ASTM D 7315 : 2017

Nephelometric non-ratio (NTU)

White light turbidimeters. Comply with U.S. EPA Method 180.1 for low level turbidity monitoring.

Detector centered at 90° relative to the incident light beam. Uses a white light spectral source.

0.0–40

0.0–40 Regulatory

Ratio White Light turbidimeters (NTRU)

Complies with ISWTR regulations and Standard Method 2130B. Can be used for both low and high level measurement.

Used a white light spectral source. Primary detector centered at 90°. Other detectors located at other angles. An instrument algorithm uses a combination of detector readings to generate the turbidity reading.

0–10 000

0–40 Regulatory
0–10 000 other

Nephelometric, near-IR turbidimeters, non-ratiometric (FNU)

Complies with ISO 7027. The wavelength is less susceptible to color interferences. Applicable for samples with color and good for low level monitoring.

Detector centered at 90° relative to the incident light beam. Uses a near-IR (780–900 nm) monochromatic light source.

0–1000

0–40 Regulatory (non-US)
0–1000 other

Nephelometric near-IR turbidimeters, ratio metric (FNRU)

Complies with ISO 7027. Applicable for samples with high levels of color and for monitoring to high turbidity levels.

Uses a near-IR monochromatic light source (780–900 nm). Primary detector centered at 90°. Other detectors located at other angles. An instrument algorithm uses a combination of detector readings to generate the turbidity reading.

0–10 000

0–40 Regulatory
0–10 000 other

Surface Scatter Turbidimeters (NTU)

Turbidity is determined through light scatter from or near the surface of a sample.

Detector centered at 90° relative to the incident light beam. Uses a white light spectral source.

10–10 000

10–10 000

Formazin Back Scatter (FBU)

Not applicable for regulatory purposes. Best applied to high turbidity samples. Backscatter is common with but not all only probe technology and is best applied in higher turbidity samples.

Uses a near-IR monochromatic light source in the 780–900 nm range. Detector geometry is between 90° and 180° relative to the incident light beam.

100–10 000+

100–10 000

Backscatter Unit (BU)

Not applicable for regulatory purposes. Best applied for samples with high level turbidity.

Uses a white light spectral source (400–680 nm range). Detector geometry is between 90° and 180° relative to the incident light beam.

10–10 000+

100–10 000+

Formazin attenuation unit (FAU)

May be applicable for some regulatory purposes. This is commonly applied with spectrophotometers. Best applied for samples with high level turbidity.

Detector is geometrically centered at 0° relative to incident beam (attenuation). Wavelength is 780–900 nm.

20–1000

20–1000 Regulatory

Light attenuation unit (AU)

Not applicable for some regulatory purposes. This is commonly applied with spectrophotometers.

Detector is geometrically centered at 0° relative to incident beam (attenuation). Wavelength is 400–680 nm.

20–1000

20–1000

Nephelometric Turbidity Multibeam Unit (NTMU)

Is applicable to EPA regulatory method GLI Method 2. Applicable to drinking water and wastewater monitoring applications.

Detectors are geometrically centered at 0° and 90°. An instrument algorithm uses a combination of detector readings, which may differ for turbidities varying magnitude.

0.02–4000

0–40 Regulatory
0–4000 other