Turbidity Meters & Sensors

Turbidity is an optical characteristic that refers to the degree of clarity of a liquid. Turbidity levels can be measured with a turbidity meter. Turbidity in water is caused by individual suspended particles or colloidal matter that scatters or obstructs light transmittance: the higher the concentration of suspended particles/colloidal matter, the higher the turbidity. Such particles are normally too small to be detected by the human eye; therefore, turbidity measurement has to be done with a turbidity meter or turbidity analyzer. The best way to control processes that require close turbidity monitoring is with an in-line turbidity meter. An in-line turbidity meter provides a continuous measurement of turbidity that can be used for process control purposes. Everyday examples of turbid liquids are:

• Milk - contains emulsified protein/oil drops in water
  
• Wastewater - contains suspended particles
• Wheat beer - contains yeast cells

A turbidity meter, also called a turbidity analyzer, is a system for measuring suspended particle concentration in a process. A turbidity meter generally consists of 3 major parts: a turbidity sensor, a turbidity transmitter and a process connection. 

Turbidity measurement, as captured by a turbidity analyzer, determines the degree that suspended particles in a liquid medium, scatter light. The scattering is influenced by:

• Particle concentration: Higher particle concentrations result in higher scattered light and therefore higher readings on the turbidity meter.
  
• Particle shape and size: Particles smaller than 1/10 the wavelength of visible light, scatter light symmetrically. Larger particles (typically with a diameter greater than the wavelength of visible light) scatter light asymmetrically. Therefore, for measuring turbidity, the scattering angle must be considered.
  
• Light wavelength: As mentioned above, the intensity of the scattered light is dependent on particle size. In addition, the presence of color in a liquid may reduce the light being measured at the detector. As a result, the use of a suitable light wavelength must be considered for turbidity measurement.
  

As a consequence of the three points above, it is only possible for turbidity to be used as a characteristic property of a sample if the measurement method is standardized. For example, in many brewery applications the liquid to be measured is yellowish in color and contains yeast particles. Therefore, to check for filter breakthrough, the amount of forward and side-scattered light is measured at an angle of 25° and 90° compared to the light source, for quality assurance purposes. Red (650 nm) and blue (460 nm) light sources are also specified in guidelines for turbidity and color monitoring.