Saturday, 8 June 2013

7 - "Under Pressure"

We were on the move to Station PAP. The equipment had been stowed, everything was tied down and we all were eagerly anticipating getting to work soon. But we weren’t quite there yet!

We recently had a meeting to discuss what depths we all wanted to collect water and it was decided that we should collect from 100 metres above the bottom to the surface. We do this using an essential piece of equipment, the CTD. The CTD, or Conductivity, Temperature, Depth rosette, is used to measure oceanographic parameters and contains large bottles called Niskins to collect water for marine scientists to use in experiments. A CTD can be sent down as deep as needed so long as there’s enough wire. The seafloor at PAP is 4800 metres deep. We would sample water all the way to 4700 metres.

Painting cups!
Image© Stephanie Wilson
A traditional memento of any cruise to the deep-sea, plus a fun lesson in physics, is to decorate a Styrofoam cup and send it down attached to the CTD. At sea level, air pressure is 14.7 pounds per square inch (psi) and increases with depth in the ocean. At 4700 metres, water pressure is at about 2108 psi. Think of what would happen if an elephant sat on the cup. Now imagine that weight pressing into the cup from all sides! There is a lot of air within Styrofoam, as ocean pressure increases with depth, the air compresses and the cups look like they have shrunk. The deeper the CTD travels, the higher the pressure. Cups at 1000 metres would not be quite as compressed as cups coming from 4000 metres.

Before and after
I had brought along some Styrofoam cups as well as many different coloured permanent marker pens. We had already been given permission from the Chief Scientist to attach a basket to the CTD to perform the experiment and so we mentioned it to the other scientists as well. Many in the science party also saved their previously used cups from earlier meals and we had a decoration party in the library the night before we were to arrive at station. Some commemorated the occasion in words such as “4700m, Station PAP, June 2013, etc..” others drew exotic deep sea creatures, the animals or plants they were there to study, the PAP station logo.. so many creative designs!

The orginal cups.
Bottom are the same cups after
having been sent to 4700 m depth
Image© Stephanie Wilson
All the cups were sent down. At attachment they were nine centimetres high. The intense pressure at 4700 metres had compressed them to four centimetres! Also not all cups are made the same so the sturdier and larger cups that I had brought did not compress quite as tightly and therefore weren’t as small. Not everything compresses equally either, some cups came back flattened, others came back stuck together and had to be cut out. On previous cruises with CTD casts to 1000 metres, cups would come up only partially compressed. Why? Perhaps it is the inconsistencies in the making of Styrofoam or maybe the cup was pressed against or stuck inside another one during its journey.

In the end, everyone who participated got one or two cool souvenirs to take back home.. and a whole lot more respect for the intense pressure of the deep.

By Stephanie Wilson

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