = average of ~60 fsw = ~ 3 ATA. Because of the linear ascent rate of 30 ft/min, the average depth can be calculated with the following steps:
   

   1. 100-15 = 85 (starting depth minus target depth)
   2. 85 / 2 = 42.5 (divide by 2 to get average)
   3. 42.5 + 15 = 57.5 (~60)(average depth plus target depth = true average depth)
3. 15 fsw ~1.5 ATA
4. 15 fsw to surface = ~1 ATA

Now multiply depth * SCR * time to convert SCR to ft3 at depth.

1. 4 ATA* 1 ft³/min * 1 minutes= 4 ft³
2. 3 ATA* 1 ft³/min * 3 minutes = 9 ft³
3. 1.5 ATA* 1 ft³/min * 3 minutes= 4.5 ft³
4. 1 ATA*1 ft³/min * 1 minute= 1 ft³

Add this all together and we see that one diver will require 18.5 ft³of gas to reach the surface with a stressed breathing rate.For two divers to reach the surface using this ascent profile, it will require twice that amount, or 37 ft³ of gas.That is almost half the volume of an aluminum 80!

The last step is to convert ft3 to PSI.This is done with the following formula:

v/wv * wp

where v is the volume of gas (37 ft³), wv is the working volume of the tank (80 ft³) and wp is the working pressure of the tank (3000 psi).

(37 ft³/80 ft3 ) * 3000 psi = 1387 psi (~1400)

Rock bottom on a 100 ft dive with an aluminum 80 is 1400 psi.

Each time you calculate a new rock bottom (if the tank or depth has changed), write it down in your dive log or wet-notes so you have it for easy reference the next time.

So, was 800 psi in an Al80 enough to get you and your out of gas team-mate back to the surface safely with a 3 minute safety stop, under stressed conditions? Probably not. In fact, you should have started your ascent 600 psi ago, long before you had an out of gas team-mate!It looks like you don’t have the gas to make it to the anchor line, so you better start ascending immediately. Your team might be able to make a controlled ascent, but chances are that you’ll end up having to increase your ascent rate beyond a safe limit or omit a recommended safety stop as a result of not spending a few minutes planning your gas supply on the surface.

With proper gas planning and a little discipline to stick to the plan, you will minimize the chances of having to choose risking a decompression injury over drowning. It also helps with peace of mind knowing that you have enough gas available to handle the situation.

Editor’s note: We liked this article so much, we created a t-shirt with its theme. It’s simple, classy, you can wear it at a club or a black tie function:

? Select your Open Water instructor carefully. Talk to accomplished local divers and ask them for a recommendation. Agency doesn’t matter much as long as the instructor is good. Bad habits are formed early and so are good habits.

? If you are going to go to a resort to learn scuba diving, complete the theory & pool classes with your local dive shop first and bring the paperwork to your resort diving center when you go on vacation. Do the open water dives there if you want but don’t waste time in a classroom or a pool while in some beautiful resort. If you can dive locally, by all means do your open water dives there. Local diving can be amazing.

? On that subject, it seems you can dive pretty much everywhere you live (I know, there are exceptions). There is always a mountain lake, a quarry or a local shore to dive at. Yes, local diving can be cheap, easy and rewarding. So dive locally!

? Forge a relationship with your Local Dive Shop (LDS). You might save some money buying gear online and sometimes your LDS will not carry the
items but when the difference isn’t much, support your local brick and mortar shop. They will rent you tanks, fill your own tanks, provide advice, organize outings, service your gear later – they’re important.

? Meet local divers, mingle, learn from them. They know the spots, they know the tricks, they know the etiquette.

? CyberDive a bit but not to excess. There are many great online communities where to exchange ideas, buy and sell used gear, meet local and remote divers. Don’t become a couch diver though, do get in the water.

? Don’t rush your recreational scuba training. Yes, you can get your Advanced Open Water certification (or equivalent) right after your Open Water but why would you ? Enjoy your diving, build some experience before opening up to new challenges.

