Regulating our Underwater Air

Photos from UAB in Antarctica group on Flickr, related to this post
By Chuck
Posted on 04/13/07

A few weeks ago I was asked by a reporter about diving in Antarctica. Part of my response was how integral it is to our work and how essential being able to dive here at Palmer Station is to me as a marine scientist. Biologically, it is a different world from anywhere else I’ve been, including other places in Antarctica. An exact quote of what I told her is:

“Being able to dive and observe the communities we are studying is exceptionally important in driving the big picture questions we have been asking scientifically. As an ecologist, there is simply no substitute for being able to literally immerse myself in the communities we are studying. All of the ecological questions we are working on this field season are the direct result of in situ observations of the communities while diving that could not have been made any other way.

“My chemist colleague, Bill Baker, and I were just talking about this two days ago and even though much of the work he is doing this year has been driven by the chemistry he has found in Palmer area sponges in his lab, the context that has put it into a meaningful picture has come from his personal observations underwater.”

Since diving is such a critical component of our research, we’ll be talking about the nuts and bolts of it in a few upcoming posts. In many ways the diving we do here is not much different from other places in the world.

In a lot of other ways, of course, diving here is very different from most places. But unlike when we are at McMurdo Station, which is a larger US Antarctic Program research station much closer to the South Pole (and which is a different world in terms of its marine communites), we have almost never been in a situation where we need to cut a hole into a solid sheet ice to get into the water here at Palmer.

Occasionally we need to dive through small chunks of ice called brash ice that can completely cover the surface. Some seasons we have had to do that a fair bit, as in the Flickr photo I’ve attached of myself and two former field team member graduate students, Kevin Peters and Yusheng Huang, diving through brash ice off the station in 2003. Particularly when you are shallow, the sound of the ice banging on itself is somewhat eerie.  But unlike the ice at McMurdo and like diving other places, you can always come to the surface through it.

Densely packed brash ice like you see in the photo is also is very hard to swim through at the surface. Fortunately, though, it usually only extends down a foot or two. Usually we are dropping out of the boat into open water. Occasionally there is a little brash ice around here or there but not so much as to impede us.

Equipment-wise, the most unusual thing we use that is different from diving in waters most anywhere else in the world other than the polar regions are our tank valves. The valves are Y-shaped (like a slingshot and about the same size) and are called “Y-valves” or “slingshot valves.” These are designed so that they can accommodate two completely separate regulators. That is unusual. The kind of regulators we use have two parts. Most places people dive with only one of the first parts (called the first stage) but have two second parts (called the second stage) attached. Having two of each is what is unusual.

The SCUBA tank on our backs contains air that has been compressed to approximately 2500 pounds of pressure per square inch (psi) pressing outwards on the tank wall when it is full. That is compared to normal atmospheric pressure of about 15 psi for the air we breathe at the surface (at least at sea level). We have two sizes of steel diving tanks available: very large (and very heavy) and super large (and ridiculously heavy!). They hold 95 or 110 cubic feet of air, respectively, compressed into a tank that we can fit on our backs. When full, the 95s weigh 45 pounds and the 110s weight 61 pounds. Fortunately, they are not nearly as heavy when in the water!

So what does a regulator do? Well, it brings the pressure of the air in your tank down to the same pressure your body is at otherwise. At the surface, that would be almost 15 psi as I said.  If the air coming out of the mouthpiece of the regulator were higher than the pressure you body is experiencing otherwise, it would be forced into your lungs and you’d have a hard or impossible time exhaling. Likewise, if it were lower, you’d have a hard or impossible time breathing the air in when you tried to inhale. The pressure the air is at coming out of the mouthpiece has to be the same as the pressure your lungs are experiencing for you to be able to take a breath.

The two stages of a modern regulator accomplish this reduction in pressure in two steps. The first stage attaches directly to the tank valve and reduces the pressure from the tank to something close to 150 psi, or about 10-times the atmospheric pressure at sea level. That connects via a hose to the second stage. The second stage has the mouthpiece that you breathe from and reduces the air pressure to exactly the pressure of the surrounding water.

If you were to breathe from your regulator at the surface, you would have air at just under 15 psi. That is what is known as one atmosphere of pressure (or one bar in a semi-unofficial version of the metric system). Every 33 feet of seawater depth increases that pressure by another atmosphere. So at 33 feet, you would be breathing air that was between 29 and 30 psi. The deepest depth we dive to is 130 feet. Can you determine what the approximate pressure is there?

I’ve gotten away from the usefulness of the slingshot valve in polar waters. When I started diving, people used regulators with only one each first and second stage (or with older, single-stage regulators that did the entire pressure conversion in one step). If your dive buddy were to run out of air on the dive (there were not submersible pressure gauges then to tell us how much air we had; those are a great safety addition to modern diving), you would share your mouthpiece with them in something called “buddy breathing.”

Buddy breathing worked, and I even had to do it once for real during an emergency on a fairly deep dive. But someone figured out that if you had two second stages attached to your first stage, you could give your buddy your extra second stage without having to deprive yourself of your air temporarily. That was a big improvement in diving safety.

Having two second stages also means that if your first one malfunctions during a dive, you have a back-up. That too is a very good thing in terms of safety. But most places, that kind of problem is rare.

Here in Antarctica, however, that kind of problem is not so rare and historically was not at all uncommon. The cold causes the regulators to freeze up sometimes, particularly when the salt water is below the freezing point of freshwater as it is here at Palmer Station now. (Saltwater freezes at about -1.9 degrees Celsius while freshwater freezes at 0 degrees.)  So particularly if any freshwater has gotten into the regulator, and that can include by the water vapor in your breath condensing and freezing, you can get ice.

Our modern regulators have lots of safeguards built in to prevent freezing, but they still can freeze once in a while. Usually when a regulator freezes, it freezes at the second stage and in the open position. So all that air is rushing out of the regulator at 150 or so psi and continuously. That can empty a tank pretty quickly.

With a slingshot valve, your buddy can close off side of the valve supplying air to the frozen regulator. That means your air is not spilling out and you can use it to make a safe and controlled ascent back to the surface while breathing on the second, independent regulator without worrying about all your air spilling out before you get there. That is a huge safety improvement.

As I wrote at the beginning, being able to dive is a critical, integral part of our work. This one small, and seemingly “plain” part of our equipment, the slingshot valve that lets the air out of our tanks, goes a long way to keeping this part of our work safe.

I love slingshot valves!


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