NOAA Teacher at Sea
Aboard NOAA Ship Okeanos Explorer
March 16-April 3rd
Mission: Caribbean Exploration (Mapping)
Geographical Area of Cruise: Puerto Rico Trench
Date: March 29, 2015
Weather Data from the Bridge: Partly Cloudy, 26.7˚C, waves 1-3ft, swells 2-4ft.
Science and Technology Log:
We launched and recovered a CTD earlier this week.
A CTD (Conductivity, Temperature and Depth probe) is used to study the characteristics of ocean water masses, as well as to insure data quality and accuracy from XBTs (Expendible Bathythermograph). In a previous blog, I discussed how the XBT is used to measure the temperature of the water to a depth of about 760 meters. That coupled with the conductivity sensors on the vessel are used to calculate salinity and pressure to derive a measure of the velocity of sound through water, an important factor when collecting sonar data.
An XBT can be launched while the vessel is underway without pausing the sonar, but it doesn’t collect data all the way to the bottom of the water column.
A CTD can go all the way to the bottom, depending on the depth of the ocean, the length of the tether cable, and the pressure rating of the frame and equipment making up the CTD. The titanium frame and equipment making up the CTD currently aboard the Okeanos can be lowered to 6500 meters. It is very large and requires the vessel to stay put during the entire process since it is tethered to the ship.
Since a CTD collects all three factors involved in the computation of speed of sound in water (salinity, temperature, and depth) and is therefore more accurate than an XBT which only collects temperature, it is used at least annually to provide comparison data for the XBT measurements. This is the reason our scientists used it on this cruise. Additionally, scientists on board a vessel may want to deploy a CTD more often if water masses are expected to change, or if they are interested in studying other features of the water column such as particulates, gaseous seeps, dissolved oxygen or oxygen reduction potential, or if they want to collect water samples at different depths.
In the above photo the small red arrow is pointing to the water sample tubes, the large blue arrow to the CTD, and the large red arrow to the altimeter which senses when the probe is within 200 meters from the bottom allowing winch operators to slow the descent to avoid damaging equipment. Scott Allen is the Survey Tech on board. His job is to maintain and calibrate the CTD. He helps launch and recover the CTD and then operates the software to collect and process the data.
The CTD software plots the temperature (green), sound velocity (pink), conductivity (yellow), and the salinity (blue) on the x-axes against depth on the y-axis. You can see locations on the graph where the values for temperature and salinity shift in a significant way with changes in depth. These shifts can indicate a boundary between different water masses. The upward spikes in the data are likely caused by some error in the equipment connections.
Let’s conduct an experiment!
Have you ever wondered what would happen to a styrofoam cup if you lowered into the water 2100 meters? The folks here tell me you get some pretty interesting results, so we had to give it a try.
Problem: Determine the effect of extreme pressure on a styrofoam cups.
Background: Styrofoam, properly called expanded polystyrene foam, is made by infusing air into polystyrene (a synthetic polymer) using blowing agents. Learn more here.
Hypothesis: What is your hypothesis? What do you think will happen to the air pockets if we send the cups to the depths of the ocean?
1. Decorate your cups, leaving one as a control for comparison after submersion.
2. Place the cups in a mesh dive bag and attach to a CTD.
3. Lower the CTD to 2100 meters
4. Raise the CTD and examine the cups.
So how much pressure was exerted on the cups at 2100 meters? We can use this formula to calculate it:
P = pgh
Pressure in a fluid = (density of water) x (acceleration due to gravity) x (height of the fluid above the object).
If the density of seawater is 1027 kg/cubic meter, the acceleration due to gravity is 9.8 m/s/s and the depth is 2100 meters, what is the pressure?
You should get 21 million Pascals (Newtons/square meters) or 21,000 kPa. If 1 kPa = 0.145 psi, how many pounds of pressure per square inch are exerted on each cup? About 3000 pounds per square inch. That’s about the weight of a compact car over each square inch! For comparison, at sea level the atmospheric pressure is 14.7 psi.
So what happened to our cups under all that pressure? Check it out!
Was your hypothesis supported or refuted? What happened to the air trapped in the styrofoam?
Air extraction is the reason that Dr. Wilford Schmidt uses iron rebar rather than cement to provide the anchor for his free vehicles. The cement crumbles as the air pockets give way and air is squeezed out. Cement is not as flexible as the polystyrene.
What other materials might change under pressure? If you don’t have access to the deep ocean or a CTD, you could always try a pressure cooker – but be safe!
I am inspired by all the people working on this vessel. They are so adventurous and have seen so much. I wondered what inspired them to do what they do. Here are some of their answers:
Mapping Intern, Kristin Mello: Took a class in scuba diving and realized she loved it and wanted to learn more. Her dive instructor encouraged her to do an internship as a research diver and she has been studying the ocean ever since.
Free Vehicle Tech, Zamara Fuentes: Built a model of a volcano in school became very interested in geology. Now she studies tsunami impacts on the Caribbean islands.
NOAA Corps Officer, Nick Pawlenko: Had never really spent much time on boats as a kid, but was inspired by Clive Cussler novels to explore the ocean.
Expedition Coordinator, Meme Lobecker: Her love of the oceans made her want to put her geography skills and interest in data collection to work in the ocean environment.
Engineer, Chris Taylor: Wanted to put his love of engineering to work for good pay. “There is never a dull moment,” he says.
Mapping Watch Lead, Melody Ovard: Just likes being near the ocean. “It’s a proximity thing. I am curious about what goes on in it,” she says.
Free Vehicle Scientist, Bill Schmidt: Loved surfing and was interested to learn what caused the changes in the surfing conditions day-to-day. Then he read Willard Bascom’s book, Waves and Beaches, and was hooked.
NOAA Corps Officer, Bryan Pestone: Swimming competitively and lifeguarding on the beach led him to a degree in marine biology.
Mapping Intern, Jossue Millan: An astrobiology poster caught his eye in his physics class, which peaked his interest in life in extreme environments. He enjoys the interdisciplinary sciences.
Teacher at Sea, Theresa Paulsen: I am inspired by the wonder in a kid’s eye or on a proud parent’s face and by the beauty that surrounds us from the depths of the oceans to the expanses of space. Life is amazing – and far too short to waste, so we have to make the most of it while we can.
What inspires you? Post a comment and let me know!
Did You Know?
For every 10 meters you go below the surface, pressure increases by one atmosphere (14.7 psi). Scuba instructors typically don’t recommend diving deeper than 40m to decrease the risk of decompression sickness, known as “the bends,” or equipment failures that could lead to drowning.
Question of the Day:
The deepest successful dive in the Guiness Book of World Records is currently 332.35 meters (1090ft)! Yikes! Read about it here.