? Many many dives later, after you complete the Rescue diver class (or equivalent), don’t bother with any of the ‘specialties’. Besides Dry Suit training, there isn’t really much to learn in the PADI specialties that you can’t learn by diving.

? Dive Master training can be fascinating but understand what you’re getting into.

? Going past your No Decompression Limit into decompression diving, diving with a ceiling (virtual or physical) such as wreck penetration diving, cave diving, deco diving are tech diving activities that absolutely require the proper training.

? Pick up a copy of the PADI Encyclopedia of Recreational Diving. It’s actually pretty good and contains most of the knowledge you’ll need in your recreational scuba activities.

? Prefer paddle fins to split fins. But that’s just my opinion. Buy a good mask as well. Later on buy a Buoyancy Control Device (BCD) and then a regulator.

? For BCD, don’t buy a jacket style, buy a back-inflated BC or a Backplate/Harness/Wing (BP/W) combination. The later is my preference and is likely to last you for a very long time. Find a used one, they’re just great.

? Learn the Frog Kick from the get go and forget about Flutter kick. You will not regret it.

? Focus on Buoyancy and Finning. Those are key skills.

? Do not get into the water without

The following was written by Cpl. Bob Teather RCMP in 1991:

Public Safety Divers are human, believe it or not – just like the rest of us. They come in various sizes and shapes and speak with as many accents and languages as there are lakes and rivers.

Public Safety Divers are found everywhere; in large cities, rural communities and at most departmental budget meetings. The best way to get one is pick up the telephone.

Public Safety Divers deliver lectures, diving equipment and bad news. They?re expected to have the wisdom of King Solomon, the abilities of Superman and the generosity of Santa Claus.

In the movies, Public Safety Divers are easy to spot. You see them hanging from helicopters and cliffs over some treacherous waterfall or rescue scene. In real life, they?re found hanging around near a telephone – even on Sundays, ?cause that?s when it usually rings … just after company has arrived and the barbecue has been lit.

Public Safety Divers are a lot of things. When they make a rescue, they?re heroes. when they complete a body recovery, they?re too late! When they?re paid for their work, they?re doing it for the money … anybody can see that! and when they?re volunteers, they?re crazy ?cause what person in their right mind would volunteer to do a job like that anyway?

Public Safety Divers see more sunrises, sunsets, lakes, rivers, swamps, cesspools and misery than anyone else. Like the mailman, they must be out in all kinds of weather. Their rubber suits change color and materials with the seasons, but their outlook on life remains the same … trying our best and hoping for the day we can make a difference.

Public Safety Divers like hot coffee on cold missions. They also like friendship, laughter and a hearty handshake. They dislike liquor in boats, children without personal flotation devices and drunks that drive their cars into the water.

Public Safety Divers get medals for saving lives, recovering children from icy waters and diving in hazardous conditions. Sometimes their widow gets the medal.

But, after all the sunrises and sunsets, the lakes and the cesspools, the training and the 4:00 a.m. call-outs, once in a while the most rewarding moment comes when the mission is over and out of the crowd a stranger walks up, embraces the Diver and whispers a heart-felt God bless you, and thank you, you have helped.

Bob summed it up greatly in 1991, and he will always be remembered by them, as a good friend, mentor, teacher, and all around great guy. He is sadly missed and fondly remembered by the many thousands of people whose lives he impacted,and changed. Sometimes for the good, and sometimes the bad (putting a felon behind bars), Bob you are sorely missed but ALWAYS remembered!!

And now I will try to take over from where Bob left off, by providing insight into what being a Pubic Safety Diver, in my case a police diver, is like. I’ll try to tell you of the training, the joys, the disappointments, the camaraderie, and the extreme sense of family shared by public safety divers around the world. I’m also adding some photos to give you, the reader, a bit of insight into the practices, training, situations encountered and the joys of the job.

Training to be a PSD is definitely NOT your standard rescue diver course as offered by the various training agencies. PSD work involves recovery, evidence recovery and the chain of custody involved to procure a successful conviction when required, it involves breaking the sad news to families, but at the same time providing much needed closure to those who have lost a loved one. It provides public education on the hazards of work and play around water, trying to educate the public on how to avoid fatal mistakes, and general safe boating and swimming practices.

