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When using ultrapure water systems, we often see the indicator "resistance value." So what exactly is resistance value, and what is considered a normal resistance value for an ultrapure water system?
Resistance value is actually one of the units used to measure water purity, and it is the reciprocal of conductivity. Water contains cations and anions; ions are conductive media. By measuring the resistance value (conductivity) of water, we can indicate the amount of ions present in the water, thus indirectly reflecting the purity of the water. We usually use resistance value to reflect the purity of ultrapure water.
The international unit of resistance is the ohm, symbol Ω. Ultrapure water has a very high resistance value, commonly expressed in MΩ. 5μS/cm = 0.2MΩ·cm; 1μS/cm = 1MΩ·cm; 0.5μS/cm = 2MΩ·cm; 0.2μS/cm = 5MΩ·cm; 0.1μS/cm = 10MΩ·cm; 18.25MΩ·cm = 0.055μS/cm.
When measuring the conductivity of water, it is related to the water's resistance value. A high resistance value indicates poor conductivity, while a low resistance value indicates good conductivity. According to Ohm's Law, at a constant water temperature, the resistance value R of water is inversely proportional to the cross-sectional area F of the electrodes and directly proportional to the distance L between the electrodes.
The resistivity of water is related to the amount of salt in the water, the concentration of ions, the charge of the ions, and the velocity of ion movement. Therefore, pure water has a high resistivity, and ultrapure water has an even higher resistivity. The purer the water, the greater the resistivity. Therefore, the resistance value usually reflects the purity of ultrapure water. Ultrapure water has extremely low ion content, and its resistance value is typically >10 MΩ·cm. Theoretically, the resistance value of the purest water is 18.25 MΩ·cm.
