conductivity – NOAA Teacher at Sea Blog

April 28, 2023April 29, 2023

Julie Hayes: Days at Sea! April 26, 2023

NOAA Teacher at Sea

Julie Hayes

Aboard NOAA Ship Pisces

April 22-May 5, 2023

Mission: SEAMAP Reef Fish

Geographic Area of Cruise: Gulf of Mexico

Date: April 26, 2023

Weather Data

Clouds: Scattered

Temperature: 77 degrees F

Wind: 12 kt.

Waves: 2-4 ft.

Science and Technology Log

Each day is started and then ended with a water sample from the ocean. The technology is called a CTD, but the procedure would be called a CTD cast (as if we were casting it in the ocean). CTD stands for conductivity, temperature, and depth. The CTD consists of a collection of electronic instruments that measure the properties of the water, including a laser that checks the clarity of the water. Sampling water bottles are connected to a metal frame called a “rosette”. This information on water characteristics is important to both the scientists and the survey mapping team that use cameras and sonar. This information lets them know how well the clarity of the water is and the speed of sound that helps with the depth finders and sonar.

The apparatus containing the conductivity, temperature, and depth probe sits on the deck of NOAA Ship Pisces, awaiting deployment. — CTD used to check water quality, conductivity, temperature, and depth.

Vocabulary Check

What is Conductivity?

Conductivity is a measure of the ability of water to pass an electrical current.

What is Salinity?

Salinity is the dissolved salt content of a body of water and is a strong conductor of water.

So why is it important for scientist to know what each of these are?

The higher salinity the water is, the higher the conductivity of electrical currents.

Temperature also plays a role in the density. Knowing each of these is important because it lets the scientists know the water quality at different depths so they can make adjustments to their cameras and sonar.

Jack Prior, Chief Scientist

Jack is a pretty “chill” guy, and I have enjoyed watching him in action the past few days. Jack is the field party chief of this mission which involves everything from planning the trip, to deciding the daily sampling locations, deploying cameras, mapping, and figuring out what to do when things go wrong. Jack is in charge of planning and submitting the protocol for the entire mission and also is responsible for the end reports of the mission. You will find Jack on this leg sitting behind multiple computers regulating and keeping a watchful eye on all of the important information regarding this mission. Jack attended the University of West Florida to get his degree in marine biology.

Jack sits at a computer desk with multiple monitors. He smiles at the camera, his right hand giving a thumbs up. — Chief Scientist Jack Prior

Student Question of the Day

Whenever I get a chance, I ask random crew members questions that my students back home were curious about. Here is how Jack answered some of the students’ questions.

Konnor, Nichole, Lillian ask: What degree do you have and what all is needed to do your job?

Jack started his major in biology and had originally planned on going on to be a pharmacist, but then moved to Florida where he ended up getting his degree in marine biology instead. Jack continued to also get his Masters at the University of West Florida, too. Jack changed his career path because he enjoyed marine life. Volunteer work is crucial to get experience, and can benefit you on becoming more diverse when it comes to getting a job in marine biology.

Alyson asks: What would be your dream job?

Someday Jack wants to explore the seafloor in a submarine.

Blake, Sailor, Lilli, Jenna ask: What is your favorite food on the ship?

Taco Tuesdays seem to be a huge hit on the ship, as well as Friday pizza day.

Auburn, Ashton M., Karson, Liam: What would you consider to be the coolest marine life you have seen?

Seeing large diverse reef habitats is what Jack says he finds the most interesting, especially uncommon invertebrates that you’d never see on the beach.

Jaxon and Dwight: Can you be on the ship if you have health issues and what happens if there is a medical emergency?

The ship is a pretty confined space with steep stairs, uneven footing, areas you have to be able to step over, and have the ability to carry heavy weight. If there is ever a medical emergency, the ship works alongside the United States Coast Guard to get them the help they need. However, the ship is great working with all issues and plans accordingly to those who may have special diet restrictions.

Personal Log

Well, I will say that I am getting better at having my sea legs but that is still a work in progress. I have really enjoyed getting to understand the life on this ship, and I am just amazed at how diverse everyone is and yet still make this an amazing environment. It has taken me a few days to get the hang of where things are and to get out of my comfort zone to ask what I feel like has to be a million questions about everything. I have really enjoyed getting to hear and learn about the crew’s background and how they ended up on NOAA Ship Pisces. I greatly appreciate their willingness to answer my questions, even though I am sure I am in their way at moments. Everyone has a job to do and work different hours and shifts. It is great to see how they all respect each other’s space and sleeping hours.