Training involves black water, swift water, ice, entanglement, recovery methods and procedures, different forms and types of search patterns, the use of SCUBA and surface supplied air operations, how to lift a vehicle, boat, airplane, etc. correctly, at the same time training a diver to be self sufficient underwater should the need arise and you find yourself in “less than optimal” dive conditions. Public safety diving is essentially a solo diver operation for the man in the water but has various backups in place such as a safety diver who is fully suited and ready to go in a moment’s notice, a tender to monitor your tether line , down time, air consumption, dive profile, and help you dress, and the 90% diver as we refer to them, who are on scene fully suited but without SCUBA on or a Surface Supplied System, to provide further back up.

You have to learn the different search patterns involved such as a jack-stay, circle, steady, arc patterns. For ice diving you have your support teams, alternate exits to plan, equipment to set up and configuration to follow. You have to determine what the best thermal protection needed for the job at hand is, and how to choose which is right for the job you are doing. You’ve got to think about whether to use air to inflate your dry-suit with argon, EANx or just compressed air, as well as the possibility of needing Trimix for depth and deco considerations.

The training list is endless.

You learn how to employ various lifting devices such as bags, cables, ropes, and yes occasionally even an extra large tractor inner tube. You learn how to secure a scene, interview witnesses to help with possible/probable locations, how to log evidence, the chain of custody, the emergency communications systems to the surface such as the rope pulls between diver and tender. As a public safety diver you hope to never have to use the 5-5-5- pull which is ” get me up NOW“.
You learn how to properly handle a body underwater, to place it in a body bag while submerged, the correct methods of ensuring that all possible remains are recovered intact.

You also learn the “list” of absolute no-nos in the recovery of human remains and evidence. An example would be a gun recovery that will be used as evidence in a court case, you must NEVER directly bring the weapon to the surface as once the metal touches air the oxidization process begins which could potentially destroy fingerprints or other useful evidence on the weapon. You bag it in the water, including filling the evidence bag with the surrounding water that the item was located in, as this halts the oxidization process, and every body of water has it’s own characteristics, such as the diatoms that are present in the water. These little single celled organisms can be the difference between a successful prosecution, or a case dismissal by the Courts. One always has to remember the chain of custody involved, from their hand, to the tenders, to the evidence tech on the scene, if you happen to have one.

Surprisingly, the difficult aspects about this job are not the various and extreme diving situations and conditions, it is dealing with the grieving family members at a recovery dive, and the most difficult is the recovery of a child.
You have to remember that we the divers are human, and a lot of us have our own children, and it really hits home. It is not uncommon to see a PSD cry at a child’s recovery, and that’s when the camaraderie of the job comes in as we all share each others sorrow’s, and believe me, it is a great feeling to have a fellow team member just come up and put his or her arm around your shoulder and let you know, “I am there for you”.

The second worse is losing a team member on a recovery as that is when the ” what if’s” start to set in, and thankfully most Department’s have a Critical Stress Incident Management team or a councilor you can lean on. Thankfully with the rigorous training PSD’s go through we don’t lose many of our own, but it happens. I remember the case a few years back when a diver from an agency not remotely close to ours lost his life giving closure to a grieving family. This hero was found with his arms still around the victim, his “end of watch” came much to early but his mission was a success, as in his final moment’s he was able to find the missing person and through his death gave closure to the family of the missing teenager.

I would have to say the most rewarding part of the job is being able to give that closure to families, the simple handshakes, the smiles, the thank you’s. We are not in this profession for the monetary or social status boosting some people associate with a PSD, we do it because ” we care” !! I recovered a child’s favorite toy once where gladly no loss of life occurred, and the best “payment” in the world was that little girl running up to me upon seeing her doll and grabbing my still wet leg and hugging it saying,” Mr.policeman, thank you, you saved Shirley”, that alone is one reason I love this job.

