Sally - the sensor
Why collect salinity data?
Salinity plays a significant role in the marine ecosystems and the measurement of salinity in the ocean is crucial due to its fundamental role in understanding environmental changes and their impacts. However, there is currently a significant lack of salinity data in coastal areas.
How does Sally work?
Sally is a customer adapted easy to use salinity sensor designed to measure the salinity of liquid. It is specially adapted to higher conductivity values, and it is therefore possible to measure conductivity in the sea. Sally consists of a Raspberry pi, a conductivity probe, a breadboard, a signal converter board, an ADC module (Analog to digital converter), connecting wires, a micro usb cable and a waterproof box. In order to calibrate the sensor, you also need to have access to a calibration liquid, which are included when ordering the conductivity probe.
The working principle of Sally involves a conductivity probe to measure the electrical conductivity in the sea. As dissolved salts in the liquid affect its conductivity, the probe determines the salinity levels. The analog signal from the conductivity probe is then converted into a digital format using the ADC module. This conversion is facilitated by the signal converter board, which acts as an interface between the ADC module and the Raspberry Pi. The Raspberry Pi processes and analyzes the digital signal to provide salinity measurements. By connecting the components and using our software, Sally enables salinity monitoring for various applications in environmental research for coastal areas and antifouling products.
Read more about the components here:
The probe
the sensor
how it is deployed
how to collect data
You place Sally on the dock at your nearest marina. The sensor is a relatively small box from which there is a cord with the conductivity meter, hanging down just below the ocean surface. Sally is easy to build and you will be able to find all material links here. Below you have also been provided with an instruction video of how to build the sensor yourself. This enables everyone to be able to build it, but specifically, boatyards with a marina will have a huge opportunity to become citizen scientists.
To collect the data you need to connect your raspberry pi to a hotspot, it can either be a wifi or a mobile hotspot. When you run the Read.py program on your raspberry pi (see links below for the complete code) The data will automatically be collected, saved in a textile and be uploaded to our website. The data is now ready for analysis.
build it yourself!
calibration
To be able to use the sensor for the first time or when the sensor has been used over a longer period of time the sensor needs to be calibrated. Sally uses a single point calibration and you need a calibration solution of 12.88ms/cm (This will be included when ordering the probe from our link). Below is a tutorial of how to calibrate Sally.
Link to complete code here
Once you have assembled Sally and all parts are connected according to the instruction video above and you have uploaded the code from the link to your Raspberry Pi, you can proceed to these following steps.
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Input the command to get to the file containing the Read function.
2. Input python3 EC_Read.py to run the program.
When the program runs, you will be able to see these values in the terminal:
3. Insert the probe into the calibration solution, stir gently, until the values become stable.
4. After the values are stable, the single-point can be calibrated. Enter control+C so stop the program.
5. Input python3 EC_Calibration.py command to enter the calibration mode.
6. The program will now identify the value of the calibration solution.
7. The sensor is now calibrated, input control+C command to exit the calibration mode.
8. Run the EC_Read.py function again and make sure that the EC value is now set to the value of the calibration solution. (In this example 12.8 mS/cm)
9. After completing these steps, the calibration is completed, and then the sensor can be used for actual measurement.
HAPPY SENSING!
using conductivity to measure salinity
The electrical conductivity of water is influenced by the concentration of dissolved salts. A conductivity meter (the probe) is used to determine the electrical conductivity which represents the water's ability to conduct electric current. Salt ions in water, such as sodium, chloride, magnesium, etc enhance the electrical conductivity of water and other solutions. The higher the concentration of these salt ions, the greater the conductivity of the water. The salinity in seawater is due to these dissolved salts. These salts originate from different sources, as an example dissolved minerals from rivers that make their way to the ocean, dissolving of rocks, volcanic activity etc. As seawater evaporates the water content decreases, leaving behind dissolved salts which contribute to its salinity. The concentration of these salts are different in different regions depending on how close you are to rivers, how much rain falls, and evaporation. This also means that the conductivity varies depending on the region you are measuring.
factors to take into account
Measuring salinity through conductivity is a commonly used method but there are challenges and limitations connected to this which is important to mention and take into account regarding the accuracy. One of the limitations with measuring salinity through conductivity is that conductivity is highly dependent on temperature, as water temperatures fluctuate the conductivity values may not solely reflect changes in salinity. Temperature can affect conductivity readings without directly impacting salinity. Temperature can affect the ability of ions to move in an electric field and therefore affect the conductivity while the salinity levels remain relatively unchanged. It is therefore important to account for these variations and apply corrections to get accurate salinity measurements. A further development of the sensor is therefore to also add a temperature meter. If each measurement of conductivity is also compared to the current temperature that is measured at the same time, the accuracy of the measured conductivity will be higher. The measured value of temperature can also be used for better and deeper analysis of the data.
Another limitation that needs to be taken into account is the instrument itself. Environmental factors, as an example fouling on the probe itself, could affect the conductivity measurements and therefore the salinity levels. The probe will also need to be calibrated with regular intervals to function properly.
In conclusion, measuring conductivity is one of the better ways we have today to measure salinity but with this said, it is important to consider the potential issues and employ the procedures and quality control measures when using conductivity to measure salinity.
