The measurement principle of laboratory instrument conductivity meter is to place two parallel plates into the measured solution, apply a certain potential (usually a sine wave voltage) at both ends of the plates, and then measure the current flowing between the plates. According to Ohm's Law, the reciprocal of conductivity (g) and resistance (r) is determined by the conductor itself. The basic unit of conductivity is Siemens (s), formerly known as Ohm. Because the geometric shape of the conductivity cell affects the conductivity value, standard measurements are expressed in units of conductivity s/cm to compensate for differences caused by various electrode sizes. Unit conductivity (c) is simply the product of the measured conductivity (g) and the conductivity cell constant (l/a) Here, l is the length of the liquid column between the two plates, and a is the area of the plates.

The conductivity of water is related to the amount of inorganic acids, bases, and salts it contains. When their concentration is low, the conductivity increases with increasing concentration, therefore, this indicator is commonly used to infer the total concentration or salt content of ions in water. Different types of water have different electrical conductivities. The conductivity of fresh distilled water is 0.2-2 μ s/cm, but after being left for a period of time, it increases to 2-4 μ s/cm due to the absorption of CO2; The conductivity of ultrapure water is less than 0.10/μ s/cm; The conductivity of natural water is mostly between 50-500 μ s/cm, while mineralized water can reach 500-1000 μ s/cm; The conductivity of industrial wastewater containing acid, alkali, and salt often exceeds 10000 μ s/cm; The conductivity of seawater is about 30000 μ s/cm. The electrode constant is often measured using a standard potassium chloride solution with known conductivity. The conductivity (25 ℃) of potassium chloride solutions with different concentrations is listed in the table below. The conductivity of a solution is related to factors such as temperature, polarization phenomenon on the electrode, and electrode distribution capacitance. Compensation or elimination measures are generally adopted in instruments.

The conductivity of an aqueous solution is directly proportional to the concentration of dissolved solids, and the higher the concentration of solids, the greater the conductivity. The relationship between conductivity and dissolved solid concentration is approximately expressed as: 1.4 μ s/cm=1ppm or 2 μ s/cm=1ppm (per million units of CaCO3). The total hardness value of water can be indirectly obtained using a conductivity meter or a total solid solubility meter. As mentioned earlier, for the convenience of approximate conversion, 1 μ s/cm conductivity=0.5ppm hardness. Conductivity is the ability of a substance to transmit electrical current, relative to its resistance value, measured in siemens/cm (s/cm). The unit is expressed in μ s/cm for 10-6 and in ms/cm for 10-3. However, it should be noted that: (1) the hardness of water is indirectly measured by conductivity, with a theoretical error of about 20-30 ppm; (2) the conductivity of the solution determines the movement of molecules, and temperature affects the movement of molecules. In order to compare measurement results, the testing temperature is generally set at 20 ℃ or 25 ℃; (3) using reagents for detection can obtain relatively accurate hardness values of water.

After water sample collection, it should be measured as soon as possible. If it contains coarse suspended substances, oil and fat that interfere with the measurement, it should be filtered or extracted to remove them.

1) Soak the platinum black electrode in deionized water for a few minutes first.

2) Adjust the head screw m to make the pointer point at zero.

3) Turn the calibration and measurement switch k2 to the "calibration" position.

4) Turn on the power switch k to preheat for several minutes, and adjust the correction regulator rw3 to make the pointer on the full scale.

5) Turn the high and low cycle switches k3 to the appropriate positions.

6) Turn the range selection switch r1 to the appropriate position.

7) Adjust the electrode constant regulator rw2 to correspond with the constant of the electrode used (this is equivalent to adjusting the electrode constant to 1, and the measured conductivity of the solution is numerically equal to the conductivity of the solution).

8) After rinsing the electrode with a small amount of the test solution, insert its plug into the electrode socket kx and immerse it in the test solution.

9) After adjusting the calibration regulator rw3 to full scale, turn the calibration and measurement switch k2 to the measurement position. The conductivity of the solution is obtained by reading the reading of the gauge needle and multiplying it by the multiple indicated by the range selection switch r1. Repeat the measurement once and take the average value.

10) Turn the calibration and measurement switch k2 to the "calibration" position and remove the electrode.

11) Measurement completed, disconnect the power supply. After rinsing the electrode with deionized water, immerse it in deionized water for later use.