Many surveys are run to measure physical properties of seawater. One of the instrument packages used aboard the McARTHUR II measures conductivity, temperature and depth, this instrument is often referred to as a CTD. CTD information is valuable in detecting how the conductivity and temperature of the water column changes relative to depth and horizontal location. The type of CTD used aboard the McARTHUR II is a Seabird 911plus.

A CTD is a group of individual sensors that measure conductivity, temperature, and depth. Conductivity and temperature information is valuable in that the salinity of the seawater can be derived from these two variables. A CTD may also have additional sensors that measure light transmission, oxygen content, and chlorophyll. The sensors are mounted on a metal frame, either a round frame, referred to as a rosette or a smaller oblong wire frame. Multiple water bottles for sampling the water can be attached to a rosette and can be used to sample the water at different depths of the cast. The metal frame is attached to a conducting wire and is lowered into the water. Information is sent back to the ship while the instrument is lowered (downcast) to a depth specified by the scientist and then brought back (upcast) to the surface.

Scientist monitors the CTD cast.

Scientists collect water samples from bottles on the CTD.

Another instrument package used aboard the McARTHUR II to measure conductivity, temperature and depth is a Seacat. The Seacat sensors are mounted on an oblong wire frame a water bottle can be attached to the wire above the frame to sample water. The frame is attached to a wire and lowered from the ship to depth and then brought back aboard. The data is downloaded to a computer after the Seacat is brought back aboard. The Seacat is smaller and easier to launch and retrieve but one of the disadvantages of using the Seacat is the data cannot be viewed until after the instrument has been brought back aboard ship.

The McARTHUR II also has a thermosalinograph (TSG) which provides the temperature and salinity of the surface water while the ship is underway. Temperature and conductivity is measured from water that is drawn into the system through an opening in the hull of the ship. Salinity is derived from the temperature and conductivty variables. A graphic readout is displayed in the Oceanographic lab on the ship so that scientists always have this information available.