Tuesday, 13 May 2014

Shrinking cups

Recovering the CTD after a cast.  The CTD itself is the horizontal silver instrument at the level of the guys' hands.  The vertical cylinders are a mix of instruments and bottles.  (Photo by Ben Moat)

Today we were working at the Mid-Atlantic Ridge.  This underwater mountain range runs from north to south in the Atlantic.  While some of the work today was on landers, we also did a CTD cast, where we send an instrument which senses the temperature and salinity of the ocean to the seabed and then pull it back up on a long wire.  This wire is about 6 kilometers long, so when the instrument is down near the seabed, there is nearly 6 kilometers of water sitting on top of it.  The weight of this water creates a unique environment for creatures living at the bottom of the sea.  It also presents an engineering challenge to instruments and sensors that we use to measure the ocean.  They need to be able to withstand incredible pressures.

Adding the cups to the CTD frame.  (Photo by Lola)

One of the fun ways to observe the effect of the pressure at the bottom of the ocean is to see their effect on polystyrene (styrofoam--for Americans) cups.  Before leaving Southampton, I took 50 cups to my daughters' nursery and school.  The children decorated their cups, and then gave them back to me.  These cups flew with me to Trinidad, and then sailed with us to the Bahamas and across the Atlantic to the Mid-Atlantic Ridge.  Today, our CTD cast was finally in the daytime (usually they start or end at night).  So Lola and I stuffed the cups with a bit of paper, and then put them into some tights.  These tights were then attached to the CTD frame with cable ties (many cable ties) and they went on an adventure to the bottom of the sea.
The CTD frame going out to sea, with the cups attached.  For the oceanograhers out there, if you notice there's a bit more on the frame that usual, the large silver cylinders are acoustic releases.  These are tested at the depths they'll be operating at before they're used for the moorings.  The smaller silver cylinders are MicroCATS (the one with the 5 circles cutout of it).  These are compared against the wired CTD sensor to improve their accuracy.

If we were to send a balloon to the bottom of the ocean, it would shrink as the weight of the water squeezed the air inside it.  When it came back up and the weight of the ocean is no longer squeezing the balloon, the air would expand again, and the balloon would look like nothing happened to it.  Polystyrene is a little bit like a balloon.  It has lots of little air bubbles in it.  Under high pressure, like 6 kilometers of water, these air bubbles collapse.  But unlike a balloon, when the cups come back up to the surface and the pressure is released, the cups don't expand again.  They are more rigid, and the keep their collapsed, squeezed out shape, permanently.   The air bubbles have popped and the cups have shrunk.

You can see the results here--teeny tiny cups where the drawings and writing of the children are now miniature.
Lots of teeny tiny cups.  The big white one is what they started out as.

Tiny cups can be used for thimbles, or tea parties.

Tomorrow, we continue with the Mid-Atlantic Ridge west side, and then a few days later will travel to the east side of the Mid-Atlantic Ridge.

You can read more about ocean pressure on Deep Sea News: http://deepseanews.com/2014/05/how-to-shrink-a-styrofoam-cup-and-other-side-effects-of-deep-ocean-pressure/ (leaves this website)

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