Check, check and triple check

by Clare Johnson

The excitement yesterday was that it was one of the scientists birthday. To celebrate the crew put up bunting in the bar, the cook made a very large and very yummy cake, and the birthday girl got a home-made card from us all. Other than that we are plodding through our mooring recoveries and deployments. We are currently steaming towards mooring number nine (I think!).

Before we put any instrument in the ocean, especially one that we are going to leave for a year collecting data, we do everything we can to check that it is working as well as it can be. Firstly the instruments are checked in the lab: do they look ok, are there any known problems from previous trips, can we connect to it ok – all the instruments are programmed using a computer. Then the instruments are checked again, this time by testing them in the ocean at the maximum pressure they will have to work at during their years deployment.

To do this we attach them to a CTD rosette. This is a large metal frame around 6 foot tall and 5 foot in diameter with various instruments and water bottles attached. It is a mainstay of marine science and a different CTD rosette was used continuously to make measurements on the Discovery cruise. To check instruments that will be used on the moorings we attach these to the rosette frame and lower it from the sea surface to just above the seabed before winching it back. This allows us to check that the mooring instruments have recorded data ok and not let in any water under the pressure the ocean exerts. It also allows a comparison of the values the mooring instrument is measuring relative to the instruments on the CTD rosette. Is there a very small (and allowable) offset or is the mooring instrument measuring incorrect values? [As an aside, we check the CTD rosette instruments are accurate in their measurement of temperature and salinity using a second reference thermometer and water samples to measure the salinity on another very accurate laboratory instrument. The CTD rosette instruments are also sent back to the manufacturer for calibration at regular intervals.] We also check that we can ‘talk’ to the hooks that tether the mooring to the weight on the seabed and are vital for the recovery of the mooring.

Finally, just before deployment of the mooring in the ocean for a year we do our final checks on the instruments, programme them how we’d like them to sample and press ‘start data recorder’. Although we do everything we can to ensure that we will get the mooring and instruments back, and collect good quality data, there is just bad luck and things we cannot control. So keep your fingers crossed for us that things are going well for our instruments down there in the cold, dark, deep North Atlantic!

pic1

CTD rosette with some instruments that will be deployed on a mooring attached (LHS) so they can be checked against the rosette instruments values. The grey bottles are closed at chosen depths in order to collect water. The yellow instrument measures current direction and speed.

pic2

The CTD rosette and attached mooring instruments going over the side. It is lowered to the seabed and then raised again on a wire which transmits real-time data to the ship.

pic3

The lab instrument that we use to very accurately determine salinity in water samples collected from the bottles on the CTD rosette. This enables us to check that the CTD sensors are working as well as possible. Running water samples through this machine is mind-numbingly dull and is referred to as ‘working in the salt mine’!

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