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GLERL's Marine Instrumentation Lab Develops The prototype for a new Autonomous CTD Profiler (ACP) was successfully tested in Lake Michigan during the 2001 field year. The ACP was programmed to mechanically drive a CTD package up
and down a cable through 40 meters of the water column at the
rate of once per hour, taking measurements at sub-meter intervals.
The ACP operated for a total of 573 cycles before the batteries
were depleted. This allowed for over three weeks of data
at the one-hour rate. We estimate that reconfiguring the
batteries will give us 720 cycles or 30 days of operation.
In addition, the sample interval can be reduced to extend the
deployment life. For example, a two-hour interval would
extend the deployment to two months. The total measurement
capability per deployment is 28,800 meters. Measurements
in shallower water will extend the deployment life while measurements
in deeper water will shorten it. The CTD package for this
prototype test consisted of a transmissometer, temperature probe,
and pressure sensor. It is capable of hosting a number of
additional sensors.
The ACP uses an electric motor to drive the CTD package up and down a wire rope cable. Rubber end stops attached to the cable determine the limits of travel and can be adjusted to meet the specific requirements of the experiment. Our prototype test was conducted at a depth of about 55 meters with the CTD travel distance of about 41 meters. The highest measurement in our prototype test was 12 meters below the surface. It is possible, however, to operate the unit closer to the surface. The lowest measurement is about 1 meter from the bottom. The operation of the ACP is controlled by an embedded microprocessor and data is stored onto a compact flash disk. We used an Onset TattleTale model 8 data logger and a 30 MByte compact flash disk. The flexibility of the programmable data logger allows specifying the interval of the cycles as well as the sample rate. The unit can sample and record data from the sensors in intervals as small as 10 cm. The following contour plot shows data from the temperature sensor:
This contour plot shows data from the transmissometer in bac (beam attenuation coefficient) units:
Most of the ACP is put together from off-the-shelf components. The housing requires fabrication from a machine shop and the wiring and some of the electronics require assembly. To duplicate the prototype unit would cost about $5,000 plus the cost of the sensors. Contact Information:
URL: http://www.glerl.noaa.gov/mil/profiler.html |
