NTU vs. FTU — What's the Difference?

NTU (Nephelometric Turbidity Units) and FTU (Formazin Turbidity Units) are both measures of water turbidity, which is a key indicator of water quality. Turbidity refers to the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in the air.

NTU is the unit of measure used in nephelometry, which is based on the amount of light that is scattered by the suspended particles in the water at a 90-degree angle to the light source. Nephelometric turbidity measurements are widely accepted and used for monitoring drinking water and other types of water quality assessments. The NTU scale is used by environmental agencies and water treatment facilities to ensure water clarity and safety.

FTU, referring to Formazin Turbidity Units, is based on a calibration standard created by mixing hydrazine sulfate with hexamethylenetetramine to produce a solution with a specific turbidity. While FTU is less commonly cited in modern standards, it historically served as a benchmark for measuring and comparing turbidity levels. FTU units were used to calibrate instruments that measure turbidity and to ensure consistency across measurements.

The transition from FTU to NTU in many contexts reflects advances in measurement technologies and standardization efforts to ensure consistency and accuracy in water quality testing. While both units serve to measure the same property—water turbidity—they originate from different calibration standards and measurement methodologies.

Modern turbidity sensors and analyzers are typically calibrated in NTU, following EPA and international standards for water testing. This standardization facilitates comparability of data and ensures that water treatment processes meet health and safety requirements.