Mechanics of the CTD
A CTD is device which is lowered into the water and relays back information about the water it is passing through. CTD stands for "Conductivity-Temperature-Depth", which, coincidentally, are the sensors they are usually equipped with. CTD's vary in both design, construction, and instrumentation. They are lowered through a depth range and the data collected is used to create a water profile of that range. Most CTD’s are also equipped with Niskin bottles that can be triggered to collect water samples at different depths.
The Kilo Moana’s CTD is cylindrical in shape. It has sensors that measure the conductivity (which translates into salinity) of the water, as well as the temperature, density, pressure, and the amount of particulates (nephels) present in the water. CTD’s are also usually equipped with Niskin bottles that can be triggered to collect a water sample. It also has 24 water sample bottles around the outer edge. They are activated by a set of magnetic latches that are controlled by the CTD’s computer which transmits data and receives instructions through a braided steel data cable.
The Kilo Moana’s CTD is simple and utilitarian in design. A welded aluminum frame protects the delicate sensors from damage. Made up of three inner and three outer hoops of aluminum (about 6 feet in diameter) and braced with 1" tubing; the instruments are placed in the center, and the 24 sample bottles are attached between the inner and outer rings. Four aluminum tubes are attached to the top hoop in a pyramid arrangement to raise the cable attachment point above the top of the CTD, increasing stability as well as providing room for the magnetic latch carousel. To further increase the stability of the CTD, both on deck, and in the water, weights are bolted to the bottom hoop.
Within the frame of the Kilo Moana’s CTD, there is actually a smaller CTD unit. The Sea Bird 911plus CTD would have worked on its own, but Stag engineers wanted to increase its capabilities. In addition to the temperature, conductivity, and pressure sensors contained within the Sea Bird 911, Stag technicians added a fluorometer, a nephelometer, a depleted oxygen sensor, and twenty four water collection bottles along with the magnetic latch system that triggers them.
The Temperature sensor uses a high speed thermistor which is housed in a pressure case along with its interpretative electronics. A Wein-bridge oscillator converts the thermistor’s varying resistance into a changing frequency which is later converted into a digital signal for processing. The oscillator works by comparing the discharge of a capacitor attached to a fixed resistor, to a capacitor whose discharge cycle is governed by the resistance of the thermistor.
The conductivity sensor works in much the same way as the temperature sensor. Instead of a thermistor, however, there is an exposed cell which provides a varying resistance to the oscillator circuitry. The cell contains separated electrodes and salinity determines the amount of current that can pass between them.
The Paroscientific Digiquarts pressure sensor is has no moving parts. A quartz crystal resonator makes up the heart of the unit. The frequency at which if vibrates is governed by the amount of stress the crystal is undergoing. The sensor is designed in such a way that the increasing water pressure increases the stress on the crystal. In addition to the crystal resonator, there is also a built in temperature sensor that compensates for the thermal instability of the crystal.
The Seapoint Chlorophyll Fluorometer measures the amount of Chlorophyll a present in the water. It has the sensing area can be left exposed to the surrounding water or be hooked up so a pump draws water through it. LED's are shined through an excitation filter into the sample area. Any Chlorophyll a present will fluoresce when it is energized by a certain wavelength. There are two additional filters on the end of the sample area that filter out everything but the fluoresced light. A light sensor measures the amount of light that makes it through, and this measurement is used to determine the floridity of the water. The LED's are modulated so that they flash at 700 Hz, and the onboard computer is synchronized so that it only reads the sensor between the flashes, reducing the amount of ambient light interference. In addition to detecting Chlorophyll, the fluorometer can be calibrated to produce and detect different wavelengths, enabling it to activate and detect other things that are cable of fluorescing. One use of this feature is to trace a fluorescent dye once it is released into the water, which allows for tracking of undersea currents.
The Nephelometer measures the amount of waterborne particles (nephels) present in the water. It has an LED and a phototransistor mounted on the outside, both aimed in the same direction. In clean water the light from the LED continues straight, and the phototransistor doesn’t ‘see’ anything. However, when there are particulates in the water, the phototransistor is illuminated by the light that bounces off them. A higher the particle density corresponds to a higher light level. The 0-5 voltage signal from the phototransistor is converted into a digital signal before being transmitted to the surface.
Altimeter and Bottom Contact Switch
Some CTD's are equipped with a sonar altimeter which allows them measure their precise height above the sea floor. This allows samples to be collected with more precision and therefore produce more meaningful data. The altimeter is mounted to the bottom of the CTD and configured so that it sends out a sonar ping, waits for the echo, and then using the time difference, calculates the distance. This is the exact same method that there shipboard depth finder uses.
