A wide variety of probes are available to measure turbidity -- the degree to which light is scattered by particles suspended in a liquid. The measured turbidity, however, depends on the wavelength of light and the angle at which the detector is positioned. For various reasons, turbidity probes do not all use the same light sources, angles of measurement to detect the scattered light, and signal processing strategies. As a result, measurements from different makes/models of turbidity probes may not be comparable to one other.
To help categorize the differences in turbidity probe design and distinguish among turbidity measurements that may not be comparable, new units of measurement were established in 2004 and assigned based on the probe design. The following table summarizes the units and the characteristics of the probes assigned to those units. The differences are based mainly on the wavelength of the light used to make the measurement, the angle at which the scattered light is detected, the number of detectors, and any special signal processing. For example, a Formazin Nephelometric Unit (FNU) is similar to a Nephelometric Turbidity Unit (NTU) in that both measure scattered light at 90 degrees from the incident light beam, but the FNU is measured with an infrared light source according to the ISO 7027 method whereas the NTU is measured with a white light according to EPA method 180.1. A great many submersible, multiparameter turbidimeters use an infrared light source.
|Units of Measurement for Turbidity
[nm = nanometers]
|Detector Geometry||Wavelength of Light Source|
|White or broadband: peak spectral output of 400-680 nm||Infrared, monochromatic: typical output in 780-900 nm range|
|Single Illumination Beam Light Source|
|90 degrees to incident beam; single detector||Nephelometric Turbidity Unit (NTU)a||Formazin Nephelometric Unit (FNU)b|
|90 degrees and other angles; multiple detectors; instrument algorithms use combination of detector readings and ratio techniques||Nephelometric Turbidity Ratio Unit (NTRU)||Formazin Nephelometric Ratio Unit (FNRU)|
|30±15 degrees to incident beam (backscatter)||Backscatter Unit (BU)||Formazin Backscatter Unit (FBU)|
|30±15 degrees and other angles; multiple detectors; instrument algorithms use combination of detector readings and ratio techniques||Backscatter Ratio Unit (BRU)||Formazin Backscatter Ratio Unit (FBRU)|
|180 degrees to incident beam (attenuation)||Attenuation Unit (AU)||Formazin Attenuation Unit (FAU)|
|Multiple Illumination Beam Light Source|
|90 degrees and possibly other angles; multiple detectors; instrument algorithms use combination of detector readings||Nephelometric Turbidity Multibeam Unit (NTMU)||Formazin Nephelometric Multibeam Unit (FNMU)|
|a NTU: limited to instruments
that comply with EPA Method 180.1.
b FNU: pertains to instruments that comply with ISO 7027, the European drinking-water protocol. This includes many of the most commonly used submersible turbidimeters.
Because suspended particles scatter light of different wavelengths with varying efficiency, turbidity data collected with infrared light sources are not directly comparable to turbidity data collected with white light sources. Always be careful when using turbidity data that have different units of measurement; data of different units are not necessarily comparable.
The U.S. Geological Survey began making distinctions in the various methods of measuring turbidity on October 1, 2004. For more information, please see Chapter 6.7 of the USGS National Field Manual, available online at https://water.usgs.gov/owq/FieldManual/Chapter6/6.7_contents.html. That resource includes helpful links to summary information as well as a link to a spreadsheet that lists many makes and models of turbidity probes along with their assigned parameter and method codes.