1.1 This test method covers the static determination of turbidity in water. Static refers to a sample that is removed from its source and tested in an isolated instrument. (See Section 4.) 1.2 This test method is applicable to the measurement of turbidities greater than 1.0 turbidity unit (TU). The upper end of the measurement range was left undefined because different technologies described in this test method can cover very different ranges. The round robin study covered the range of 0–4000 turbidity units because instrument verification in this range can typically be covered by standards that can be consistently reproduced. 1.3 Many of the turbidity units and instrument designs covered in this test method are numerically equivalent in calibration when a common calibration standard is applied across those designs listed in Table 1. Measurement of a common calibration standard of a defined value will also produce equivalent results across these technologies. TABLE 1 Summary of Known Instrument Designs, Applications, Ranges, and Reporting Units Design and Reporting Unit Prominent Application Key Design Features Typical Instrument Range Suggested Application Ranges 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