Conductivity meter introduction and working principle

Conductivity is the ability of an object to conduct current. The measuring principle of the conductivity measuring instrument is to place two parallel plates into the solution to be measured, add a certain potential (usually a sine wave voltage) at both ends of the plate, and then measure the flow between the plates. Current. According to Ohm's law, the conductivity (G), the inverse of the resistance (R), is determined by the conductor itself. The basic unit of conductivity is Siemens (S), originally known as ohms. Because the geometry of the conductivity cell affects the conductivity value, the standard measurement is expressed in terms of unit conductivity S/cm to compensate for the difference in electrode size. The unit conductivity (C) is simply the product of the measured conductivity (G) and the cell constant (L/A). Here L is the length of the liquid column between two plates, and A is the area of ​​the plate .

The conductivity of water is related to the amount of inorganic acids, alkalis and salts it contains. When their concentrations are low, the conductivity increases as the concentration increases. Therefore, this index is often used to estimate the total concentration or salt content of ions in water. Different types of water have different conductivities. The conductivity of fresh distilled water is 0.2-2μS/cm, but after standing for a while, due to absorption of CO2, it increases to 2-4μS/cm; the conductivity of ultrapure water is less than 0.10/μS/cm; the conductivity of natural water is much In 50-500μS/cm, the mineralized water can reach 500-1000μS/cm; the conductivity of industrial wastewater containing acid, alkali and salt often exceeds 10000μS/cm; the conductivity of seawater is approximately 30000μS/cm.

The electrode constant is often determined using a standard potassium chloride solution of known conductivity. The conductivity of different concentrations of potassium chloride solution (25°C) is listed in the table below. The conductivity of a solution is related to its temperature, the polarization on the electrode, and the capacitance of the electrode. The instrument is generally compensated or eliminated. The water sample should be determined as soon as possible after collection. If it contains coarse suspended substances, oils and fats, interference measurements should be filtered or extracted.