Selection starts from the principle
A turbidity meter is an instrument that measures the degree of light scattering or absorption by suspended particles in a liquid, and is widely used in water quality monitoring, food and beverage, chemical and environmental testing. The first step in choosing the right turbidity meter is to define its measurement principle, which determines the instrument's suitability, sensitivity, and maintenance needs. At present, the mainstream measurement principles include scattered light, transmitted light, scattering-transmission ratio and non-optical methods (such as ultrasonic methods), but optical methods are dominant.
1. Scattering light method
The scattered light method is based on the principle that light scatters after irradiating suspended particles, and the intensity of scattered light is directly proportional to the concentration of particles. It typically uses a 90° detection angle and has high sensitivity for low to moderate turbidity (0–1000 NTU) samples and meets ISO 7027 standards. The formula is expressed as:
Among themIsis the scattered light intensity,ksis the scattering coefficient (related to particle size and wavelength),Nis the particle number concentration,I0This is the intensity of the incident light. This principle is suitable for clean water quality monitoring, but it is important to note that high-chromaticity samples may interfere.
2. Transmitted light method
Transmitted light method measures the attenuation of light intensity after passing through a sample and is suitable for samples with high turbidity (above 100–4000 NTU). The decay rate follows Bill-Lambert's law:
Among themTis the transmittance,εabsorbance coefficient for molarity,cis the particle concentration,LIt is the optical process. This method is effective for high turbidity samples, but the sensitivity is insufficient at low turbidity. The selection should be based on the expected turbidity range of the sample.
3. Scattering-transmission ratio method
This method measures both scattered and transmitted light, calculates turbidity by ratio, extends the scale range (e.g., 0.5–10000 NTU) and reduces chromatic interference. It is suitable for occasions with complex water quality or requiring a wide range, such as environmental monitoring stations.
4. Non-optical principle
Non-optical methods such as ultrasonic attenuation or capacitance method are suitable for high-density suspensions or special liquids, but they are less popular, and optical principles are prioritized in selection.
Consider the measurement range and accuracy
Different principles have advantageous measurement ranges. Please refer to the following concise comparison (table width 100%):
| Measurement principle | Typical Range (NTU) and Characteristics |
| 90° scattered light method | 0–1000, low turbidity high sensitivity, need to avoid bubbles |
| Transmitted light method | 100–4000+, good linearity for high turbidity |
| Scattering-transmission ratio method | 0.5–10000, wide range anti-interference |
Light source wavelength and standards compliance
The wavelength of the light source affects the measurement results. Common light sources are white light (tungsten lamp), near-infrared light (860nm), or LED. International standards such as ISO 7027 recommend the use of 860nm LEDs to reduce chromaticity interference, while EPA methods such as US EPA 180.1 allow for white light. Confirm that the instrument meets the target industry standard, such as ISO 7027 or EPA methods commonly required for drinking water testing. The scattering coefficient in the formulaksWavelength-dependent, long wavelengths scatter less fine particles but are more conducive to penetration.
Sample conditions and interferences
Evaluating sample properties is key: high chromaticity (e.g., wastewater) absorbs light and affects the accuracy of the scattering method, where the scattering-transmission ratio method is more reliable. Bubbles, bubbles, or scaling can introduce noise, consider in-line instruments with defoaming or self-cleaning systems. Temperature changes have an impact on the stability of electronic components and sample viscosity, so it is necessary to purchase a temperature repair model or laboratory constant temperature environment.
Instrument structure and environmental adaptability
Most of the turbidity meters used in laboratories are benchtop, which requires stability and repeatability. On-site or online testing requires portable or waterproof types. Consider the probe material (e.g., 316 stainless steel or PTFE), degree of protection (IP67 or above), mounting method (immersion or flow-through), and electromagnetic compatibility. When the sample contains strong acids, alkalis, or high temperatures, it is necessary to choose resistant materials.
Routine maintenance and calibration
Regular calibration is required during use, and formazine standards are commonly used as reference materials. Some instruments support automatic calibration to reduce human error. Use a non-woven cloth or a special swab before cleaning the light window to avoid scratches. Focusing on instrument storage and longevity, electronic components are sensitive to humidity, and it is recommended to store them in a dry environment.
Comprehensive selection suggestions
According to the actual needs: first, determine the measurement principle (low turbidity focuses on scattering, high turbidity focuses on transmission, complex sample selection ratio method); secondly, check the standard requirements (ISO or EPA); Consider sample conditions (chromaticity, particle distribution), installation environment (lab or field), and budget. It is recommended to ask the manufacturer for test data or trial instruments.
Cited references
1. Review of turbidity measurement techniques, Journal of Water Quality Technology, 2018
2. Application of optical scattering in suspension analysis, Advances in Analytical Chemistry, 2020
3. International standard ISO 7027-1:2016, Water quality-turbidity determination
4. Laboratory Instrument Selection Guide, Environmental Monitoring Technology, 2019