The other method for determining distance to the bottom is much simpler. The bottom contact switch uses a weighted wire attached to a switch. When the weight touches the bottom, reliving the pressure from the wire, the switch activates and the CTD knows exactly how high it is (length of the wire) off the bottom.
Different from either the altimeter or bottom switch, a pinger is a simple device that is somewhat complicated to use. A pinger sends out a short burst of sound (a "ping") every second or so. There is a transducer mounted on the hull of the ship that is calibrated to detect the pings. When it receives a ping, it displays it as a point, located by the time of reception and an arbitrary distance scale. The ship actually ‘hears’ two pings for every one the pinger sends off. This is because the sound burst from the pinger travels in all directions at exactly the same speed. The ship is first hit with the signal coming directly from the pinger, and then it receives the ping that bounced off the bottom. By knowing approximately how deep the water is, and comparing the time difference between the two pings, you can figure out how far off the bottom the CTD is. As the two points get closer together, you know that the difference between the two distances is lessening, and therefore, the CTD is getting close to the sea floor.
The Kilo Moana’s CTD is equipped with twenty four sample bottles for taking water samples at different depths. Niskin bottles, as they are called, are basically big plastic tubes with end caps that are held in place by a large spring. The end caps are cone shaped, and fit snugly into the end of the tube, pressing against an O-ring to create a watertight seal. When armed, they end caps are pulled out and to the side, where they are held in place by a one of the carousel’s magnetic latches.
The SBE Carousel is a set of twenty-four latches arranged in a circular fashion so that each latch points to one of the sample bottles on the outer edge of the CTD. Each of the twenty four latches has an electromagnet underneath it. When activated, it moves a small tab, causing the latch to release. With this system of electromagnets, there is no direct connection between the electronics and the latching mechanism, which makes the whole system much more robust and reliable.
Once the command to "fire" a Niskin bottle is given, the shipboard computer sends a signal to the CTD, which processes it and then activates the correct electromagnet. Once the latch is released, there is no longer anything holding the bottle’s end caps to the side, and they snap back into place, trapping a water sample for future study.
Because of the way the sample bottles are configured, as depth increases, so does the pressure on the end caps, this causes them to push harder against their seal, making them more watertight. When coming to the surface, the pressure inside the sample bottle increases with relation to the water outside; the end caps act as one-way valves, allowing some of the sample water to flow out, reliving the pressure differential, but preventing any backflow which could contaminate the sample.
Once on deck, the water sample is retrieved by opening an air valve on the top of the Niskin bottle and a valve on the bottom. Once the a sufficient sample has been gathered, the bottles can be opened, cleaned, and redeployed.
The CTD sends and receives information along a data cable. The cable is made up of a four conductor coaxial cable inside a braided steel sheath that supports the weight of the CTD. The onboard microprocessor converts the various signal types from all the sensors into one digital signal. It then amplifies this signal and sends it back to the ship via one of the four copper wires imbedded within the cable.
On the shipboard side, there is a SBE 11plus Deck Unit that provides an interface between the CTD and a regular computer. Installed on the controlling computer is a program that allows you to see the data in real time, view the status of the sample bottles, and allows you to fire them. When a bottle is fired, it logs precise the time and depth, storing it along with the sensor data. Once the CTD profile is complete, the data can be converted into several different formats so that it can be analyzed by different programs.
If there is no need for active data logging, or if you don’t have an expensive cable with data transfer capabilities, there are memory units that attach directly to the CTD, that log sensor data. Of course, you have to wait until the CTD is retrieved and the data downloaded before you can view the data.
Deployment and Recovery
The CTD is kept in a specially designed hanger on the fantail. It rests on a movable platform which slides in and out of the hanger on tracks. Once it is outside the hanger, the hydraulic winch and crane are powered up. Tag lines are attached to the outer ring of the CTD to stabilize it while it is lifted clear. Eight hundred pounds is a lot to have swinging around the back deck. Once the CTD is off the pad, is swung over the side and lowered into the water.
The operation is then moved into the control center, from where the CTD is lowered to its target depth and then back up. The winch operator can monitor the wire out, tension, and speed. The science crew monitoring the incoming data can trip the sample bottles if anything interesting comes up.
Once the CTD is out of the water, tag lines are clipped on to the outer ring. While tension is kept on the lines, the crane maneuvers the CTD over its landing pad, and then lowers it. Once the tension on the wire is relived, the CTD is secured to its pad, and moved inside the hanger, at which point water samples can be retrieved, and the CTD serviced.
Blake English onboard the R/V Kilo Moana.
25 April, 2005
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