D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Friday 31 July 2009

Pelagra – how it works

In an earlier blog entry Chris and Peter described how they collect marine snow, using Pelagra free-floating, or more precisely, neutrally buoyant sediment traps that sink to a certain depth and remain there for several days. So, how do we design such a device that can do this?

The clue is in the name: neutrally buoyant. So what does that mean? Well, consider what would happen if you held a ping-pong ball under water and let it go. It would rise to the surface and float, right? And then if you did the same with a pebble of the same size it would sink straight to the bottom. We would say that the ping-pong ball is “positively buoyant” and the pebble is “negatively buoyant”. It follows then that if you held a “neutrally buoyant” object under water and let it go it would stay just where it is. This is similar to trying to make something ‘float’ at a fixed height in mid-air. In fact, the obvious example of something that can do this (more or less) is a hot-air balloon and the reason this is possible is the same reason we can make a sediment trap ‘float’ at a particular depth in the ocean. The reason is density. A ping-pong ball will rise to the surface because it is less dense than the water; conversely a pebble will sink because it is denser. So, for an object to be “neutrally buoyant” its density must be the same as that of the water.

OK, so how does that help the Pelagra traps to become neutrally buoyant at a particular depth? Fortunately for us engineers the density of ocean water tends to increase with depth which means that each depth has a unique density creating density ‘layers’ all the way down. Three factors combine to create this density gradient: salinity, temperature and pressure. Salinity increases with depth and temperature decreases with depth, both of which result in an increase in density. Pressure also increases with depth and, contrary to popular belief, water is compressible and so the higher the pressure the higher the density. In fact the density of the ocean here at the PAP site varies from 1.024 kg per litre at the surface to 1.032 kg per litre at 1000m deep.

So, all we have to do is to make sure that the density of the Pelagra trap is equal to the density of the water at the depth we want to be at. Sounds easy, but in practice it can be quite tricky.

Before we come out to sea the traps are weighed to the nearest 0.1 gram. This is not easy for a device that weighs around 130kg and requires a special, highly accurate balance to achieve. They are then floated in a freshwater test tank of known density and temperature and small weights are added gradually until the trap is fully submerged with the very top just level with the water surface. We can now say that the trap is neutrally buoyant in the tank and this enables us to calculate its exact volume.

When we arrive at the deployment site we use the CTD instrument (see blog entry of 16 July) to measure the seawater density and then use the figures we obtained in the test tank as a starting point to calculate how much weight we need to add to make the trap density equal to the seawater density. This calculation is complicated by the fact that the density of the trap is affected by temperature and pressure, just as the seawater is, so this also needs to be taken into account.

Once deployed over the side, the Pelagra trap will sink until its density and the seawater density match. However, due to the dynamic nature of ocean water, a particular density ‘layer’ does not necessarily remain at a constant depth or temperature. If the density layer were to move to slightly deeper water for instance, then the pressure would increase and both the water and the Pelagra trap would be compressed slightly and their respective densities would increase - but not at the same rate. This would lead to a mismatch in densities and the trap would deviate from the intended depth. The Pelagra trap can detect these situations with onboard sensors and compensate by using its ‘buoyancy engine’. This consists of an internal pump that pumps hydraulic oil to and from an external bladder. This way the trap can control its own volume and hence its density.

At the end of the mission a 2kg weight is released, the buoyancy engine pumps to maximum volume and the trap ascends to the surface. To aid recovery, the trap is fitted with a satellite positioning beacon that relays its coordinates to the research ship. A brightly coloured flag and a flashing light help to make the trap visible from the ship’s bridge, day or night.

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