ASTM D7726-11(2016)e1

5.1.1 Turbidity is also used in environmental monitoring to assess the health and stability of water-based ecosystems such as in lakes, rivers and streams. In general, the lower the turbidity, the healthier the ecosystem.

5.2.1 Turbidity measurements are based on the instruments’ calibration with primary standard reference materials. These reference standards are traceable to formazin concentrate (normally at a value of 4000 TU). The reference concentrate is linearly diluted to provide calibration standard values. Alternative standard reference materials, such as SDVB co-polymer or stabilized formazin, are manufactured to match the formazin polymer dilutions and provide highly consistent and stable values for which to calibrate turbidity sensors.

5.2.2 The traceability to calibrations from different technologies (and other calibration standards) to primary formazin standards provides for a basis for defined turbidity units. This provides equivalence in the magnitude of the turbidity unit between the different measurement technologies when they are all calibrated on standards that are traced to primary formazin. This means that a TU is equivalent in its magnitude to a nephelometric turbidity unit (NTU), and all other units as described in this guide. See Table 1.

5.2.3 Turbidity is not an inherent property of the sample, such as temperature, but in part is dependent on the technology used to derive the value. Even though the magnitude of turbidity units are equivalent and are based on turbidity standards, the units do not maintain this equivalence when measurement of samples is practiced. Turbidity standards are generally free of interferences and samples are not. Depending on the type of technology employed for measurement, the magnitude of the different interferences on a given sample can differ significantly with respect to the different measurement technologies. The user of a turbidity technology should expect to observe a lack of measurement equivalence across different turbidity measurement designs when common samples are analyzed. See Section 6 on interferences.

5.2.4 Depending on the application, some instruments are calibrated on a sample that has been characterized (or defined) by some independent means. The calibration may include one or more samples that have been characterized with respect to the application of its use. See Test Methods D3977.

5.3.1 The actual reporting units, signified by a two to four-letter code, are based upon distinguishing design criteria for each of the common measurement technologies. The intent of attaching the measurement unit to the determined turbidity value is to indicate the type of technology used.

5.3.2 Even though various instrument designs may be grouped by technology type (that is, FNU, NTU, FBU, etc., and refer to Table 1), instruments within a group should not be considered to be identical nor it is proposed that sample values obtained will be alike. Instruments within each technology may still have other design differences whereby samples give different results. For example, pathlength differences between two instruments with the same reporting units can impact measurements and the relative difference in results.

5.4.1 Table 1 provides a summary of technologies and their respective reporting units that are in the different ASTM test methods. The reporting unit is a two to four letter-code that has been assigned to a unique type of technology. The reporting unit follows every reported turbidity measurement and serves as metadata to the respective measurement.

5.4.2 The key design features are based on three criteria: (1) type of light source used, (5.4.2.1 If the measurement unit begins with an “F” then the light source is a near-IR wavelength. Most designs will encompass a light source that is in the 860 ± 60 nm range. The strength of this wavelength is that most natural colors do not absorb at this level, which reduces or eliminates color interference. Two things that interfere at the near-IR are carbon black and copper sulfate. Second, the incident light beams are easily collimated, which extends the overall operational range. Third, the output of the light source can be regulated to provide a stable output over time. The weakness is that longer wavelengths are less sensitive to smaller particles with respect to response at very low turbidities.

5.4.2.2 If the measurement unit either begins with an “N” or is a two-letter unit (for example, BU, AU), the incident light source will be in the 400–680-nm range. The strength of this wavelength range is increased sensitivity to smaller particles when compared to longer wavelengths (such as those in the near infared (IR) range). The weakness of this wavelength range is that color that absorbs at the same wavelengths, as those that are emitted by light source will cause a negative interference. Second, if the source is an incandescent light source, additional optics is required to maintain collimation and stability over time. The light source will typically need to periodic replacement over the life of an instrument.

5.4.2.3 If the measurement unit includes an “R” it is a nephelometric method that utilizes a 90-degree detector plus one other detector. This is referred to as a ratio metric technique and helps to compensate for color interference, regardless of the wavelength of the incident light source. The technique also helps to linearize the response to turbidity at higher levels and can provide an extended measurement range. The technique can also help to stabilize measurement outputs. The technique is the most flexible across different applications because of the combination of sensitivity to low turbidity ranges and the ability can measure very high turbidity levels.

5.4.2.4 If the measurement unit has a “B” it indicates a backscatter technique. These techniques typically have a wide range, but are not sensitive at low turbidities. They are also more susceptible to color and particulate absorbance interferences.

5.4.2.5 If the measurement unit has an “A” it indicates an attenuation or absorbance measurement. The measurement is a combination of light that is attenuated and absorbed, in combination. Color is a significant interference, except for applications that require color to be considered part of the overall turbidity measurement. The method is very sensitive to wavelength and thus, the reporting unit should also include the wavelength of the incident light beam.

5.4.2.6 If the measurement unit contains an “M” it indicates a technology in which at least two incident light beams and two detectors are employed. The method also encompasses a ratio technique. These designs are very similar to ratio techniques as are the advantages and limitations.

5.4.2.7 Other Units: 

1.1.1 In principle there are three basic applications for on-line measurement set ups. The first is the bypass or slipstream technique; a portion of sample is transported from the process or sample stream and to the turbidimeter for analysis. It is then either transported back to the sample stream or to waste. The second is the in-line measurement; the sensor is submerged directly into the sample or process stream, which is typically contained in a pipe. The third is in-situ where the sensor is directly inserted into the sample stream. The in-situ principle is intended for the monitoring of water during any step within a processing train, including immediately before or after the process itself.

1.1.2 Static covers both benchtop and portable designs for the measurement of water samples that are captured into a cell and then measured.

1.4.1 This guide does not provide calibration information but rather will defer the user to the appropriate ASTM turbidity method and its calibration protocols. When calibrated on traceable primary turbidity standards, the assigned turbidity units such as those used in Table 1 are equivalent. For example, a 1 NTU formazin standard is also equivalent in measurement magnitude to a 1 FNU, a 1 FAU, and a 1 BU standard and so forth.

1.4.2 Improved traceability beyond the scope of this guide may be practiced and would include the listing of the make and model number of the instrument used to determine the turbidity values.