Measuring conductivity and salinity
Conductivity is a measure of the ability of water to conduct an electric current. In fish farms this is important in order to evaluate the ability of the water to inhibit pH fluctuations, i.e. the buffering capacity. In seawater, Na+ and Cl− ions dominate and here the instrument is used to measure the salinity.
The probe consists of two electrodes and is lowered into the water. A small electric potential (voltage) is applied across the electrodes. An electric current will occur between the electrodes, the size of which depends on the ion concentration in the water. To prevent the establishment of a layer on the electrodes which affects the current, it is necessary to use an alternating current as pre-voltage.
Conductivity is affected by temperature, so it is important to compensate for this parameter when taking measurements. Each instrument should have a special table setting out the effect of temperature on the conductivity. Advanced instruments incorporate automatic temperature compensation.