It is generally accepted that resistivity is the opposite of conductivity. Since the conductivity and resistivity of water are special, we feel it is important to clarify what conductivity is in order to properly define the subject.
Conductivity represents the capacity of water to conduct an electric current. It is closely related to the total dissolved solids concentration in the water.
Although there are many different ways to express it, it is generally expressed in microsiemens per centimetre (µs/cm).
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Let's recall that the resistivity of a material is its characteristic to resist an electric current. As we discussed earlier, electricity is conducted by the dissolved solids in water, i.e., the ions (cations/anions). This means that the higher the resistance of a water sample, the purer it is since pure water hardly conducts electricity.
Unlike conductivity, which is expressed in microsiemens, resistivity is usually expressed in ohms. In fact, resistivity is the resistance (R) of a material multiplied by its cross-sectional area (A) and divided by its length (l). The equation used to express the resistivity is as follows:
- ρ = RA/l
The interchangeability of conductivity and resistivity
When we talk about interchangeability, we mean that both concepts can be used to define the ability or not to conduct electricity. For example, ultra-pure water, at 25°C, has these characteristics.
- Resistivity: ~ 18.2 MΩ × cm.
- Conductivity: ~0.055 µs/cm
These two measurements represent the same conductivity/resistivity capacity of water.
In short, although resistivity can be used to express the ability of water to conduct an electric current, conductivity and microsiemens are generally used.