NTU means Nephelometric Turbidity Unit. It reports a turbidity result from a nephelometric method that measures light scattered near 90° to the incident beam. NTU is not a particle concentration or a universal physical unit: the result depends on the light source, detector geometry, calibration and sample.
What does NTU measure?
NTU expresses the optical response of the sample relative to turbidity standards under the stated method. It can track changes in suspended and colloidal material, but it does not identify particles or directly report TSS in mg/L.
NTU, FNU, FAU, FTU and JTU
| Unit | Meaning | Key limitation |
|---|---|---|
| NTU | Nephelometric Turbidity Unit, commonly white/broad-spectrum source with approximately 90° detection | Instrument/method details still matter |
| FNU | Formazin Nephelometric Unit, commonly near-infrared source with approximately 90° detection | Real samples can differ from NTU results |
| FAU | Formazin Attenuation Unit, based on transmitted-light attenuation | Not the same detector geometry as nephelometry |
| FTU | Generic/historical Formazin Turbidity Unit | Does not identify optical design sufficiently |
| JTU | Historical Jackson Turbidity Unit using a visual candle method | Not comparable as a modern instrument method |
ISO 7027-1 notes numerical equivalence of the NTU and FNU formazin-based nephelometric scales. USGS nevertheless uses different reporting units for white/broad-spectrum and near-infrared instruments because they can respond differently to real environmental samples.
Direct answer: Can NTU and FNU be used interchangeably?
Not as a universal conversion for real water samples. If two methods must be related, compare them with representative paired samples over the required range and keep the instrument/method identity with the data.
ISO 7027-1 and EPA Method 180.1
EPA Method 180.1 specifies a tungsten source and a detector centered at 90°, with NTU reporting. The commonly used ISO-style nephelometric measurement uses near-infrared light around 860 nm; ISO 7027-1 also includes an attenuation method reported in FAU. Different wavelengths and optical designs can respond differently to color and particle properties.
Use the method required by the monitoring or regulatory program. A sensor that merely displays NTU is not automatically EPA Method 180.1 compliant, and an 860 nm LED alone does not establish full ISO conformity.
What does an NTU value mean?
A higher NTU generally means a stronger optical turbidity response for that instrument and method. It does not by itself state the mass of solids, identify pollution or prove that water is safe or unsafe. Interpret it against a defined treatment target, site baseline, product/process specification or applicable regulation.
Can NTU be converted to mg/L?
No universal factor exists. A site-specific relationship may be built from paired turbidity and laboratory TSS or suspended-sediment results. The relationship must be checked when the water matrix, particle source or instrument changes.