PSD diving is far from glamorous!! We dive in some of the worse conditions known to man, in sewers, in hazardous waste spills, in biological event’s where you face risk of catching disease, in black water, in swift water, in freezing condition’s that would make the most hardened Eskimo shiver. In gasoline tanker spills, where getting any of the containment on your skin can cause burns, blindness, etc. This is where you thank your lucky stars that somebody had the know how to invent a positive pressure full face mask and super vulcanized dry suit.

Another downside of the job is the ever current struggle to find funding for training, equipment, seminars, etc.
Most people in the USA and Canada are not aware that most PSD agencies are run by volunteer department’s who rely on public funding and donations. It is only the “luckier” larger Department’s that actually have funding set aside for their Underwater Recovery Team (URT). The countless towns and smaller cities rely on their volunteer teams to provide this service to them and to me THEY are the real heroes, having to rely on their own initiatives to obtain equipment, training, and even storage space.. They hold fundraiser’s, dances, Bar-B-Que’s, etc. to raise funds as these folks are the backbone of the PSD community in little places all over these great countries of Canada and the USA.

The next time your area has a fundraiser, please support it, you never know when or if you’ll be needing the services they can provide.

If you are a diver and want to get involved with this sort of diving contact your local PD, Fire Dept., or EMT stations. They’ll be more than happy to provide you with the information and set you in the right direction. I’m also going to provide you with some links to training agencies that provide PSD training. I myself prefer IADRS, PSDA, and ERDI, but don’t let me influence you. If you are interested in doing this job either professionally or on a volunteer team find an instructor you are comfortable with, has the know-how and experience and will take the time to instruct you to the fullest of your capabilities.

There are leaders in this community of PSD’s that you may have already heard of. I consider these guys to be the cream of the crop, but there are many others out there that are too numerous to list, they know who they are: “Blades” Robinson, Mark Phillips, Bob Kinder, Butch Hendricks. The list is long.

I do hope this has shed some light on the “art” of public safety diving, has enlightened you on the job, and hopefully stirred you to go and make inquiries on how you too can become a member of the “best job in the world”.

Safe diving always!!

Tom

Some more links: PSD Diver Monthly | Lee’s Summit Underwater Rescue and Recovery | San Marcos Area Recovery Team (SMART) | South Carolina Public Safety Diver International |Midwest Technical Recovery Team | CPA International Investigations

It’s worth reading completely but here is the conclusion: “This investigation has demonstrated that significant improvements in drysuit thermal protection can be achieved when using argon instead of air as a drysuit inflation gas. It should be noted however that these improvements can only be achieved by carefully and repeatedly purging (a minimum of 6 purge cycles) with pure argon prior to water entry”

The default set up for my Dive Rite O2ptima rebreather is two Hammerhead handsets. The primary is used to drive the Solenoid (which injects oxygen into the loop) and provides decompression information (Buhlman GF) as well as a read on the O2 Cells 1,2 and 3. The secondary is used to drive the Diva HUD (Head Up display with vibrating capabilities) displaying information from the same cells there as well as on its display. A lot of O2ptima divers dive their rebreather this way with one handset on each arm and the HUD.

Some choose to get the optional deco capability on the secondary handset, a $500 pin code from Hammerhead and a reasonably good choice which gets you a backup decompression computer should the primary fail (and you find yourself manually running your rebreather, using SCR mode or bailing out on Open Circuit).

The O2ptima has a neat feature which allows you to install a fourth sensor that can be read from a third party computer connected via a fisher cable. That’s what I was doing with some degree of success and failure with my VR3. This gives you a second deco computer, possibly a different deco algorithm and a fourth cell to get O2 readings from. Unfortunately a fourth cell can fail or behave erratically and as such I didn’t trust the VR3 as much as I would have wanted to. It also always seemed to be reading .10 higher than cell 1,2,3 which bothered me. One could also use a Liquivision X1, Shearwater Pursuit or Delta-P VRX with this scheme, reading from one sensor.

