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Conductivity Sensors / Resistivity Sensors Frequently Asked Questions (FAQ)
What is conductivity
Electrical conductivity is the ability of a material to carry an electrical current.
What is resistivity?
Electrical resistivity is the reciprocal of conductivity. Resistivity is the intrinsic property that quantifies how strongly a given material opposes the flow of electric current.
Why do we measure conductivity?
Electrical conductivity has been measured for many years and it is still an important and widely used analytical parameter today. It is an easy, simple and economical way to provide an indication of the purity of the measured medium, typically, water (the higher the conductivity reading, the higher is the concentration of dissolved ions in the water). The excellent reliability, sensitivity, response and relatively low cost of the equipment make conductivity a valuable, easy-to-use tool for quality control. In some applications the purity measurement is made as resistivity (the reciprocal of conductivity).
What does a conductivity sensor measure?
A conductivity sensor measures the ability of a solution to conduct an electrical current. It is the presence of ions in a solution that allow the solution to be conductive: the greater the concentration of ions, the greater the conductivity. You can find more information about METTLER TOLEDO conductivity sensors:
What is the measuring principle of a conductivity sensor?
A conductivity probe consists of an electrode pair, to which a voltage is applied. The conductivity sensor measures the flowing current and calculates the conductivity.
What unit is conductivity measured in?
Conductivity is measured in siemens per cm (S/cm). A conductivity of 1 S/cm is actually quite high, so most conductivity measurements involve solutions where conductivity is measured in mS/cm (thousandths of a S/cm) or in μS/cm (millionths of a S/cm).
How many types of conductivity sensors are there?
There are three technology types of a conductivity probe used for process conductivity measurement:
How does a 2-electrode conductivity meter work?
The classical 2-electrode conductivity meter consists of two parallel plates. An alternating current voltage is applied across the two electrodes, and the resistance between them is measured. The 2-electrode conductivity meter is used for water conditioning and purification stages where it is capable of detecting minimal levels of impurities in ultrapure water.
How does a 4-electrode conductivity meter work?
The 4-electrode conductivity meter works with an additional electrode pair. The outer electrodes are the current electrodes to which alternating current is applied; they are driven in the same manner as the 2-electrode conductivity meter. The inner measuring electrodes are placed in the electric field of the current electrodes and measure the voltage with a high impedance amplifier. The current flowing through the outer electrodes and the solution can be accurately measured by the circuit. If the voltage across the inner electrodes and the current are known, the resistance and conductance can be calculated. The advantage of the 4-electrode conductivity sensor lies in the fact that there is negligible current flowing through the inner electrodes where the measurement is made. Therefore, no polarization effects occur which would otherwise influence the measurement. The 4-electrode conductivity sensor is also less sensitive to measuring errors through electrode fouling. The 4-electrode sensors are for middle to high ranges.
How does an inductive conductivity probe work?
The METTLER TOLEDO inductive conductivity probe is constructed like a pair of transformer coils where the solution to be measured is the core of the transformer. The parallel coils are closely spaced and embedded within a polymer body like a donut which is immersed into the solution. There are no electrodes and usually no metal contacting the solution. One coil is energized with alternating current and the signal induced into the second coil is related to the conductivity of the solution flowing through and around the sensor. The cell constant is determined by the diameter of the hole, among other factors. The inductive conductivity sensor covers mid to very high conductivity ranges, and is particularly resistant to fouling. Because it is non–contact, it is particularly suitable to be used in chemical (corrosive) applications where metal electrodes could be damaged by the media.
What does cell constant mean?
The cell constant is the ratio of the distance between the electrodes to the area of the electrodes in 2- and 4-electrode conductivity sensors. The smaller the cell constant, the more precise the sensor will be in determining changes of the conductivity in media. However, a small cell constant reduces a sensor's measuring range. Accurate conductivity measurement requires an accurate measurement of the cell constant, which is determined by calibration. For METTLER TOLEDO sensors, the cell constant is precisely measured and is documented on each sensor's Quality Certificate. Calibration solutions can be traced back to the National Institute of Standards and Technology (NIST).
How do you calibrate a conductivity sensor?
A METTLER TOLEDO conductivity sensor can be calibrated against a solution of known conductivity (much like calibrating a pH sensor against a solution of a known pH). Alternatively, a device can be used that contains a range of very precise resistors that duplicate known conductivity measurements.
When do you need to perform a conductivity sensor calibration or verification?
In general, the cell constant of the sensor will not change; however, if the sensing elements are altered in some way (e.g. solid deposition or other fouling of the electrodes or insulator of the sensor, loss of electrode material through corrosion) the cell constant will change. METTLER TOLEDO conductivity sensors are factory calibrated and the cell constant is precisely determined. Therefore, calibration is typically not required. However, it is recommended to verify the sensor or make calibration adjustment if necessary on an annual basis. The frequency of verification or calibration is very much dependent on the applications or on the plant standard operating procedure requirements.
Does temperature affect the conductivity measurement?
Conductivity is strongly temperature dependent. As the temperature of a sample increases, the viscosity of the sample decreases which leads to increased mobility of the ions. Therefore, the observed conductivity of the sample also increases even though the ion concentrations may remain constant.
In good practices, every conductivity result must be specified with a temperature or be temperature compensated, usually to the industry standard of 25 degrees Celsius. Because temperature is also dependent to various samples, the appropriate temperature compensation algorithms must be chosen carefully.
2-electrode sensors specially for low conductivity and 4-electrode sensors for medium to high conductivity.
Easy-to-use sensors for measuring under the harshest conditions.
Thornton offers a wide variety of conductivity / resistivity sensors to accommodate pure water applications.
Wide range of applications
from biopharmaceutical to chemical processes
at very low to medium conductivities
Compliance through certification package
due to robust design
Long lifetime through
excellent chemical resistance
Reliability thanks to
individual testing of cell constant to ensure highest possible measuring accuracy
Highest accuracy available
for pure water measurement
Sensor diagnostics and Plug and Measure for models
with Intelligent Sensor Management (ISM)
Wide choice of fittings and certified materials
to meet process requirements