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.

The Mysterious NMF Fellows on DY029

Jon Short, National Marine Facilities Sea Systems, National Oceanography Centre

In other blog posts, both from this cruise and from previous cruises in the SSB programme, there have been references to the National Marine Facilities technicians but few added details. So just who are these mysterious fellows and what do they do?

National Marine Facilities Sea Systems (NMFSS) is the organisation who manage the RRS James Cook and the RRS Discovery and the National Marine Equipment Pool as well as providing technicians and engineers providing specialist support to NERC research cruises on both the NMFSS ships and other vessels.

Jon Short preparing the trace metal rosette and Niskin sampling bottles. Photo: Callum Whyte.

There are seven technicians from NMFSS on board Discovery for DY029; Rob (who looks after the mooring deployments and instrumentation), Alan (our mechanical engineer who looks after equipment ranging from deck winches to the machine that produces liquid nitrogen at -300^oC), Jon (our IT expert, who makes sure that all of the vital data, from numerous instruments, is logged and recorded), Sam (who prepares and deploys the autonomous gliders) Robin and Colin (who are learning how to operate and maintain the two CTD systems on board) and me, another Jon (also looking after the CTD systems and, very loosely, in charge of the team).

The NMF team preparing to deploy a mooring. Photo: Alex Poulton.

For each cruise supported by NMFSS the preparation starts at least six months before the sail date when we meet with the senior scientists involved and discuss with them what they want to achieve and which pieces of equipment from the pool are best suited to gather the data. This equipment is then prepared for use on the required research cruise. For DY029 this involved the design of moorings and the procurement of hardware for these moorings, payloads for the autonomous gliders to be identified and fitted, laboratory containers to be fitted out to the specification of the scientists involved and instruments, fitted to the CTD frame and on the moorings, to be calibrated to very precise standards.

The NMF team and deck crew recovering a glider. Photo: Callum Whyte.

Once this is all complete the technical team and the ship's crew "mobilise" the vessel. This involves loading all of the equipment required (including everything the scientists bring), installing it on board and commissioning it for use. After the ship sails we provide 24 hour support, operating, maintaining and deploying equipment and making sure the scientific team have everything they need for a successful cruise.

The NMF team preparing the anchor chains for the moorings. Photo: Callum Whyte.