Having switched to the Liquivision X1 from the VR3 and following closely the announcements for the PPO2 interface, the X-Link, I have been working on a solution to connect three cells to a Fisher cable. Kevin from HammerHead set my head up with a split harness and 5 wires Fisher cable so I could continue monitoring Sensors 1,2,3 with the HammerHead handsets in addition to Sensors 1,2,4 with my Liquivision X1/X-link. Of course the same could be done with a Shearwater Pursuit or Delta-p VRX.

So what do I like about this setup ?

• O2 readings from 4 cells instead of 3 with voting logic on cells 1,2 and 4 for the external PPO2 reading
• Different software and hardware. If the HH primary handset was to freeze up due to an odd software issue (as opposed to electrical) it’s entirely possible the same thing could happen on my secondary. With the Liquivision X1, I am on a different hardware and software platform, one altogether different bugs maybe but _different_ is the key. Redundancy through additional and different hardware/software.
• VPM algorithm vs Bulhman GF. Always good to have two different profiles from two different algorithm
• Different voting algorithm on a different group of cells
• Possibility to fall back to 2 cell, one or constant PPO2 if I need to
• Very easy to configure the bailout gases (not that the HH or VR3 were that hard just not very quick to configure)
• Same deco computer, parameters, profile and as such deco plan as my usual dive buddy
• the X1 is very bright which is great for dark water diving (hello Boston). Also, I can see my buddy’s PPO2 from 10 feet away
• Same software for planning dives (V-planner) and diving those plans. Plan your dive, dive your plan

And while Fisher cables can be problematic, when well maintained they are reliable. Now, I still have and use my two HammerHead handsets. With the secondary tucked away in a belt pocket, the HUD and primary remain my main dive instruments.

The Liquivision is a very good backup and could be the primary if I had a different setup (MCCR or OC, for example) and could be substituted by a Shearwater Pursuit or a Delta-p VRX. In addition to tables, of course and a deco plan written on white electrical tape.

Every year dive gear manufacturers come out with new features for their regulators, wetsuits, and BCs. There’s a good business reason for this: manufacturers need to differentiate their product and get consumers to buy more each year. It’s hard to build a business around a commoditized simple item. I suspect golf is like this too.

The irony is that diving is one of the few gear-intensive sports where the best stuff is the simplest and often cheapest. By the time you get to technical diving you know this, but meanwhile you’ve wasted a fortune on recreational junk which you could have avoided had there been a nearby technical diver to ask.

So here is the first of my “I wish I’d known that” gear posts.

Fins

Fins are important. They are how you control your movement under water: forwards, backwards, and turning. (Yes, turning – your hands are for handling gear not swimming.)

I recommend two things to consider when choosing fins. One is obvious, the other maybe not.

Fins Shape – Split or Paddle?

Oh, I wish someone had pointed this out to me before I’d started buying gear.

A very popular design these days is the “split” fin. (I used a set of these for several years.)

What split fins do well: high-rpm flutter kicking. When you flutter kick, each “leaf” twists creating a propeller-like effect. This makes it quite good for fast flutter kicks and straight-line speed tests. Sounds great, right?

However, here’s the kicker (sorry, couldn’t resist!) — you are not high-rpm flutter kicking when scuba diving!

What split fins lack: the stiffness and body you need for frog-kicking, turning and backing up.

The time has come where, at least here in the frozen wasteland of the northeast, we are diving with air temperatures well below the freezing point and often with wind chill factors below zero. This, compounded with water temperatures in the mid to low 30s can have serious consequences if we don’t take care to dive with the proper equipment and procedures. Regulators can freeze, hands rapidly become numb, and hypothermia can set in quickly if diving wet or with a leaking drysuit. There are several equipment and procedural techniques we can use to help combat these problems.

Equipment Considerations

Exposure protection

Dive dry. While there are many high-quality wetsuits available and cold tolerance varies from individual to individual, a well maintained drysuit with a proper undergarment will keep you warmer for a longer period of time. Hypothermia is extremely dangerous. It slows the thinking process down and also is a major contributor to decompression-related injuries.