There is so much science around me that I never knew existed, and I am shocked on how much technology is actually being used and heavily relied upon. Today was the first day the waves were calm enough that I was able to go out on the stern (learning names of different areas of the ship) to work on the blog and soak up a little bit of Sun. It was nice to be able to get some fresh air. The food has been amazing on the ship. I love how everyone is so courteous by thanking the cooks, as well as cleaning up after themselves before leaving the mess. The mess is the area in which we eat and the kitchen is called the galley. It has taken me a few days to understand the boat “lingo” but I am starting to catch on. The stairs are pretty steep, and everyone on board says to use 3 points of contact when walking. This is so that if they hit a wave while walking you are more stable. I could definitely see this being an issue going up and down the stairs. The doors are super heavy and I am still learning how to get those twisted and sealed tight the first time I close it (I am getting there).

A view of the mess: that is, the ship's the dining area. At the moment, it is unoccupied. There are five long tables, bolted to the floor, covered in blue vinyl or plastic table clothes. Black chairs surround each one. The chair's legs are all capped in cut-open tennis balls. The tables are supplied with condiments and paper towel holders. A large television screen mounted on the wall shows a football game. — The mess where we eat. It is spotless and a great size to fit everyone on board.

A staircase, as seen looking down from the top stair. There are railings and anti-slip applications.

A heavy metal door, with rounded edges, and a steering-wheel shaped handle. The door is stamped "Keep Closed."

August 6, 2018

Stephen Kade: Conductivity, Temperature, and Depth, August 5, 2018

NOAA Teacher at Sea

Stephen Kade

Aboard NOAA Ship Oregon II

July 23 – August 10, 2018

Mission: Long Line Shark/ Red Snapper survey Leg 1

Geographic Area: 30 54 760 N, 76 32 86.0 W, 40 nautical miles E of Cape Lookout, North Carolina

Date: August 5, 2018

Weather Data from the Bridge:

Wind speed 11 knots,
Air Temp: 30.c,
Visibility 10 nautical miles,
Wave height 1 foot

Science and Technology Log

While our main mission aboard the NOAA Ship Oregon II is to survey and study sharks and red snapper, it is also very important to understand the environmental conditions and physical properties of the sea water in which these animals live. The CTD instrument is used to help understand many different properties within the water itself. The acronym CTD stands for Conductivity (salinity), Temperature, and Depth. Sensors also measure dissolved oxygen content and fluorescence (presence of cholorphyll).

The CTD instrument itself is housed in a steel container and is surrounded by a ring of of steel tubing to protect it.

Conductivity is a measure of how well a solution conducts electricity and it is directly related to salinity, or the salt that is within ocean water. When salinity measurements are combined with temperature readings, seawater density can be determined. This is crucial information since seawater density is a driving force for major ocean currents. The physical properties and the depth of the water is recorded continuously both on the way down to the ocean floor, and on the way back up to the surface. There is a light, and a video camera attached to the CTD to provide a look at the bottom type, as that is where the long line is deployed, and gives us a good look at the environment where our catch is made. These data can explain why certain animals gather in areas with certain bottom types or physical parameters. This can be particularly important in areas such as the hypoxic zone in the Gulf of Mexico. This is an area of low oxygen water caused by algal blooms related to runoff of chemical fertilizers from the Mississippi River drainage.

Ready to deploy the CTD

Calibrating the CTD

The CTD instrument itself is housed in a steel container and is surrounded by a ring of of steel tubing to protect it while deployed and from bumping against the ship or sea floor. Attached water sampling bottles can be individually triggered at various depths to collect water samples allowing scientists to analyze water at specific depths at a particular place and time. The entire structure is slowly lowered by a hydraulic winch, and is capable of making vertical profiles to depths over 500 meters. An interior computer display in the ship’s Dry Science Lab profiles the current location of the CTD and shows when the winch should stop. We have found this to be a tricky job, during large wave swells, as the boat rocks quite a bit and changes the depth by a meter or more. The operator must be very careful that the CTD doesn’t hit the ocean floor too hard which can damage the equipment.

