What happens exactly during a CTD cast

The end of the cruise is fast approaching, and I realise I have not explained you what I was doing on board Oden. Well, you know I stare at the ocean a lot and write blogposts. I also eat very well, sleep in a comfy bed, go to the gym and the sauna, read and take pictures. And occasionally, I work.

Let’s be clear about it: it is much more exciting to do it than to read about it. It all starts with a telephone or radio call asking me to turn up to the CTD container. Time to put on my CTD gear: hat and big socks for the cold; hard hat, steel-toed boots and gloves for protection; bright orange overalls to not get wet. We check that the CTD is ready to be lowered (i.e. that the bottles are “corked” properly), and when the bridge allows us to do so, we bring it on deck by pushing it on a trolley. Now, the CTD needs to be lowered from the ship into the water. That has to be done outside, and it has not been very fun yesterday when it was snowing and windy. The CTD is brought overboard by manoeuvring a hydraulic A-frame, then lowered in the water by getting the wire out using a remote.

CTD fashion by Jari (also holding the remote) and Rob

CTD fashion by Jari (also holding the remote) and Rob

Once the key sentence “Bridge, the CTD is in the water” has been said in the radio, one person goes back inside and switches on the CTD deck unit, i.e. what powers the sensors via the wire. Apparently, if we do it earlier than that and then touch the wire, we can die electrocuted, so we have been careful. Finally, when the pump status indicates “on”, the real cast begins, and we control the winch from inside. Although someone has to stay outside to check the wire angle and whether ice is approaching, and again that is great on a sunny day –one of the few moments where you can be really on your own on the ship; but less so when it is very cold. Meanwhile, inside the container, we watch this screen:
fig2
In fact, the screen is much larger because as the CTD travels through the water column, we can see the temperature, salinity and oxygen concentration that it measures, live. But I just took this picture on deck right now, so no data to show you. This is not what we watch primarily anyway. Instead, on the way down, especially in regions like here where the depth of the water column is not well-known, we keep our eyes fixed on the line which says “altimeter”. The altimeter is a kind of sonar, pinging towards the sea floor, but only detecting said sea floor when we are getting close to it. About 100 m more precisely. So when we see this value changing and decreasing, we get ready to stop the winch. On the way back up, we also press the “fire bottle” button: that triggers the closing of the large bottles that are attached to the CTD.

Then the CTD is out of the water, and we bring it back to our container using again the remote, A-frame and trolley (except that it’s much harder to push now that the bottles are full). Once “the CTD is back on deck and secured” has been said to the bridge, we can proceed and sample the water. Basically, we fill small bottles using the water coming from our large bottles, in order to get this water analysed in the lab in a few months. We empty the large bottles –my favourite moment, and the reason why we need waterproof overalls- and then go get a cup of tea to warm our hands. Or if we’re finished for the day, we watch our underwater footage and see this:

Approaching the bottom (left) and the mysterious blue creature (right) – more to be published soon as high-quality real exciting videos!

Approaching the bottom (left) and the mysterious blue creature (right) – more to be published soon as high-quality real exciting videos!

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