Shelf Sea Biogeochemistry blog

Showing posts with label CTD. Show all posts
Showing posts with label CTD. Show all posts

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.

Friday, 17 July 2015

Meanwhile, science continues

By Mark Moore

Around the drama of Wednesday afternoon (see Lucie’s recent blogpost), the science on board continues apace. Over the 5 intensive days of science to date we have already completed 72 ‘Events’ each of which effectively corresponds to a deployment of one of the many pieces of equipment which you may have read about during previous log entries, including deployments of moorings and gliders, CTDs, net sampling (see picture below) alongside snow catchers, in situ pumping systems, etc, etc ....  

Picture: The ongoing event log in the main laboratory
Others: A CTD coming on deck
Net sampling
Mooring deployments
A glider being ballasted on board before being deployed

With all this activity occurring on board it is important that we keep detailed, accurate and up to date records. The first stage in making sure everyone knows where we are with planned activities is a running event log which we keep in the main lab of the ship (see picture). Here we provide a rough record of what has happened to date, allowing the person leading the next activity to confirm the event number and providing a record for checking against more detailed logs. Simultaneously the officers on watch also keep a separate ongoing record of all activities from up on the bridge. One of my jobs as principal scientist then involves keeping a check to make sure that all these logs are aligned. It pays to have multiple redundancy as although a research ship is a reasonably small space, with everyone working in different areas of the ship and at different times of the day and night effective communications can sometimes still be a challenge!

Work wise we are currently around half way through another of our CTD (Picture) sampling transects down the shelf edge aimed at understanding the processes by which iron (Fe) may be transported off the shelf. Working where the water depth shoals so steeply has its own challenges. We have to be extra careful lowering sampling equipment near to the bottom as there are regions of the shelf break where the water depth can change by 1000m in under a km, i.e. a >45% slope!

Friday, 8 May 2015

Rough seas and science finally starts

Deploying the CTD. Credit: Gary Fones

The core aim of DY030 is to collect samples and data to understand how the chemistry and biology of the Celtic Sea link together to drive healthy and productive conditions, as well as how those conditions might change with climate change. After sailing we experienced some weather more associated with March than May – a number of scientists took to their cabins or just sat on deck staring at the horizon wishing the waves away! Those with their sea legs carried on and continued preparations in their various laboratories on the ship waiting for the science to start.

CTD. Credit: Torben Stichel
We eventually started work on the 6th May at one of our main Benthic Process sites – Benthic G. First up is always some CTD work even on a benthic sediment sampling research cruise. CTD (Conductivity, Temperature and Depth) is the stock instrument of any oceanographic cruise and enables us to understand the water column structure using a number of on-board sensors and collecting water samples from per-determined depths for subsequent analysis. After a slow start it is always nice to get the first sampling underway.

CTD. Credit: Richard Cooke

Tuesday, 21 April 2015

Game of Filters: A Song of Filters and Water

Clare Davis and Calum Preece, University of Liverpool (Westeros)

The University of Liverpool team (picture 1) is responsible for determining the composition and relative concentrations of dissolved and particulate organic nutrients, namely carbon, nitrogen and phosphorus. This is a key part of understanding both nutrient cycling and the fate of carbon fixed by primary production in the shelf system.  

Picture 1. The Liverpool team with [Jon] Snow Catcher enjoying some afternoon sunshine. Photo: Jose Lozano.

In real terms, this equates to an awful lot of filtering during the SSB cruises. To achieve this we travel down from Filterfell in the North to Southampton where we join the ship. From then on, we employ all of the Seven Filtrations to collect a wide range of samples. But first of all, we trot our little legs over to whichever device we are using for sampling that day, be it Jon Snow Catcher, CTD or Ned SAPS, armed with Tygon Lannister tubing and fill our bottles with as much seawater as we can get our hands on. There is one exception however, when we are working alongside the Fe Island team we aren’t trusted in the clean lab so they sample their fancy CTD on our behalf and deliver the water to us.

During transects and at designated stations we collect water samples from the CTD which we analyse for dissolved organic nutrients, including dissolved organic phosphorus (DOP), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), amino acids (AA) and coloured dissolved organic matter (CDOM). We define these nutrients as those which pass through what is arguably the king of filters; King GFFrey with a pore size of 0.7μm.

We collect a selfish amount of water from the CTD for sampling particulate nutrients, including particulate carbon, nitrogen, phosphorus, lipids, amino acids, stable nitrogen isotopes and pigments. We define the particulate fraction as anything stuck to King GFFrey after filtering a couple of litres of seawater (picture 2).  We also collect particulate samples from the now infamous Jon Snow Catcher. 

Picture 2. A [King] GFF[rey] filter covered with particulate material. Photo: Chata Seguro.

A personal favourite for sampling particulate nutrients is the honourable and reliable Ned SAPS. With the help of Lord Commander Jon Short (picture 3), his Men of the NMF Watch, and good old Ned SAPS we can filter hundreds of litres of seawater in situ, separating out large particles from smaller ones which can give us useful insight into the composition and variability of the different sized particles in the water column.

Picture 3. [Lord Commander] Jon Short of the NMF [Watch] and good old [Ned] SAPS. Photo: Chata Seguro.
After all the samples have been filtered most are frozen in the freezer room which lies beyond the great hangar, but the Cercei CDOM samples must be analysed on Hodor Horiba…Horiba before they degrade. This is helps us calibrate the CDOM sensors on Samuel ‘Tarly’ Ward’s sea gliders that roam the Celtic Sea.

While many are currently playing in the Game of Filters, there is no denying that the North is a force to be reckoned with as they rule over their Seven Filtration rigs across the not-so-narrow Celtic Sea.

The bloom is coming! And soon the seabed will be covered with marine snow…