Dry Lab — An interior computer display in the ship’s Dry Science Lab profiles the current location of the CTD and shows when the winch should stop.

The data collected while deployed at each station is instantly uploaded to NOAA servers for immediate use by researchers and scientists. The current data is also available the general public as well, on the NOAA website. Once safely back aboard the Oregon II, the CTD video camera is taken off and uploaded to the computer, The CTD must be washed off and the lines flushed for one minute with fresh water, as the salt water from the ocean can damage and corrode the very sensitive equipment inside. The instrument is also calibrated regularly to ensure it is working correctly throughout all legs of the long line survey.

Personal Log

I am having such a great time during my Teacher at Sea experience. In the 9 days aboard ship so far, we have traveled the entire coasts of Mississippi, Arkansas, Florida, South Carolina, and North Carolina. Never in my life did I think I would get an opportunity to do something like this as I’ve dreamed about it for decades, and now my dreams have come true. I’m learning so much about fishing procedures, the biology of sharks, navigational charting, and the science of collecting data for further study while back on land at the lab. I can’t wait to get home and spread the word about NOAA’s mission and how they are helping make the world a better place, and are advocating for the conservation of these beautiful animals!

Animals Seen: Sharpnose shark, Tiger Shark, Grouper, Red Drum fish, Moray Eel, Blue Line Tile fish

July 8, 2018July 15, 2018

Pam Schaffer: Oceanographers Toolbox: What is a CTD? July 7,2018

NOAA Teacher at Sea

Pam Schaffer

Aboard NOAA Ship Bell M. Shimada

[July 2-10, 2018]

Mission: ACCESS Cruise

Geographic Area of Cruise: North Pacific: Greater Farallones National Marine Sanctuary, Cordell Bank National Marine Sanctuary

Weather Data from the Bridge

Date	July 7 2018
Time	1200 (noon)
Latitude	37° 58.3’ N
Longitude	123° 06.4’ W
Present Weather/ Sky	Cloudy
Visibility (nm)	10
Wind Direction (true)	341°
Wind Speed (kts)	18
Atmospheric Pressure (mb)	1018
Sea Wave Height (ft)	3-5
Swell Waves Direction (true)	330°
Swell Waves Height (ft)	3-5
Temperature Sea Water (C)	13.2°
Temperature Dry Bulb (C) Air Temperature	13.1°
Temp Wet Bulb (C )	12.1°

Science and Technology Log

Marine life is not evenly distributed throughout the World’s oceans. Some areas contain abundant and diverse life forms and support complex food webs whereas other areas are considered a desert. This variation is due to environmental factors like temperature, salinity, nutrients, amount of light, underlying currents, oxygen levels and pH. Some of these variables, such as temperature, oxygen levels, and pH, are experiencing more variability as a result of climate change. In order to understand the health of marine environments, scientists explore the chemical and physical properties of seawater using a set of electronic instruments on a device called a CTD. CTD stands for conductivity, temperature and depth and is the standard set of instruments used to measure variables in the water column.

Source: ACCESS www.ACCESSoceans .org — Source: ACCESS http://www.ACCESSoceans .org

The CTD is the bread and butter of oceanography research. It is primarily used to profile and assess salinity and temperature differences at varying depths in a water column. But the device can also carry instruments used to calculate turbidity, fluorescence (a way to measure the amount of phytoplankton in the water), oxygen levels, and pH. Conductivity is a way of determining the salinity of water. It measures how easily an electric current passes through a liquid. Electric currents pass much more easily through seawater than fresh water. A small electrical current is passed between two electrodes and the resulting measurement is interpreted to measure the amount of salt and other inorganic compounds in a water sample. Dissolved salt increases the density of water, and the density of water also increases as temperature decreases. Deeper water is colder and denser. Density is also affected by water pressure. Since water pressure increases with increasing depth, the density of seawater also increases as depth increases.

Optical sensors are used to measure the amount of turbidity, fluorescence, and dissolved oxygen at various depths in the water column. Dissolved oxygen levels fluctuate with temperature, salinity and pressure changes and is a key indicator of water quality. Dissolved oxygen is essential for the survival of fish and other marine organisms. Oxygen gets into the water as gas exchange with the atmosphere and as a by-product of plant photosynthesis (algae, kelp etc.).