Wear a proper undergarment under your drysuit. A proper undergarment is a garment made out of a warm material that, in the event of a flood, retains most of its insulation value when wet. 3M?s Thinsulate is an example of this kind of fabric and it retains over 80% of its insulation value when wet. Most manufacturers make Thinsulate lined undergarments including DUI, Viking, Diving Concepts, Bare, and Andy?s. In the winter months, adding a thin wool or polypro layer underneath the drysuit undergarment can help wick moisture away from your skin and keep you much more comfortable during the dive. Personally, I wear a thin Under Armour long sleeve, compression fit shirt . Do not wear cotton in the drysuit. Cotton is a horrible insulator when wet and will actually hold the water against your skin, which when combined with a drysuit leak will quickly lead to hypothermia.

Wear dry gloves. Just like a drysuit will keep you warmer than a wetsuit, dry gloves will keep you warmer than wet gloves. It is important to use a dry glove system that allows you to keep the inner seal intact in case you get a leak in your glove. Without a wrist seal, a small leak in the glove can rapidly lead to a very wet and cold dive. To equalize the gloves, slip a long, thin piece of surgical tubing or shock cord underneath the wrist seal. When possible a hands-off approach to diving is the best way to prevent a leak from developing.

Regulators

Dive with a 1^st stage diaphragm regulator that is environmentally sealed. As a general rule, diaphragm regulators are less prone to freezing problems than piston regulators. An environmental seal prevents water from entering the chamber and freezing.

Use a ?downstream? regulator. Downstream regulators will universally freeze in the open position, free-flowing the regulator. While this means loss of gas until you can shut down tank, it also means you have gas to breathe until you can switch to an alternate gas source. Some ?upstream? regulators can freeze in the closed position, suddenly depriving you of gas to breathe. Most regulators manufactured today are of downstream design, with the exception of a few such as Poseidon, which are upstream. Poseidon regulators have an over pressure valve that helps reduce the possibility of lock-up, but my opinion is that it is just another part that can fail when a simpler design works just as well.

Always have a redundant gas supply in case the regulator freezes. Redundancy comes in many forms including a good buddy, doubles, or a pony bottle.

Procedures

Even if it?s going to be above freezing on the day of the dive, keep your tanks indoors the night before. The pressure drop from a tank pressure of 3000 psi to the regulator?s intermediate pressure of 140 psi causes a phenomenon called adiabatic cooling. This can cause a temperature drop of up to 40-50 degrees. If the gas in your tank is at room temperature, the gas you end up breathing is likely coming out at 20 or 30 degrees. Now consider that if your tanks have been sitting in a car overnight, the gas in the tanks might be 10 or 20 degrees. This means, due to adiabatic cooling, the gas you are breathing is 30 or 40 degrees below zero. Not only will this chill your body from the inside much faster, but it will also greatly increase the likelihood of regulators freezing.

If the outside air is below or near the freezing mark, before the dive, do not exhale into the regulator until both the first and second stages are submerged. The moisture in your exhaled breath can collect in the second stage and cause freezing before you even get in the water. This means when you test breathe your regulator, only inhale through it; do not exhale back into it. It is also advisable to even minimize breathing from the regulator at all until it is submerged.

Plan your dive with the assumption that gear will leak and regulators will freeflow.

Plan your dive times shorter. It isn?t enough to say, ?we?ll turn around when one of us gets cold.? By that time, it?s too late. You are now uncomfortably cold and may have a long swim and ascent remaining which equates to getting closer and closer to hypothermia.

Be more conservative. That tough dive you did last August suddenly becomes a whole lot more challenging when it?s 15 degrees outside and the water temperature is 36 degrees.

Conclusion

Make sure you are well prepared before embarking on that cold-weather, cold-water dive. Using the right equipment and procedures can do a whole lot to mitigate the risk of this type of diving.

Photos courtesy of Jeff Schreiber (c) 2006

However, one thing I learned is that when you have that much time to prepare for a trip, you should just ship as much stuff ahead of time as you can!