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Fresh and Seawater Density

What is this sensor?

CTD is an acronym for Conductivity, Temperature and Depth. However, this is somewhat misleading as this device actually measures pressure to calculate the depth. Furthermore, oceanographers use the conductivity measurement, in conjunction with temperature and pressure, to calculate the salinity of the ocean. Both temperature and salinity are used by oceanographers as the optimal variables for understanding ocean processes.

This device is one of the most common instruments used by oceanographers, as differences in salinity, temperature and depth are the primary parameters distinguishing different water masses. CTDs can be used to profile a body of water in several different ways. For example, a CTD can be sent through a water column (surface to bottom, or bottom to surface) to create a vertical profile of the water column. As the CTD descends (or ascends) it records how the water changes, allowing researchers to see layers in the water column. CTDs can also be fixed at a particular location and depth, allowing researchers to monitor water properties over time. For example, a CTD fixed at 100 metres might record a change in salinity; this would indicate that the water around the CTD has changed, perhaps due to tides, upwelling, or other means. Finally, a CTD may be pulled through the water to identify horizontal variations.

How does a CTD work?

Each component of the CTD takes different measurements, and these measurements are often processed through computer software to create specific parameters. For example, the CTD will take a measure of conductivity, temperature and pressure, and process these into a measure of salinity. All variables are available to researchers, should they want to work with raw data or use the measurements for other purposes. CTDs can be cast from the side of a ship to create a vertical profile (variable vs. depth) or fixed at a specific depth to create a horizontal profile (variable vs. time), depending on which measurements the researcher wants to collect.

Each component functions differently, and is listed below:


Conductivity is a measure of the water’s ability to conduct electrical current. Pure, distilled water is a very poor conductor and thus, will have a low conductivity. When natural minerals, such as salts dissolve in water, some of the ions conduct electricity. Conductivity is used to determine how much inorganic material is dissolved in the water and is measured through the use of electrodes. The water sample being measured passes between two plates and a current is passed between these plates. Dissolved inorganic ions, such as salts and other minerals, will conduct the electrical current from one side of the probe to the other. Depending on the resistance of the water being sampled, researchers can determine the quantity of dissolved salts present in the water. Through calculations, salinity measurements can be derived from conductivity, temperature, and pressure. 


Temperature is measured with a special temperature-sensing device called a thermistor or resistance temperature detector. Unlike a standard classroom thermometer that gives a reading through the thermal compression or expansion of a liquid, thermistors measure temperature by detecting changes to the electrical resistance of a metal. Different metals will resist an electric current differently at different temperatures, so the change in voltage measured will reflect the temperature. A current is passed through a metal wire, and the resistance is measured. Using conversion coefficients, this change can be used to determine the temperature of the water with an extremely high degree of accuracy.  


Pressure is measured by a pressure gauge.  Pressure gauges operate on the principle that a small coil of wire or tube of fluid will compress or change shape depending on the external pressure exerted on that gauge. In the ocean, pressure and depth are directly related, so the amount of pressure being exerted on the gauge can be used to determine the depth of the reading. The pressure sensors in stationary CTDs are so sensitive that they can accurately measure the tidal cycles—the alternating high and low tides every 24 hours—or even waves alternating crests and troughs on the time scale of seconds.


Density, like salinity, is not a direct measurement but is calculated based on other parameters derived by the CTD measurements. The density anomaly (denoted Sigma T) of sea water is calculated based on the salinity (calculated from conductivity) and temperature of the water, as these are the driving factors of density. As temperature decreases and salinity increases, the density of sea water will increase.  Density is an important variable in oceanography as the density of the water can be an important factor in water’s ability to uptake or hold other nutrients, such as oxygen. So that we concentrate on the most significant digits, we often subtract 1000 from the density in kilograms per cubic metre, and denote this as Sigma (σ = density - 1000).

A simple relationship to help you remember is: As salinity ↑ and temperature  ↓  =  ↑ in density

How will the data appear?
Which units are used?

  • Conductivity:  siemens per metre (S/m)
  • Salinity: Practical Salinity Unit (PSU)
  • Temperature:  Celsius (°C)
  • Pressure: decibars (decibars)
  • Depth: metres (m)
  • Density: kilograms per cubic metre (kg/m3)


Conductivity is measured in siemens per metre (S/m), which is the conductance of a substance. Conductance is an object’s ability to pass or conduct an electrical current. As an electrical current is passed through a water sample, its conductance is measured in S/m. In general, the higher the conductance, the more dissolved salts there are in the given sample and therefore, the higher the salinity.