Photo Credit: Julie Chase/ACCESS/NOAA/Point Blue

Typically, CTD instruments are attached to a large circular metal frame called a Rosette, which contains water-sampling bottles that are remotely opened and closed at different depths to collect water samples for later analysis. Using the information and samples collected, scientists can make inferences about the occurrence of certain chemical properties to better understand the distribution and abundance of life in particular areas of the ocean.

Scientist Carina Fish collects samples from CTD

On our mission, scientists deploy the CTD to a depth of 500 meters at most stations. On the shelf break, the researchers deployed the CTD to 1200 meters (more than 3/4 of a mile below the surface) to collect samples. The pressure is so great at this depth that a 1 foot by 1 foot square of Styrofoam is crushed to a quarter of its size(3″x 3″).

Personal Log

Around 01:30 last night we lost our Tucker Trawl net as it was being re-positioned. The winds had picked up to around 20 knots and the sea height was around 5-8 feet according to the bridge log. The sea state complicated the retrieval and as best we can conclude the wind and seas pushed the net bridle into a prop blade which swiftly and effortlessly cut the 1/3” thick metal wire cable and separated the net from its tether. Mishaps at sea are part and parcel of working in a harsh and variable environments. Even the very best and most experienced captain and crew encounter unforeseen issues from time to time. Dr. Jaime Jahncke quickly stepped into action and made contact with onshore colleagues to arrange for another net for the next research cruise. In the meantime, we plan to use the hoop net to collect krill samples, weather permitting.

Did You Know?

According to NOAA scientists, only about 5% of the Earth’s oceans have been explored.

June 29, 2017June 29, 2017

Dawn White: Sampling the Pacific, June 24, 2017

NOAA Teacher at Sea

Dawn White

Aboard NOAA Ship Reuben Lasker

June 19 – July 1, 2017

Mission: West Coast Sardine Survey

Geographic Area of Cruise: Pacific Ocean; U.S. West Coast

Date: June 25, 2017

Weather Data from the Bridge

Date: June 25, 2017 Wind Speed: 22 kts

Time: 4:00 p.m. Latitude: 5026.55N

Temperature: 14.3^oC Longitude: 12808.11W

Science and Technology Log

Although the scientists have not performed any fishing trawls since departing San Diego, there is a survey crew on board that has continuously been monitoring the water column for a variety of factors using acoustics and an instrument called a Conductivity/Temp/Depth (CTD) probe.

Last night I was able to observe the launch and retrieval of a small, handheld CTD probe. It looks very much like a 2 ft torpedo. The electronics and sensors built into the probe measure such factors as salinity, sound speed, depth, and water temperature. This smaller probe is launched off the tail of the boat and let out on a line of filament from a reel that appears very similar to a typical fishing reel. It does not take more than a couple of minutes for the probe to sink to a depth of about 300 meters. Data is collected from the probe at various depths on the way down. Once the probe has reached its target depth, it is simple reeled back in using a winch to retrieve it. This requires quite a bit of energy as the probe is deployed with enough line for it to end up about 3 miles behind the ship. The data from this probe is then blue-toothed to the program used by those monitoring the water column acoustically. It help the techs make corrections in their acoustical readings.

White_scientists deploying probe_R — Surveyor Jian Liu and scientist Juan Zwolinski deploy the smaller CTD probe off the stern of NOAA Ship *Reuben Lasker*

The Reuben Lasker also carries a larger version of the CTD probe with the additional capabilities such as water collection at various depths. However, this version requires the ship to be stationary. Taking measurements with the unit slows down the work of the day as each stop takes about 30 minutes from launch until retrieval. The launch of the larger CTD can be seen below.

CTD launch

CTD surfaces after test

CTD being landed back on deck

White_CTD probe in basket — CTD Probe in steel protected basket

The data from the CDT probe is recorded real-time on the survey team’s computers. Below you can see how this data presents itself on their video screens.

Several variables plotted against temperature (x-axis) and depth (y-axis)

Key for CTD Data: Temperature is in red, Salinity in blue, Fluorescence in green

On the left video display you can see that there are several variables that are plotted against a depth vs. temperature. The green line tracks fluorescence (a measure of the chlorophyll concentration); the light blue line tracks dissolved oxygen; the red line represents temperature; the blue line is for salinity.

Extension question for my students reading this: What correlations or relationships do you see happening as you observe the change in variables relative to changes in depth?

