The curse of the XCTD

Personal blogging was not allowed while we were on the ship. Although the official Polarstern blogging team did a great job, we thought we’d complement with info (and photos) specific to our part – the physical oceanography. We’ll try to do that once to twice a week throughout August. We are starting this week with some fairly unhappy memories about one specific instrument: the XCTD. Just pretend that you don’t know we’re back in Gothenburg 😉

Elin deploying an XCTD. Picture by Elisa Bravo Rebolledo.

The XCTD is an eXpendable probe that measures Conductivity (salinity) and Temperature along with Depth as it free-falls through the water column. The probe, i.e. the small torpedo-like black thing visible on the picture above, is connected to the computer via a copper wire. When we reach the bottom of the ocean or the end of the wire, the wire is cut and that’s the end of the measurements. Nothing is brought back up onboard. The theory is that you save a lot of time compared to a normal CTD deployment:

  • only one scientist is needed to do an XCTD, and no crew member;
  • the ship does not need to stop;
  • the whole procedure takes no more than 10-15 min for 1000 m depth – a CTD cast at that depth takes over 1 h.

The practice during this cruise has been a bit different…

1) The cables

The setting involves a lot of cables and components, and as we discovered each one of them can cause trouble. There is a battery, connected to a power converter, connected to a deck unit, connected to a computer and to the throwing “gun”, connected to the probe. I’m not 100% sure how that happened, but the power converter died during the first XCTD. Luckily, electrician Andreas was around during that cast and provided us with a different plug and a huge power extension cord plugged somewhere on the ship. Good news was, no need to carry the heavy battery anymore!

2) The drivers

Fast forward a few hours onto the second cast, and this time the computer and deck unit refuse to talk to each other. Switch everything off and back on again, unplug, replug, reboot… and the computer’s battery dies after these two hours of standing outside in the snow. For safety reasons, it is forbidden to run on the working deck. So I have to walk briskly, accutely aware that everyone on deck, bridge and close to a window, is at that time staring at us in disbelief of how long the deployment takes. Ten extra minutes of annoyed unplugging-replugging later, it works, and the second XCTD cast occurred.

3) The probes

After that, we were in deep waters for several hours and used the normal CTD. But we did not sleep – we were doing the CTD transect madness, which will be the topic of an upcoming post. So, fast forward to 4am, i.e. 25h after I had gotten up, and onto the third XCTD for yet another error: unrecognised probe! Everything looks ok, but the software will not detect that you have loaded a probe, hence will not record anything. I try with a different probe, and another one, and another one, and a final one, and I go to bed in one last wave of French swear words.

 

You know you’re in trouble when even Tomas the fitter can’t fix the XCTD. Picture by Sebastian Zeppenfeld

Who was faulty in the end? The extremely wobbly “gun” cable that would not stay connected? Or this particular box of probes? We will never know for sure. After the CTD transect madness, we stayed in deep waters for a while and performed only normal CTDs. So much time had been wasted with the XCTDs that we figured out that would save time by not even trying. And we did not trust the XCTDs anymore. We gave them a second chance two weeks later, using probes from the second box. Results were not perfect, but it worked better… and faster. And that’s when the chief scientist decided that since the XCTD does not record fluorescence, i.e. how much chlorophyll (algae) there is in the water, we would do normal CTDs instead from now on…

 

 

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