- Published on May 09, 2007
Being put under pressure is a fact of life in many aspects of our lives and a part of many professions. Lately, though not at all unusually, I’ve been feeling the pressure of lots of deadlines, obligations, and other commitments to things I really want to contribute to in my profession.
I guess you could say that I’ve volunteered to put myself under those pressures and you’d be correct. And I do not want to sound as if I am complaining since the deadlines, obligations, and commitments are all part of letting me do what I love doing. I do very much love being a marine scientist, a leader of a group of scientists, and an Antarctic scientist involved at local, national, and international levels.
One small addition to “the pile” that came today was one that I was expecting, a call to come to the National Science Foundation’s office in the Washington DC suburbs soon after I return home in early June. I am part of the board that oversees scientific diving safety for the National Science Foundation Office of Polar Programs and we usually meet in early June. Helping to oversee diving safety for my students, colleagues, and others working here is one of the professional services that I truly value the opportunity to contribute to.
Those sorts of pressures are really just “mental” pressures that come from trying to pack too many professional activities into too little time. Being a diving scientist, however, a common part of my job is putting myself under real pressure. That is, the increased physical pressure on my body that comes from submerging beneath many feet of water that gravity is pulling down on top of me.
As a diver this is one kind of pressure that I cannot ignore. For one thing it would be hazardous to my health since there are many diving related medical considerations associated with it, for example, problems commonly called the “bends” that come from not letting your tissues and circulatory system adjust to the rate at which gasses from the air you breath enter and exit them under pressure. It is also critical that our equipment provide air to us at the same pressure our lungs are under. I talked about that in my “Regulating our underwater air” post.
Another aspect of diving under pressure that we cannot ignore is particularly noticeable because we are wearing dry suits. We simply can’t ignore it. It is the law. Boyle’s Law.
To quote Wikipedia, Boyle’s Law was named for the Irishman Robert Boyle who published it in 1662 and it states: “for a fixed mass of ideal gas at fixed temperature, the product of pressure and volume is a constant.” In easier terms, that means that as pressure goes up, the volume of a gas like those in air goes down and as the pressure decreases, the gas volume increases.
What does that mean to me as a diver? Several things. As mentioned in previous posts, the suits we wear to keep us warm are called dry suits. They cover several layers of thermal underwear and a heavy insulated jumpsuit. What really keeps us warm is the air that our body warms up and which is trapped by the underwear and jump suit. As we go deeper into the water we are under increasing pressure from the water overhead. So the air in the suit compresses. But it is all that air which is insulating us from the cold water!
The air in our suits is also buoyant. We wear lead weights to counteract that buoyancy. Otherwise we would not be able to get below the surface. Ideally, we want to be what is called neutrally buoyant which means that we neither sink down nor float up. With our “little” 95 cubic foot tanks (see also my post on “Regulating our underwater air”) I wear 40 pounds of lead on a belt around my waist and another 5 pounds on my ankles. With the huge 110 cubic foot tanks I decrease the weight on the belt to 32 pounds.
As I said, as we go deeper in the water, the air in the dry suit compresses. In addition to providing less insulation, that also means that it provides less buoyancy. So if we were neutrally buoyant at the surface, as we go deeper we become more and more negatively buoyant. That means that we sink at faster and faster rates.
So, both to maintain our insulation and our neutral buoyancy, in order to counteract the change in air volume predicted by Boyle’s Law we need to add air to the suit. Fortunately we have hoses coming off our regulators that attach to valves mounted in the dry suits over the center of our chests. As we go down, we are constantly pressing a button on the valve to add air to the suit so that we can maintain insulation and buoyancy.
Great, we are neutrally buoyant and (relatively speaking) warm when we get to our deepest dive depth. But eventually we start up. Boyle’s Law says that air expands now as the water pressure is decreasing. From a warmth perspective, that really isn’t a problem. But it sure is from a buoyancy perspective. Coming back up to the surface has to be done at a particular rate and coming too fast can cause a number of medical problems.
We definitely need to be able to control ascent rates and that means letting the expanding air out of the suit so that we can maintain something very close to neutral buoyancy. Built into the left arm of each suit is another valve. Rather than letting air in, it lets air out and is called the “dump valve.”
As we are ascending, particularly during the last portion closest to the surface when the relative change in water pressure is the greatest per foot of ascent, we are constantly holding our left elbows high to allow air to vent out of he dump valves. It looks kind of like the “I’m a little teapot” routine. But it gets the excess air out of the suit and allows for a safe ascent.
Another place this all comes into play is our gloves. Most places in the world where people dive in dry suits, they still wear thick wetsuit gloves for simplicity. So the suits have a seal at the wrist that keeps water from getting in. We wear dry gloves for extra warmth but the insulation problem with changing pressure is the same. We need a way to get air into the gloves as we descend. Otherwise the air would be so compressed that there would be no insulation at all (and warmth aside, having the gloves squeezed against our hands would be very uncomfortable).
To get air into the gloves, we put small plastic tubes under our wrist seals. That allows the air we add to our suits to equalize with the air in our gloves. And when our hands get cold, we can raise them above out bodies. That lets air warmed against our bodies flow up into the glove (which is at slightly lower pressure). It is a small bit of warmth, but nice nonetheless.
So when we dive, the pressure goes up, then the pressure goes down. Our equipment lets us adjust regardless. I’m looking forward to my professional time pressures going down. They will, just like the diving ones. And regardless, I know how to adjust.