Conductivity: conductivity is the ability of a substance to transmit current, which is relative to the resistance value. Conductivity is the ability of conducting current in water solution. The conductivity of water solution is directly proportional to the concentration of dissolved solid quantity. Moreover, the higher the solid concentration, the greater the conductivity. The relationship between conductivity and dissolved solid concentration is approximately expressed as follows: 1.4 μ S/cm=1ppm or 2 μ S/cm=1ppm (CaCO3 per million units) measured in siemens/cm (s/cm), 10-6 of this unit is μ S/cm, ms/cm at 10-3. Among them, 1ppm is equal to 1mg/l, which is the unit of measurement of total solid solution. The conductance (L) is the reciprocal of the resistance (R). Under certain conditions (temperature, pressure, etc.), the resistance of conductor is not only determined by the nature of matter, but also depends on its cross-sectional area and length. The conductivity (L) of the conductor can be expressed as follows: Where: K-1/ ρ， It is called conductivity or specific conductivity;        Q-1 / A, known as electrode constant or conductivity cell constant. For electrolyte solution, conductivity refers to the conductivity of 1cm3 solution filled between two parallel electrodes with a distance of 1cm. It can be seen from the above formula that when the electrode constant (q) is known and the solution resistance (R) is measured, the conductivity can be obtained. The electrode constant is usually determined by the standard potassium chloride solution with known conductivity. The conductivity of potassium chloride solution with different concentrations (25 ℃) is shown in table 3-13. The conductivity of the solution is related to the temperature, polarization phenomenon on the electrode, the capacitance of the electrode distribution, etc. the compensation or elimination measures are generally adopted in the instrument. Table 3-13 conductivity of potassium chloride solution with different concentrations The conductivity of water body is mainly measured by conductance meter (hj/t97-2003), which is composed of conductivity pool system and measuring instrument. The conductivity cell is a container for holding or sending the solution under test. In the conductivity cell, the conductivity electrode and temperature sensing element are installed. The common flat electrode in laboratory, such as type 260 conductivity electrode, is to make two plates with an area of 5 × The smooth platinum plate of 10mm2 or platinum sheet coated with platinum black is fused to the ring glass, and the distance between poles is 6mm. The smooth platinum electrode is used to measure the solution with low conductivity, and the platinum electrode with platinum black plating is used to measure the solution with high conductivity. The electrode of industrial conductance meter is made of stainless steel or graphite in cylinder or ring; For the determination of conductivity of strongly corrosive media, non-contact electrodes can be used. According to the principle of measuring conductance, conductance meter can be divided into balanced bridge conductometer, resistance voltage divider conductivity meter, current measuring conductivity meter, electromagnetic induction conductivity meter, etc.