Rosette with Niskin Bottles and CTD. Note the CTD cable attached to the center of the Rosette. Photo Credit: Andrew Cogswell.

CTD, eh? – Part 1 by Andrew Cogswell, Guest Contributor

Andrew Cogswell of the Bedford Institute of Oceanography has submitted this guest scientist entry from the R/V Atlantis.

Time: 2032 UTC
Longitude: 59 05.1 W
Latitude: 66 19.5 N
Conditions: wicked good (5-10 kts wind, fog)
Upcoming Conditions: ughhhhhh (Monday – 40 kts wind)

The SSSG Technician manages the deployment of the Rosette and CTD from the deck of the R/V Atlantis. Photo credit: Lorenza Raimondi.
The SSSG Technician manages the deployment of the Rosette and CTD
from the deck of the R/V Atlantis. Photo credit: Lorenza Raimondi.

What is a CTD, you say? Right now, it seems like a self-induced water torture device designed to make the victim experience piercing pain in each finger as you purposefully collect bone numbing water sample after bone numbing water sample, but I digress. The acronym stands for Conductivity, Temperature and Depth; each parameter represented by a different sensor which collects these data and either stores them internally or transmits them through a cable back to the ship where they are viewed live and stored by operators. OK, Coles Notes version (you can stop here if you want).

The name, “CTD”, vastly understates the complexity of this bundle of oceanographic wizardry. It was originally branded the STD (Salinity, Temperature, Depth – Wow, poor acronym choice!) in 1960 but was later sold as the CTD in 1970 (Phew, that was close! Find out more by reading Instrumentation and Metrology in Oceanography by Marc Le Menn). Since that time, the CTD has gained momentum, with new sensors occasionally being developed to add to the ever growing arsenal of this “Swiss Army Tool” of water sampling. As such, a world wide community of CTD users and data providers are established and require high levels of instrument precision and accuracy based on a generally agreed upon set of international standards. Data generated by CTD’s are sent by organizations the world over to central data repositories like the World Ocean Database (formerly, the National Oceanographic Data Center).

Niskin Bottles with the tops open connected to the Rosette Carousel, ready for deployment. Photo credit: Lorenza Raimondi.
Niskin Bottles with the tops open connected to the Rosette Carousel, ready for deployment. Photo credit: Lorenza Raimondi.

A modern CTD, like the one aboard the Atlantis, is truly amazing! When combined, the bundle of probes which measure water column characteristics, is about the size of a large microwave oven. The list of sensors on the CTD aboard the Atlantis is extensive and includes a primary and secondary set of temperature and conductivity (salinity) probes, oxygen, fluorometer, pH, Photosynthetically Active Radiation (PAR), pressure,(depth) and an altimeter. Whoa, that’s a lot going on at the same time! Each instrument requires it’s own form of baby sitting that could involve: soaking in fresh water between casts, bathing in a standard solution, covering a sensor head, etc… To further complicate the issue (or compliment probe measurements I suppose), this hyper-complicated mass of temperamental electronics is housed in a large cylindrical frame, upon which mounts 24 – 20L Niskin bottles. These vertically positioned bottles employ an ingenious series of high tension bands that run through the center of the bottle to caps on either end. Small but strong plastic wires at the top of the bottles radiate to ridged hooks on a central “carousel”, holding the bottles open.

A large electro-magnetic cable is “terminated” to the “CTD Rosette”. Many kilometers of cable (~6 km in our case), snakes its way through a “block” or wheel on the crane which protrudes from the starboard side of the ship. The cable is wound tightly on a winch drum, which pays out the desired amount of cable required for the depth of deployment. The data from the CTD makes it’s way up the cable during the cast and is visualized using software at the “Command and Control Center” in the Computer Room on the Atlantis. The bottles are fired at pre-determined depths, and each bottle contains enough water (hopefully) to meet the sampling demands of our scientists in the lab.

 

Rosette with Niskin Bottles and CTD. Note the CTD cable attached to the center of the Rosette. Photo Credit: Andrew Cogswell.
Rosette with Niskin Bottles and CTD. Note the CTD cable attached to the center of the Rosette. Photo Credit: Andrew Cogswell.
Computer screen with underway information provided by the ship. Note the Time to Station. This reading is used in the laboratory to prep for deployment. Photo credit: Andrew Cogswell.
Computer screen with underway information provided by the ship. Note the Time to Station. This reading is used in the laboratory to prep for deployment. Photo credit: Andrew Cogswell.

Alrighty then, here is our typical order of events. You might want to grab a drink or get up and stretch a little before reading. Long before the cast, a spreadsheet of required sampling is produced along with the depths samples are required from. Depending on the area being sampled, different questions are being asked and require a unique subset of our analytical suite. Each bottle on the rosette is then labelled with a unique sequential numerical identifier (a little sticker with numbers). All samples drawn from a bottle inherit this identifier for analysis. As we transit towards our stations, we are watching a screen which provides us with an approximate arrival time. We typically use this time to prepare our sample bottles and label them with stickers. Approximately 20 minutes from station, we make sure the rosette bottles are labelled with stickers, cock the bottles, remove sensor coverings and tubing and tie on tag lines to rings on the rosette frame. Once on station, the ship’s SSSG (Shipboard Scientific Support Group) technicians correspond with the bridge to acquire approval for launching the gear over the starboard side of the ship. The technician coordinates with the winch operator (housed in the next level up from the working deck) and the deck crew to deploy the CTD over the rail of the ship and into the water (science staff BTW – we don’t sit at computers or stand in labs with coffee cups all day discussing the latest pocket protectors and episodes of Big Bang Theory. OK, that happens sometimes, but who doesn’t love a good pocket protector, right!)

The Computer Room CTD command center on board the R/V Atlantis. Photo credit: Andrew Cogswell.
The Computer Room CTD command center on board the R/V Atlantis. Photo credit: Andrew Cogswell.

An operator in the Computer Room at the CTD control center (me) turns the CTD on via the “Deck Unit”. Some time is required for pumps to engage that will move water through the CTD sensor package, and then the system is brought to the surface. The CTD acquisition software is turned on to log the data and the descent is initiated by a command to the winch operator to descend to 100 m at 30 m/min.  At 100 m, control of the winch is assumed by the SSSG technician in the Computer Room and descent to the final depth is initiated at 60 m/min until near the bottom where the speed of descent is reduced to minimize the likelihood of impact (and making lots of people very agitated).  The SSSG technician and the CTD computer operator can acquire the water depth from a sounder or on board multibeam echosounder system.  At 5 m off bottom, the CTD is stopped and the sensors are given time (~1 min) to come to equilibrium prior to firing the first bottle.  The SSSG technician then brings the CTD up to the next required depth and the process is repeated at predefined intervals until the CTD is at the surface.  At the surface, the science crew and SSSG tech prepare for receiving the CTD on the starboard side of the ship.  The CTD deck unit is shut off and the deck crew use long poles with hooks on the end to latch on to the CTD and use small tugger winches in coordination with the crane to bring the CTD back up over the rail and into position on the deck.  Once on deck, the CTD is ratcheted down and the science staff assume their positions to acquire samples from the rosette. OMG, I’m tired just thinking about it!

End of Part 1  –  Rosette Blog – Andrew Cogswell – Bedford Institute of Oceanography

 

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