Conductivity Analyzer

Overview

A conductivity analyzer measures electrical conductivity of dissolved salts, acids, or bases by applying an AC voltage across a Electrode Pair and measuring the resulting current. Conductivity is the inverse of resistivity and is proportional to dissolved ion concentration, so it serves as a fast proxy for total dissolved solids (TDS). Conductivity analyzers are standard in water treatment, boiler makeup control, cooling tower chemistry, and desalination, anywhere a continuous measurement of salinity or ionic strength is needed without chemical analysis.

The system comprises a Excitation Signal Generator driving the electrode pair at 1–10 kHz (the frequency chosen to minimize electrode polarization and cell charging artifacts), a Measurement Amplifier detecting the resulting AC current via synchronous demodulation, a Temperature Compensation Module compensating for temperature-dependent conductivity drift, and a Transmitter Electronics linearizing and outputting 4–20 mA.

How it works

Two electrodes separated by a fixed distance form a conductance cell. When AC voltage is applied, ions in the solution carry current from anode to cathode at a rate proportional to ion mobility and concentration. The current I is given by Ohm's law: I = V × G = V × (σ × A / L), where σ is conductivity (S/cm), A is electrode area, and L is distance between them. The ratio K = L/A is the cell constant; the transmitter must know K to calculate conductivity from measured conductance.

The Measurement Amplifier uses synchronous demodulation (lock-in detection) to suppress noise: it detects AC current only at the excitation frequency, filtering out noise at other frequencies. The Phase Detector multiplies the cell current by the excitation signal (phase-locked), then low-pass filters to extract DC magnitude proportional to conductance. High excitation frequency (e.g., 10 kHz) minimizes electrode double-layer capacitance effects that would otherwise distort measurement at low frequencies; DC would cause electroplating and cell damage.

Conductivity is temperature-dependent: pure water becomes more conductive as temperature rises because ion mobility increases (~2%/°C for electrolytes). The Temperature Compensation Module measures sample temperature via an RTD Sensor and applies a temperature coefficient correction. Many analyzers normalize to a standard reference temperature (e.g., 20 or 25 °C) so that readings are comparable across time and location.

The Transmitter Electronics stores a cell constant K (measured during factory calibration or field calibration using a conductivity standard) and a temperature coefficient. From the lock-in magnitude and temperature, it calculates conductivity and outputs 4–20 mA. Two-point calibration is supported: zero (pure water or low-conductivity buffer) and span (known standard, e.g., 10 mS/cm). Graphite electrodes are inexpensive but wear over years; coated titanium electrodes last longer but cost more. The Flow Cell Union allows quick sensor replacement without breaking threaded connections.