A conductivity probe measures a solution’s ability to carry an electrical current. This measurement provides information about the total concentration of dissolved charged particles, or ions, within the liquid. The probe works by submerging a sensor into the solution and converting the measured electrical flow into a standardized value. This technique is widely used in fields like water quality monitoring, agriculture, and industrial process control.
Understanding Electrical Conductance in Liquids
Electrical conduction in a liquid solution, unlike in a metal wire, is carried by dissolved ions—atoms or molecules that possess a net positive or negative charge. When compounds like salts, acids, or bases dissolve in water, they separate into these charged particles, creating an electrolyte solution. The concentration and mobility of these ions directly determine the solution’s capacity to conduct electricity.
Conductivity is defined as the inverse of electrical resistance. A high concentration of mobile ions results in low resistance and high conductivity. The standard unit of measurement is the Siemens per meter (S/m), but the most common practical unit is the microsiemens per centimeter (\(\mu\)S/cm). This unit represents the conductance of a theoretical one-centimeter cube of the solution.
The Standard Two-Electrode Measurement System
The most common type of conductivity probe utilizes a standard two-electrode system. This design features two parallel electrodes, often made from durable, conductive materials like platinum, stainless steel, or graphite, encased in a protective housing. When the probe is submerged, an alternating current (AC) voltage is applied across these two electrodes, and the solution completes the electrical circuit between them.
The applied voltage causes positively charged ions to migrate toward the negative electrode, and negatively charged ions move toward the positive electrode. This coordinated movement constitutes the measurable electric current. The instrument measures the current passing through the solution, which is directly proportional to the solution’s conductivity, following Ohm’s Law. AC voltage is necessary to prevent polarization, a phenomenon where ions accumulate on the electrode surfaces and artificially alter the resistance reading.
A physical parameter known as the “cell constant” converts the raw electrical measurement into the standardized conductivity unit. The cell constant is a fixed ratio determined by the precise geometry of the probe, specifically the distance between the two electrodes and their surface area. This constant is unique to each probe and is multiplied by the measured conductance to yield the specific conductivity (\(\mu\)S/cm) of the solution, independent of the probe’s shape. Standard cell constants (e.g., 0.1, 1.0, or 10 cm\(^{-1}\)) are selected based on the expected conductivity range of the sample.
Why Temperature Correction is Essential
A solution’s electrical conductivity is highly sensitive to temperature changes, requiring all raw measurements to be corrected. As the temperature of a liquid increases, the ions gain kinetic energy, allowing them to move faster and more freely. This increased mobility results in lower resistance and a higher conductivity reading, even if the ion concentration has not changed.
To be meaningful and comparable, the measured conductivity must be adjusted to a single reference temperature, conventionally 25°C. Most modern probes include an integrated thermistor, a temperature-sensitive resistor, positioned near the electrodes, which continuously measures the solution’s temperature.
The probe’s internal electronics use a pre-programmed algorithm, often a linear or non-linear compensation formula, to apply a correction factor to the raw conductivity value. This calculation simulates what the conductivity would be if the solution were at 25°C, providing the standardized, final conductivity reading.
Alternative Probe Designs
While the two-electrode system is standard, alternative designs address challenges in specific applications, particularly those with high conductivity or fouling potential.
Four-Electrode Probes
Four-electrode probes improve accuracy, especially in solutions with higher ion concentrations, by employing a different measurement technique. These probes use an outer pair of electrodes to apply the AC current and an inner pair to measure the resulting voltage drop. Separating the current-carrying function from the voltage-sensing function minimizes errors caused by polarization or the accumulation of deposits on the electrode surfaces.
Inductive (Toroidal) Probes
The inductive, or toroidal, probe measures conductivity without any direct electrical contact between the sensor and the liquid. This probe consists of two wire-wound coils—a drive coil and a receive coil—encased in a chemically resistant plastic body. An AC voltage applied to the drive coil creates an alternating magnetic field that induces a current in the surrounding solution. The magnitude of this induced current, which is proportional to the solution’s conductivity, is measured by the receive coil. This non-contact method makes inductive probes ideal for measuring highly corrosive, dirty, or high-conductivity solutions where traditional electrodes would quickly foul or corrode.