Measurement principle
Turbidity is an optical index that characterizes the effect of suspended particles in a liquid on light scattering and absorption. Its measurements are mainly based on two classical optical principles: scattering and transmission. Both methods quantify turbidity values by detecting the interaction between light and particulate matter in the sample, but there are significant differences in their physical models, detection angles, and applicable scenarios, which directly affects the accuracy and application scope of measurement.
Scattering method
The scattering method usually refers to measuring the intensity of scattered light at a specific angle to the incident light (commonly 90°). According to the Rayleigh scattering and Mie scattering theories, the intensity of scattered light is related to particle concentration, particle size distribution and refractive index. For low-turbidity samples, the scattering signal has a good linear relationship with the turbidity value over a wide range. Its measurement model can be simplified to:
Is = ks ⋅ Φ ⋅ I0 ⋅ C
Among them, Isis the intensity of scattered light, ksis the coefficient related to the instrument geometry and particle properties, and Φ is the scattering function of the particles, I0is the incident light intensity, and C is the concentration of particulate matter. This method is sensitive to low concentrations, small particle size particles, and is relatively unaffected by sample color, and is the basis for many standard methods (e.g., EPA 180.1, ISO 7027).
Transmission method
Transmission method, also known as attenuation, evaluates turbidity by measuring the degree of attenuation of incident light as it passes through a sample. According to Lambert-Beale's law, light intensity attenuation is directly proportional to the concentration of particulate matter and the extinction cross-section. Its basic relationship can be expressed as:
It = I0 ⋅ e-σ ⋅ C ⋅ L
Among them, Itis the intensity of transmitted light, σ is the total extinction coefficient (including scattering and absorption) of the particle, and L is the optical path. This method responds well to samples with high concentrations and larger particles, but has low sensitivity in the low turbidity range and is easily disturbed by sample color (absorbent substances).
Comparative analysis
The accuracy of the measurement is constrained by a variety of factors, and it needs to be comprehensively compared in terms of principle, interference, range, and standard compliance. The following table summarizes the key comparisons:
| Compare items | Scattering method (90° as an example) |
| Sensitivity and low range | High sensitivity to low turbidity (such as <1 NTU) and good signal-to-noise ratio |
| High turbidity measurement range | At high concentrations, it is easy to be saturated by multiple scattering, and the upper limit of the volume range is limited |
| Particle size influence | It responds well to small particle sizes (e.g., <0.1 μm) and is sensitive to particle size distribution |
| Color interference | It is relatively little affected by the color of the sample (absorbent substance). |
| Standard compliance | Widely comply with international standards in the fields of drinking water, wastewater and other fields |
| Compare items | Transmission method |
| Sensitivity and low range | The sensitivity is insufficient at low turbidity, and the signal-to-noise ratio is poor |
| High turbidity measurement range | Suitable for high turbidity measurements with a high upper range |
| Particle size influence | The response to large particle size particles is more significant |
| Color interference | It is significantly disturbed by the color of the sample, which can easily lead to high readings |
| Standard compliance | It is mostly used for process monitoring or specific industries, and there are few standard references |
Application scenarios
The method you choose depends on your specific measurement needs. For potable water, low-turbidity ultrapure water, or testing that requires strict compliance reporting, scattering is often the preferred choice due to its sensitivity at low scales, immunity to color interference, and compliance with standards. For the monitoring of high-turbidity fluids (e.g., some wastewater, slurries) in industrial process control, transmission methods may be applied due to their range advantages. In practice, it is also necessary to consider the calibration method of the instrument, the stability of the optical components, and the complexity of daily maintenance.
Conclusion
The accuracy of scattering and transmission methods cannot be separated from specific application scenarios. In the low to medium turbidity range, especially where high sensitivity and standard compliance are required, scatterometry often provides more reliable and accurate measurements. In the field of high turbidity, the transmission method may show its range advantage. Users should make comprehensive judgments and selections based on the typical turbidity range of the sample to be tested, particle characteristics, color interference, and the technical standards followed. Regular calibration and good maintenance of the instrument are essential to maintain the long-term measurement accuracy of either method.