Salinity is the amount of dissolved salts in water. Salinity is usually determined as a ratio of the measured conductivity to the conductivity of a specific known concentration of dissolved ions. Salinity was formally expressed as ppt (parts per thousand, i.e. grams of dissolved salts per kilogram of seawater), but PSU (Practical Salinity Units) is considered more accurate as it considers more variables. PSU and ppt are nearly the same, and can be considered a reasonable approximation for one another.  


Temperature is measured in degrees Celsius (°C), and can now be determined accurately to 3 decimal places. Temperature readings will be heavily dependent on where the CTD measures in the ocean. As depth increases, temperatures become colder. At higher latitudes, seawater is colder, and at lower latitudes seawater is warmer. Fixed CTDs detect changes in temperature that indicate changes in water masses.   


Pressure is measured in decibars, which are very nearly equivalent to metres of depth. For example, 550 decibars can be reasonably approximated as 550 metres of depth.


Density is expressed as kilograms per cubic metre (kg/m3). Density typically increases as temperature decreases and salinity increases. Warm, fresh water will have a lower density than cold, salty water. When a CTD is in a fixed position, fluctuations in density usually reflect a change in the water mass. For example, an influx of low density water may indicate that a large volume of fresh water has been added to the water (i.e. runoff from a large river) near a shallow CTD.   ​

What is the normal range for these data?
What variables influence it?

Quick general reference:
Conductivity 5 S/m
Salinity ~32–37 PSU
Temperature Location and depth dependent
Pressure Depth dependent & fluctuates with the tidal cycle
Density 1020–1035 kg/m3

Detailed explanation of Data Ranges

Variables and trends affecting CTD data and their general significance are discussed below. Each variable on its own may have a limited effect on the ocean, but shown in tandem with other data they can help researchers get a "big picture" of oceanic conditions. 


Conductivity can change depending on the amount of dissolved solids in a sample of water, but there is also a very small temperature and pressure dependence. Generally, conductivity of one water mass will be consistent. Changes in conductivity could indicate in a change in water mass or an influx of salts or other minerals, possibly caused by seasonal changes, pollution or other activity.

Variables affecting conductivity:  Temperature (conductivity is always corrected to 25°C), pressure (increased pressure increases very slightly the conductivity), and dissolved inorganic compounds (salts).

In the data, you may notice seasonal changes in salinity as freshwater is added or removed from the surrounding seawater. In shallow locations, you may notice changes in salinity that can be linked to atmospheric changes such as solar radiation, rain, or evaporation. This conclusion can be supported with data from the meteorological station.      


Salinity is a measure of the total salts dissolved in water. Animals and plants are adapted for a certain range of salinity, and the increase or decrease of salinity can negatively affect them. Different environments will have “normal” levels of salinity, and seasonal changes are not uncommon. For example, areas near streams and rivers may experience a change in salinity as spring run-off increases the amount of freshwater entering the water. In polar latitudes, freezing ice pushes the salt out of the ice, increasing the salinity of the surrounding water; of course the reverse is true as icebergs melt, a large volume of freshwater is then added and salinity decreases.

Variables affecting salinity: Temperature, pressure, evaporation, freezing, freshwater influx.

In the data, you may notice seasonal changes in salinity as freshwater is added or removed from the surrounding seawater. In shallow locations, you may notice changes in salinity that can be linked to atmospheric changes such as solar radiation, rain, or evaporation. This conclusion can be supported with data from the meteorological station.    

Extended explanation

Conductivity and Salinity

Different plants and animals have adapted different tolerances to different salinity levels, and can be negatively affected by salinity changes. Salinity can change depending on the rainfall, mixing, or evaporation of water in the area. For example, as water evaporates due to sun or wind, this can cause salinity levels to increase. Equally, in polar latitudes, where low temperatures turn freshwater to ice, this can cause salinity levels to increase in the surrounding waters. Influxes of fresh water, such as rain, ice melt, or streams and rivers can cause salinity levels to decrease.


Temperature is very dependent on depth and location (i.e. latitude). In shallow locations, water temperature will be affected by atmospheric conditions. In some deep locations, the temperature may be affected by local geological processes such as volcanoes, but it will generally be very cold and near freezing. Latitude will also affect temperature as tropical latitudes will have considerably warmer temperatures than polar ones.

Variables affecting temperature:  Location, depth, and atmospheric interactions (e.g., solar radiation)

In the data, you may notice an inverse relationship between air and water temperature. You may also notice variability in temperature that can be linked to solar radiation or air temperature.

