MJ-WQS-SC is a digital output water salinity sensor developed and manufactured by MJ Instruments. With sensor body filled internally which acts as sensor weights as well as protecting the electronic components, the sensor is well suited for long-term deployment. The body is built with corrosive resistant material, allowing it to be deployed at industrial discharge for pollution monitoring. Different housing is also available upon request. With standard RS485 Modbus output, it is easy to integrate the sensor into the commercial controllers.
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Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely. Conceptually the salinity is the quantity of dissolved salt content of the water. Salts are compounds like sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate which dissolve into ions. The concentration of dissolved chloride ions is sometimes referred to as chlorinity. Operationally, the dissolved matter is defined as that which can pass through a very fine filter (historically a filter with a pore size of 0.45 μm, but nowadays usually 0.2 μm). Salinity can be expressed in the form of a mass fraction, i.e. the mass of the dissolved material in a unit mass of solution.
Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical near coasts where rivers enter the ocean. Rivers and lakes can have a wide range of salinities, from less than 0.01 g/kg to a few g/kg, although there are many places where higher salinities are found. The Dead Sea has a salinity of more than 200 g/kg. Rainwater before touching the ground typically has a TDS of 20 mg/L or less.
Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few percentage (%). Physical oceanographers working in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five significant digits. A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.