Industrial water testing provides evidence for process control, equipment protection, microbial risk management and discharge compliance. Incoming supplies, process water, boiler feedwater, boiler water, condensate, open evaporative cooling systems, closed circuits and effluent have different objectives and specifications. Appropriate measurements may include hardness, alkalinity, pH, conductivity, silica, dissolved oxygen, treatment residuals, microbial parameters and identified contaminants. Sodium base-exchange softening does not remove dissolved solids overall, so conductivity alone cannot demonstrate hardness-removal performance. Boiler operating conditions and sampling hazards require competent assessment and appropriate equipment. HSE advises routine Legionella testing at least quarterly for open systems such as cooling towers, with more frequent sampling where circumstances require it. Chemical and microbial controls are not interchangeable. Closed circuits need tests appropriate to their materials, inhibitors and formulation. Effluent monitoring follows the applicable permit or consent, and Environment Agency guidance cited here applies to England. Surface-water and groundwater discharges differ from trade-effluent discharges to sewer. Oxygen-demand parameters and dissolved or total metals are not interchangeable. Representative sampling needs defined locations, appropriate spot or composite methods, recorded process conditions and method-specific preservation and transport. Field instruments and continuous monitors need calibration, verification and failure arrangements. Laboratory accreditation covers specified tests and matrices, with MCERTS requirements relevant to certain regulated effluent programmes. Interpretation requires correct units, methods, reporting limits, uncertainty and the correct specification. Trends and documented action plans connect findings to proportionate decisions. Reviewed 7 October 2026 using Environment Agency, HSE, UKAS and Spirax Sarco guidance.