White_Lasker route — Route of NOAA Ship *Reuben Lasker*

Here is the route taken by the Reuben Lasker during the past 24 hours or so. As you can see from the chart, the ship has now reached the northern-most end of Vancouver Island. This is where the CDT recordings, marine mammal watching, deployment of two sets of plankton nets (to be explained later) and fish trawling will begin along the predetermined transect lines.

Note at the base of the screen the other parameters that are continuously recorded as the ship moves from place to place.

Personal Log

The action on-board is increasing dramatically today. We have arrived at our outermost destination today, along the northernmost coast of Vancouver Island. The sights from the bridge are amazing…all this blue water and rugged, pine covered coastline. I am still waiting for that orca whale sighting!

The waves are up today but I’m holding my own. Yeay! Especially as the night fishing will begin in a few hours.

Unique activity of the day – I just finished a load of laundry! The ship possesses 3 small washer/dryer units so we can redo our towels and whatever else we have used up during the course of this first week. How serviceable can you get! I’ll retrieve mine as soon as dinner is over. We have set meal hours and if you miss…it’s leftovers for you! Best part of this is I am actually ready to eat a normal meal, even with the ship rocking the way it is today.

I have now been assigned deck boots and a heavy duty set of rain gear to cover up with when the fish sorting begins. I can’t wait to see what all we pull up from these nutrient rich waters!

Did You Know?

Much of the data collected by the CTD and acoustic equipment from the Reuben Lasker is entered into a large data set managed by CalCOFI (California Cooperative Oceanic Fisheries Investigation). Anyone interested in utilizing and analyzing this data can access it via the organization’s website located here. There is an incredible amount of information regarding the work and research completed by this group found on this site. Check it out!

June 17, 2016August 21, 2021

Spencer Cody: What Remains Unseen, June 17, 2016

NOAA Teacher at Sea

Spencer Cody

Onboard the NOAA Ship Fairweather

May 29 – June 17, 2016

Mission: Hydrographic Survey

Geographical Area of the Cruise: along the coast of Alaska

Date: June 17, 2016

Weather Data from the Bridge:

Observational Data:

Latitude: 55˚ 10.643′ N

Longitude: 132˚ 54.305′ W

Air Temp: 16˚C (60˚F)

Water Temp: 12˚C (54˚F)

Ocean Depth: 30 m (100 ft.)

Relative Humidity: 81%

Wind Speed: 10 kts (12 mph)

Barometer: 1,013 hPa (1,013 mbar)

Science and Technology Log:

106_0507 (2) — Hydrographic Senior Survey Technician Clint Marcus is cataloguing all of the discreet hazards and objects by location and by photographic evidence that will be available for the new nautical charts once the survey is complete.

Uncovering potential dangers to navigation often requires more that acoustic equipment to adequately document the hazard. Many hazards are in water that is shallow enough to potentially damage equipment if a boat were to be operating in that area and may also require special description to provide guidance for those trying to interpret the hazard through nautical charts and changing tides. This is one of the key reasons so much planning must be placed into assigning survey areas determining the size and extent of polygons for mapping. Depending on the complexity of the area’s structures, the polygon assignment will be adjusted to reasonably reflect what can be accomplished in one day by a single launch. Near-shore objects may require a smaller boat to adequately access the shallow water to move in among multiple hazards. This is where a smaller boat like the Fairweather’s skiff can play a role. The skiff can be sent out to map where these near-shore hazards are using equipment that that will mark the object with a GPS coordinate to provide its location. Additionally, a photograph of the hazard is taken in order to provide a greater reference to the extent of the object and what it looks like above or below the water. This information is collected and catalogued; so, the resulting nautical chart will have detailed resources and references to existing nautical hazards.

104_0445 (2) — Ensign Pat Debroisse covers nautical hazards such as rocks and kelp indicated throughout a very shallow and hazardous inlet.

Nautical hazards are not the only feature found on charts. Nautical charts also have a description of the ocean bottom at various points throughout the charts. These points may indicate a rocky bottom or a bottom consisting of silt, sand, or mud. This information can be important for local traffic in terms of boating and anchoring and other issues. In order to collect samples from the bottom, a launch boat drops a diving probe that consists of a steel trap door that collects and holds a specimen in a canister that can be brought up to the boat. Once the sample is brought up to the boat, it is analyzed for rock size and texture along with other components such as shell material in order to assign a designation. This information is collected and catalogued so that the resulting nautical chart update will include all of the detailed information for all nautical hazards within the survey area.

