Under Pressure

Hello Fellow MIST-chief Makers,

The science portion of the cruise wrapped up a couple days ago, and we have all been resting and catching up on some well-deserved sleep. We are now in transit, heading for our final destination of Kaoshiung, Taiwan. 10 days of transit (~4 left!) is a long amount of time, so to keep entertained we have all been reading, working on papers, playing ping pong (Sylvain was crowned champion of the MIST Open), cards, and other board games. We also recently held a trivia night, with categories ranging from ‘Leviathans of Literature’ to ‘Roger Revelle: The Man, The Ship, The Legend.’

 

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Assembled trivia teams and Quiz Master Robert

  Since we have finished our work, I thought I would take today to talk a little more about the CTD operations that Riley mentioned a couple days ago.  When we send the CTD (Conductivity, Temperature, and Depth) instrument down into the ocean, it is attached to a metal frame that also holds a collection of 24 bottles. The top and bottom of the bottles are held open by elastic monofilament that is attached to a clip. Each clip can be tripped remotely while the CTD/Rosette is in the water, closing the top and bottom of the bottles. This allows us to take water samples from anywhere in the water column.   After we lower the entire apparatus into the water, one of the winch operators for the ship lets out wire until the CTD/Rosette is at the deepest target depth that we have decided to sample. We then trip the first bottle, and begin to haul the wire in until we reach the next depth that we want to sample. It is important to plan ahead of time at which depths you want to sample, because if you ascend past a target depth you cannot go back down to it. Can you guess why?

 

Rosette

Riley checking the bottles of the CTD Rosette – you can see the monofilament attached to the bottle tops that allow the bottles to ‘fire’ and close at depth

  I’ll give you a hint: Think about pressure at depth. At the surface, we have an atmospheric column above us – in other words we experience 1 atmosphere of pressure. 1 atmosphere of pressure is the rough equivalent of 1 elephant standing on you (specifically, the weight of the elephant distributed over the area of its feet is roughly equivalent to one atmosphere of pressure). This seems like a lot, but if think about the fact this is the weight of the entire atmospheric column above you, its not that much! Now compare that with the pressure you would feel in the water column: The air immediately around you has a density of ~1.2 kg/m3 (depending on temperature and elevation). The density of sea water is ~1025 kg/m3 — meaning that its ~1000 times denser than the air at standard temperature and pressure conditions.

Sampling

Starting to sample from the rosette bottles

  Pretend you are moving downwards in the water column while hanging on to the CTD/Rosette. At the surface of the ocean you feel only the atmospheric pressure above you. However, as you move downwards you start to feel the weight of the water above you… which we know is 1000 times more dense! At 10 meters depth you experience the weight of 2 atmospheres above you (1 atmosphere of air and 1 atmosphere of water). At 20 meters you experience 3 atmospheres of pressure (1 atmosphere of air and 2 atmospheres of water). The last CTD cast that we did went down to about 4100 meters — and so you would have experienced roughly 410 atmospheres of pressure at the bottom.

 

Now back to the question from before — imagine that you sealed a bottle shut with air before you head down to the bottom of the ocean. The pressure in the interior of the bottle would be 1 atmosphere, while the pressure around the exterior of the bottle would be 410 atmospheres. It would implode!!  To demonstrate this, we attached a styrofoam cup to the CTD/Rosette apparatus. As the cup moves downwards in the water column, it experiences more and more pressure and begins to compress. This causes the cup to shrink. We did this experiment – and its actually pretty impressive how much the cup shrinks!

 BigCup

SmallCup

 

Thats all for now … tomorrow I’ll talk a bit about the characteristics of the water column.   As always, thanks for reading!

–Alan