CD 157: Investigating the submarine canyons off Portugal


The great shrinking polystyrene experiment

What effect will pressure have on polystyrene?


Most of the compression will occur in the top 10m of water. Beyond this the amount of compression will become less for every 10m increase in depth.



Air pressure that we experience at sea level is 1 Atmosphere. Under water the pressure increases by one Atmosphere every 10 m, so at 10m the pressure is twice that found at the surface  and at 20m the pressure is three times the atmospheric pressure. The increase in pressure from 10m to 20m is from a factor of 2 to 3, ie an increase of 50%, whereas in the top 10m the increase is 100%. The relative increase in pressure is more the nearer to the surface so the relative change in size will be more in this zone.


Some slabs of polystyrene were cut to roughly equal sizes. Each was measured using a ruler to the nearest 1mm [see photos below]. In total there were seven slabs of polystyrene.

The slabs were tied securely into an old pillowcase which in turn was secured to the cable at intervals. The range would be from 10m to as deep as possible. [The intervals selected at the site were 10, 100, 2400, 3700 and 4700m depth.] Each slab would be re-measured immediately on arrival at the surface. At three depths we used one slab only, but it was possible to have two slabs in the pillowcase at two of the depths.


Before immersion

After immersion [click to enlarge]


Was the prediction correct?

Use some data from the graph to support your statement.

What would you conclude from the same data?
[Build your statement around some data from the graph. A good starting point is to select values of the independent variable, pressure, which increase in a pattern e.g. doubling each time.]

Can you think of any scientific reasons that support any new conclusions?


Was the test fair?

How could we improve the accuracy? Which measurements are most important to be precise with?

How could we improve the reliability?

If you treated this as preliminary work, what changes would you make to the plan.
[Look particularly at the intervals].

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April 2004
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