Shelf Sea Biogeochemistry blog

Friday 24 July 2015

Diagnosing Transmission Problems

By  Julie Wood, NMF Technician

Thursday marked the end of the second iron transect of the cruise and for the technicians, it certainly was an eventful transect.

As technicians, one of the most important pieces of equipment we are responsible for is the CTD. This is a short name for the large metal frame carrying conductivity, temperature and pressure (measuring depth) sensors along with a whole suite of other instruments such as sensors to measure current, turbidity and fluorescence. It also carries large water bottles which can capture water from any depth visited.

On this cruise, we have two CTDs. One is a normal stainless steel frame with 20L bottles, while the other is made of titanium with 10L water bottles. Apart from titanium, this second frame contains as little metal as possible because it is used to collect water for investigating trace metals. The 10L water bottles are kept in the trace metal laboratory on the ship. Before each trace metal CTD, they are individually carried out to the frame to limit exposure to the metal on the ship.
Clean Sampling room with bottles

The CTD is lowered in the water by a wire of over 7000m long stored on a large drum. The cabling from the CTD is joined to the wire by an electrical splice near the mechanical termination (this is the conical part between the wire and the CTD frame). This allows real-time data from the sensors to be transmitted from the CTD along the wire. This means we can see profiles of ocean parameters while the CTD is in the water which can help the scientist select the depths that they would like to take water samples.

The first CTD of the iron transect was to commence on Tuesday morning at around 4am. Nick and Tom, the technicians on duty, prepared the CTD as usual for its journey down to 2400m. At around 1050m, the sensor readings indicated that the communications between the deck computer unit and the CTD had failed. The CTD was brought back on deck and the sensor readings all returned to normal. A second deployment was attempted for diagnostic purposes, however once the CTD was back in the water, the sensor readings stopped again confirming that there was a problem with the termination.
Julie and Dougal working on the CTD

The senior technician, Dougal, was called to assist in diagnosing and rectifying the fault. Based on the observations, initially 2m of cable was removed from the end of the wire. However, when the wire was tested, the electrical characteristics were found to be unsatisfactory. A further 400m of wire was removed and then the wire performed perfectly.

With assistance from Andy, the mechanical engineer, and Steve from the Glider group, the team started to build a new termination which is time consuming and requires attention to detail. A new mechanical termination needed to be put on along with a new electrical splice in order to communicate with the sensors. Both activities required concentration to ensure they were correctly and safely attached.

The final test, the load test, was performed on the new termination. This involved attaching the termination to the deck and progressively applying increasing force to a final weight of 1.2 tonnes. This ensures that it is well able to hold the CTD frame.

By 7:30pm, the titanium CTD was back in the water. Despite passing the load test, the first deployment following a new termination is always a nervous affair. The frame safely made it down to 2430m, just 20m shy of the bottom. All bottles were filled successfully with recovery of the CTD at 9:15pm.

The Metal Free CTD Winch

Unfortunately, this incident did caused delay to the science program. Some careful re-jigging of the timetable by the Principle Scientist meant that the iron line was still completed successfully. We deployed the titanium CTD at seven stations along the iron transect.

The final titanium CTD was retrieved on Thursday at 2pm, amid much excitement from the team of iron scientists collecting these water samples. With a completed transect, we hope they find lots of interesting features about iron on the shelf.

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