109_0712 (2) — Bottom samples are taken with a heavy steel torpedo-shaped probe that is designed to sink quickly, dive into the ocean bottom, clamp shut, and return a sample to the boat. Credit Ensign Joseph Brinkley for the photo.

Personal Log:

Dear Mr. Cody,

The food on the cruise ship is great. They have all of our meals ready and waiting. There are many people who prepare and serve the food to us to make our trip enjoyable. (Dillion is one of my science students who went on an Alaska cruise with his family in May and will be corresponding with me about his experiences as I blog about my experiences on the Fairweather.)

Dear Dillion,

The food onboard the Fairweather is also very good. Much of the work that they do happens so early in the morning that most never see it take place. Our stewards take very good care of us by providing three meals a day, snacks, and grab bag lunches for all of our launches each day. They need to start early in morning in order to get all of the bagged lunches for the launches prepared for leaving later that morning and breakfast. They start preparing sandwiches and soup for the launches at 5 AM and need to have breakfast ready by 7 AM; so, mornings are very busy for them. A morning snack is often prepared shortly after breakfast for those on break followed by lunch and then an afternoon snack and finally dinner. That is a lot of preparation, tear down, and clean up, and it all starts over the next day. The steward department has a lot of experience in food preparation aiding them in meeting the daily demands of their careers while preparing delicious and nutritious food that the crew will enjoy.

106_0469 (2) — What are you doing at 5:15 in the morning? Mornings are very busy for the steward department preparing lunches for the day’s hydrographic launches and breakfast for the entire crew. From left to right, Chief Steward Frank Ford, Chief Cook Ace Burke, Second Cook Arlene Beahm, and Chief Cook Tyrone Baker.

106_0477 (2) — Chief Steward Frank Ford is preparing a delicious mid-morning snack for the crew.

Frank Ford is the chief steward. He has been in NOAA for six years. Before joining NOAA he had attended culinary school and worked in food service for 30 years in the restaurant and hotel industry. “I try to make meals that can remind everyone of a positive memory…comfort food,” Frank goes on to say, “Having good meals is part of having good morale on a ship.” Frank and the others in the steward department must be flexible in the menu depending on produce availability onboard and available food stores as the mission progresses.

106_0473 (2) — Chief Cook Tyrone Baker helps prepare breakfast.

Tyrone Baker is the chief cook onboard. He has been in NOAA for 10 years and has 20 years of food service experience in the Navy. Ace Burke has been with NOAA since 1991 and has served in many positions in deck and engineering and has been a steward for the last 15 years. He came over from the NOAA ship Thomas Jefferson to help the steward department as a chief cook. Arlene Beahm attended chefs school in New Orleans. She has been with NOAA for 1 ½ years and started out as a general vessel assistant onboard the Fairweather and is now a second cook.

Did You Know?

Relying on GPS to know where a point is in the survey area is not accurate enough. It can be off by as much as 1/10 of a meter. In order to increase the accuracy of where all the points charted on the new map, the Fairweather carries horizontal control base stations onboard. These base stations are set up on a fixed known location and are used to compare to the GPS coordinate points. Utilizing such stations improves the accuracy of all points with the survey from 1/10 of a meter of uncertainty to 1/100 of a meter or a centimeter.

Can You Guess What This Is?

A. an alidade B. a sextant C. an azimuth circle D. a telescope

The answer will be provided in the next post!

(The answer to the question in the last post was D. a CTD. A CTD or Conductivity, Temperature, and Depth sensor is needed for hydrographic surveys since the temperature and density of ocean water can alter how sound waves move through the water column. These properties must be accounted for when using acoustic technology to yield a very precise measurement of the ocean bottom. The sensor is able to record depth by measuring the increase of pressure, the deeper the CTD sensor goes, the higher the pressure. Using a combination of the Chen-Millero equation to relate pressure to depth and Snell’s Law to ray trace sound waves to the farthest extent of an acoustic swath, a vertical point below the water’s surface can be accurately measured. Density is determined by conductivity, the greater the conductivity of the water sample running through the CTD, the greater the concentration of dissolved salt yielding a higher density.)