K-Axis Day 6: CTD training sessions

The weather has remained unpleasant all week and we’ve been jostled around constantly in the ship by the waves that seem to come at us from all directions. This has meant that we haven’t been able to do a test CTD deployment and instead we’ve had to get creative with our training sessions.

A little about the CTD:
CTD stands for Conductivity, Temperature, Depth and relates to a specific sensor that is lowered down into the water on a long wire and takes constant measurements for salinity (conductivity), water temperature, and how deep it is from the surface. A fluorometer is also commonly used to detect chlorophyll a fluorescence, which is a measure of phytoplankton abundance. These instruments are attached to a large metal circular cage, around which we have 24 10L Niskin bottles (that look like long plastic cylinders with a lid at the top and bottom) that are used for taking water samples at pre-determined depths. Both ends of all the bottles are open on the way down and as the CTD ascends through the water column the winch is stopped and a Niskin bottle is ‘fired’ (the lids are closed), which traps water from that particular depth and brings it back to the surface. This way we get to take water samples throughout the entire water column for testing. All the data is relayed back to a control room onboard the ship in real time. The bottle firing is also performed in the control room and requires effective communication between the CTD operator and the crew members manning the winches. That is the general makeup of a CTD but it does have space to attach other sensors and instrumentation. On this voyage we also have some very interesting imaging equipment on our CTD and I’m looking forward to seeing the data come back from that.

With so many projects and people all vying for water from the Niskin bottles a ‘water budget’ has been devised to ensure that everyone gets the volume of water they need and every single millilitre is allocated appropriately. This water budget has been discussed, negotiated, and agreed on over the months of planning that were involved before the ship even left Hobart. With very little room for mistakes and no one wanting to miss out, we have been filling up the Niskin bottles with seawater from the underway line and having training sessions for everyone involved in taking water samples from the CTD.

The water samples are being used for both chemical and biological experiments, some of which require a lot of care to be taken to ensure the water samples are not ‘contaminated’. This could be from foreign particles in bottles or air bubbles that may modify the dissolved gasses in the sample. Therefore, lots of practice runs have been undertaken to ensure that all sampling methods are properly adhered to and there is no wastage of water above what has been accounted for in the ‘water budget’.

Lastly, we did a test run today with everyone taking their respective water samples from one Niskin bottle to be certain that we wouldn’t run out of water before everyone got what they needed. We were very happy to find that no one missed out on getting the water they required. This is very good news as we prepare for our first CTD deployment. We’ve still got a few days to go but our training seems to be on track.

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