Ask A Scientist: Under the Ice

From Tucker High School in Atlanta, Georgia:

When you dive under the ice, how long do you stay under the ice with each dive? Even with the special scuba diving gear, do you still feel cold when you are under the ice?

Each dive has it’s own mission and time can vary, but our average time has been 35-40 minutes. How long we stay underwater is a function of the amount air we take down and the depth at which we are diving. Since we only use steel 95 tanks, which hold about 96 cubic feet of air at 2400 psi, all of our dives times are regulated by how deep we are diving. We use the some of the same gas laws developed by scientists around the end of the 18th century to plan our SCUBA dives in order to make sure we have plenty of air. As you take these tanks down under water the deeper you go the more pressure there is from the water column, and as Boyle’s law states, the volume of a given mass of gas is inversely proportional to its pressure, if the temperature remains constant. Thus, the deeper you go the more ambient pressure there is and the mass of gas per breath increases because the air gets compressed to a smaller volume, which means you have fewer breathes off that one tank at 100 feet verses 20 feet. Just to be clear here, the density of the gas you are pulling out of your tank immediately goes to ambient as you are breathing through your regulator. So at depth you breathe more gas per unit volume due to the increased density, and since density increases as a function of depth, the time your gas supply will last decreases proportionally. However, Boyle’s law doesn’t account for temperature and that water is COLD. So we have to use Charles’s gas law to account for an even greater reduction of volume when we dive in ice water. There are also safety concerns about nitrogen dissolving into your blood at higher pressures, which is another important item we take into account. We have been trained thoroughly on how to take into account all these factors so that we stay well within our safety guide lines when we go below and always come back safe and sound.

Cynthia Spence photo of Rory climbing out after a dive

Our SCUBA dry suits and special dry suit underwear, a sort of fleece onesie, help keep us “warm” and dry in the ice water.  There are certain tricks you can do to move some of that warm air around your core to your hands and feet which really help to keep you from getting too cold. Stay down for any amount of time and you can feel that cold starting to sink in, but again a small price to pay for this kind of science.

 

 

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