Electrical conductivity (EC) is the ability of water to conduct an electrical current, directly proportional to the concentration of dissolved ions. Making it one of the fastest and most cost-effective ways to assess water purity, ionic loading, and treatment performance. Conductivity measurement underpins quality control in boiler and cooling water systems, ultrapure water production, desalination operations, environmental monitoring, and food and beverage processing. Salinity and total dissolved solids (TDS) are derived from conductivity using established conversion factors, extending the utility of a single measurement across multiple regulatory and process contexts.
The Relationship Between Conductivity, Salinity, and TDS
These three parameters are fundamentally linked:
- Electrical conductivity (EC): Measured in microsiemens per centimetre (µS/cm) or millisiemens per centimetre (mS/cm). Pure deionised water has conductivity approaching 0.055 µS/cm at 25°C; seawater is approximately 53,000 µS/cm; Singapore tap water typically measures 50–150 µS/cm.
- Total dissolved solids (TDS): Estimated by multiplying conductivity by an empirical factor, typically 0.5–0.7 depending on ion composition. Expressed in mg/L (ppm). This conversion is an approximation. Gravimetric TDS determination (evaporation at 180°C per Standard Methods) gives the true value.
- Salinity: Calculated from conductivity and temperature using defined algorithms (Practical Salinity Scale, PSS-78, for marine water) or approximated using the TDS conversion. Expressed in parts per thousand (ppt) or practical salinity units (PSU).
Conductivity Measurement Principles
Two-electrode (contacting) cells place two electrodes in the liquid and apply an AC signal, converting measured resistance to conductivity via the cell constant (K, in cm⁻¹, ranging from 0.01 for ultrapure water to 50 for high-conductivity brines) — simple and cost-effective but susceptible to polarisation errors at high conductivity and electrode fouling. Four-electrode cells use separate current-injection and voltage-sensing pairs, eliminating polarisation since no current flows through the voltage electrodes, making them more accurate above ~5 mS/cm and standard in wastewater, seawater and brine applications. Inductive (toroidal) cells use two magnetically coupled toroids with no direct electrode contact, eliminating fouling and polarisation entirely — ideal for aggressive, high-particulate or fouling-prone streams (concentrated acids, bases, seawater, wastewater effluent), measuring 20 µS/cm to 2,000 mS/cm. Conductivity is highly temperature-dependent (roughly 1.5–2% change per °C), so all industrial meters apply temperature compensation referenced to 25°C, matched to sample chemistry: linear compensation (2%/°C) for most process waters, non-linear (natural water) compensation for environmental samples with unknown ion composition, and a pure water algorithm for ultrapure water where water's own non-linear behaviour dominates.
Industrial Applications in Singapore
Ultrapure water and semiconductor manufacturing needs UPW with resistivity typically above 18 MΩ·cm (conductivity <0.056 µS/cm), with online high-purity monitors throughout the distribution system detecting contamination, ion exchanger breakthrough or membrane failures critical to yield. Boiler water and steam systems set conductivity limits per the boiler manufacturer and ASME code, since high conductivity accelerates carry-over and corrosion — continuous monitoring triggers blowdown, and EMA boiler regulations require water quality monitoring records for registered pressure vessels. Cooling water systems track the concentration factor of dissolved minerals as water evaporates, with automatic blowdown valves discharging concentrated water once conductivity exceeds the limit to prevent scale and reduce Legionella risk (see our cooling tower monitoring guide). Wastewater and NEA compliance uses conductivity as a rapid indicator of load changes and unauthorised high-strength discharges, with NEA TDS limits (typically <1500 mg/L for sewer discharge) making TDS derived from conductivity a relevant compliance parameter trackable in real time. Desalination and water reclamation: PUB's NEWater and desalination programmes rely on conductivity and TDS as primary RO membrane performance indicators, with rising permeate conductivity signalling fouling, scaling or O-ring failure. Food, beverage and pharmaceutical facilities use conductivity to verify DI, RO and distillation purification systems are performing correctly, with Ph. Eur. Chapter 2.2.38 defining a conductivity-based in-line test for Purified Water and WFI that replaces more laborious ionic tests.
Selecting the Right Conductivity Instrument
| Application | Recommended Cell Type | Measurement Range |
|---|---|---|
| Ultrapure/semiconductor water | High-purity 2-electrode, low K | 0.05–200 µS/cm |
| Potable/process water | 2-electrode, K=0.1–1 | 1–5000 µS/cm |
| Cooling / boiler water | 4-electrode or 2-electrode, K=1 | 10 µS/cm–20 mS/cm |
| Wastewater / brine | 4-electrode or toroidal | 1–2000 mS/cm |
| Seawater / environmental | 4-electrode or toroidal with salinity algorithm | 1–80 mS/cm |
Calibration of Conductivity Meters
Conductivity meters are calibrated using NIST-traceable standard solutions at known conductivity values (e.g. 147 µS/cm, 1413 µS/cm, 12.88 mS/cm). Key calibration considerations:
- Allow standard solutions to equilibrate to room temperature before use
- Rinse the cell three times with the standard before filling for measurement
- Calibrate the cell constant (K) when a new cell is installed or after cleaning
- Verify calibration at the beginning of each shift for compliance monitoring
Formal traceable calibration with an ISO/IEC 17025 certificate is available from Unitest Instruments' SAC-SINGLAS accredited laboratory (accreditation LA-2023-0845-C). This calibration is accepted by NEA, PUB, HSA, and other Singapore regulatory agencies as proof of measurement traceability.
Maintenance of Conductivity Sensors
Contacting electrodes accumulate scale, biofilm, or chemical deposits that alter the cell constant and introduce measurement error. Maintenance practices include:
- Visual inspection: Check for deposits, pitting, or physical damage monthly
- Chemical cleaning: Soak in dilute acid (0.1 mol/L HCl) for scale; dilute detergent for oils/biofilm; rinse thoroughly with deionised water
- Cell constant verification: After cleaning, verify the cell constant against a traceable standard; replace the cell if constant has shifted by more than ±2%
- Toroidal cells: Wipe with a damp cloth to remove surface deposits; no disassembly required
Contact Unitest Instruments for genuine replacement conductivity cells, electrodes, and calibration standards for your installed Hach instruments. Our team responds to all enquiries within two business hours.
Environmental Monitoring and Salinity in Singapore Waters
Singapore's unique geography (surrounded by marine waters, with freshwater reservoirs, estuaries, and heavily used drainage channels), creates varied conductivity monitoring requirements. Environmental scientists monitoring Singapore's waterways must consider the salinity gradient from inland freshwater systems (conductivity typically <500 µS/cm) to estuarine mixing zones (1–10 mS/cm) and open coastal waters (50–60 mS/cm). Multiparameter sondes deployed in tidal drains must use the appropriate salinity algorithm to correctly interpret conductivity readings at varying tidal stages. See our water quality testing guide for broader context on environmental water monitoring in Singapore.