Extended explanation

Temperature can be largely affected by atmospheric conditions, location, and depth. For example, solar radiation and air temperature can directly affect the temperature of the water in shallow conditions and can be related to seasonal atmospheric conditions. Since cold water is denser than warm water, the colder it is the deeper it sinks. So the deeper one goes in the ocean, typically the colder it gets. The coldest seawater is formed at the surface of the ocean in the highest latitudes of the Arctic and Antarctic oceans. Seawater is very slightly compressible, and at the greatest depths of the ocean (i.e. >3000m), there is a small increase in the effective temperature due to compression. This is only a very small correction, but must be taken into account when comparing deep water masses.     

Water has a higher heat capacity than air and as such, takes much longer to heat up or cool down. In coastal environments, the same atmospheric heating will cause the land to warm many degrees, while the ocean only warms a few. Similarly, in the winter, the land will cool off quickly, while the ocean takes a long time to cool. For example, it is not unusual for open water temperatures to be cooler than air temperatures in the summer and warmer than air temperatures in the winter. This phenomenon explains why coastal areas tend to have milder winters and cooler summers (maritime climate), whereas inland locations (such as the Canadian Prairies) have a much larger variability between summer (hot) and winter (cold and dry).  Sea water tends to absorb energy slowly over the summer and release this energy over the winter.  This in turn can affect the weather conditions in this area. 


Pressure changes with the amount and density (total weight) of water over the sensor. Daily changes in pressure are dominated by sea-level changes due to the tides.

In the data you may notice the daily tide changes as a steady increase and decrease in pressure. On the West Coast, we have a semi-diurnal tide resulting in 2 high tides of different height and 2 low tides of different height each day. The pressure data will reflect a high-high tide, a low-low tide, a low-high tide, and high-low tide.  


Density is dependent on temperature, salinity, and pressure. Colder, saltier water will have a higher density than warm or fresh water. Seawater is also very slightly compressible, so water becomes slightly higher in density as it sinks very deep in the ocean (>3000m).

Variables affecting density: Temperature, salinity, and pressure.

In the coastal ocean data, you may notice changes in density caused by increases in fresh water. These data may be collaborated with data from the meteorological station or ice profiler. 

Extended explanation

Changes in coastal (shallow) seawater density will be reflected in changes in temperature and salinity.  In shallow locations, you may notice that the density is much more variable than in deep locations.  This is due to the large variations in the salinity and temperature of the upper ocean.

Depth will also play a role in density, providing the CTD is not at a fixed location. When the CTD is fixed in place, the density is a result of the relationship between temperature and salinity.

Ideas for classroom explorations

This section is intended to inspire you and your students to explore different ways of accessing, recording, and interpreting data. These suggestions can be used ‘as is’, or can be freely modified to suit your needs. They can also be used to generate discussion and ideas, or as potential staring points for projects.

  • Record the temperature and salinity on a daily basis. Watch for seasonal trends to appear. Inquire, are atmospheric changes reflected in the water data at the same time, or is there a delay?
  • Compare CTD readings after specific atmospheric events. For example, do the data change during or after a heavy rain?
  • Compare local CTD readings to those in other locations. Will one predict or precede the trend of another?

Ideas for projects

This section contains suggestions for long-term projects you and your students may be interested in investigating using the data. These projects may require support from multiple data sources, experts in the field, or additional experimentation.

  • Compare the data and inquire about relationship among other seasonal trends. For example, during fish migrations is there a trend in the data? In successful years, are there differences observed from unsuccessful years?  
  • Compare data from several years. Can a seasonal or annual trend be determined?  Can this trend be attributed to something using additional data, anecdotal evidence, or both?
  • Compare multiple stations using the same variables. 

Common misconceptions or difficult concept elements

This section is intended to help you anticipate where students may struggle with difficult concept elements or ideas. We’ve noted content that may require additional support for students to fully understand, or content that may lead to misconceptions.  

  • Multiple factors can be at play. For example, salinity may be affected by atmospheric evaporations and freshwater influxes at the same time.
  • Although conductivity is used to determine salinity and pressure is used to determine depth, they are not exclusive to those measurements. For example, a researcher may want to know conductivity and salinity to compare them and reach different conclusions.
  • The relationships among these variables may be impacted by other factors as well; they are not exclusive to each other.
  • CTD equipment often contains other sensors as well, so that these data can be collaborated and compared with CTD data. For example, fluorometers and dissolved oxygen sensors are often included on the CTD apparatus.
  • CTD data will be different depending on how they are collected. For example, stationary CTDs will sample water over time, while CTDs that move up and down in the water column will sample water in a vertical fashion as they travel.