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Conductivity is an important parameter for weighing water quality. It represents the concentration of electrolytes in the water. Its value has a certain relationship with the amount of inorganic acid, alkali, and salt contained in it, and is directly proportional to the concentration of dissolved solids. Higher, the greater the conductivity. Different types of water have different electrical conductivity. By measuring the electrical conductivity, the water quality can be preliminarily determined, and then the appropriate water treatment method can be selected.
Conductivity refers to the ability of a solution to conduct current. The unit is Siemens per meter (S/m). TDS total dissolved solids, the unit is ppm. Under normal circumstances, the conductivity is about twice that of TDS.
The principle of conductivity measurement is to place two polar plates (or cylindrical electrodes) parallel to each other and a fixed value L in the measured solution, and add a certain potential to both ends of the polar plates (in order to avoid solution electrolysis, Usually it is a sine wave voltage, frequency 1~3 kHz), and then measure the current between the plates, and calculate the resistance value by Ohm's law, and then calculate the conductance G. Combined with the electrode constant K, calculate the conductivity S (S=K*G).
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The specific calculation process is as follows:
Ohm's law: U=I*R
I: current, the unit is ampere (A)
U: Voltage, the unit is volts (V)
R: Resistance, the unit is ohm (Ω)
The resistance R depends on the volume between the two metal plates:
R=ρ*K
ρ: resistivity, unit Ω*m or Ω*cm
K: cell constant, cm-1
Conductivity G=1/R
Conductivity S=1/ρ=K*G
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There are two common conductivity sensors: two-electrode type and four-electrode type.
Two-electrode conductivity sensor：
The conductivity cell of the two-electrode conductivity sensor consists of a pair of electrodes. A constant voltage is applied to the electrodes. The change in the resistance of the liquid in the conductivity cell causes the current of the measuring electrode to change, and it conforms to Ohm’s law. Instead of resistivity, conductivity is used. Use conductivity instead of resistance in metals, that is, use conductivity and conductance to express the conductivity of the liquid, so as to achieve the measurement of liquid conductivity.
At present, the measurement range of the two-electrode conductivity sensor is 0~20000μS/cm, and the electrode constant has different ranges. The common ones are: the electrode constant K=0.01, the measuring range is 0.02~20μS/cm; the electrode constant K=0.1 , The measuring range is 0.2-200μS/cm; the electrode constant K=1.0, the measuring range is 2.0-2000μS/cm. The cell constant K=10.0, and the measuring range is 20-20000μS/cm.
Four-electrode conductivity sensor：
The four-electrode conductivity cell is composed of 2 current electrodes and 2 voltage electrodes. The voltage electrode and the current electrode are coaxial. During measurement, the liquid to be measured passes through the gap between the two current electrodes. An AC signal is applied to both ends of the current electrode. Through the current, an electric field is established in the liquid medium, and the two voltage electrodes induce a voltage to keep the voltage across the two voltage electrodes constant. The current passing through the two current electrodes has a linear relationship with the conductivity of the liquid. At present, the largest measurement range of the common four-pole conductivity sensor can reach 500ms/cm.