Significance of Use

Turbidity is one of the most important parameters used to determine the quality of drinking water. Once considered as a mostly aesthetic characteristic of drinking water, signi˜ficant evidence exists that controlling turbidity is a competent safeguard against pathogens. In natural water, turbidity measurements are taken to gauge general water quality and its compatibility in applications involving aquatic organisms. The monitoring and treatment or wastewater was once solely based on the control of turbidity. Currently, the measurement of turbidity at the end of the wastewater treatment process is necessary to verify that the values are within regulatory standards.

Turbidity of water is an optical property that causes light to be scattered and absorbed, rather than transmitted. The scattering of light that passes through a liquid is primarily caused by the suspended solids present. The higher the turbidity, the greater the amount of scattered light. Even a very pure liquid will scatter light to a certain degree, as no solution will have zero turbidity.  

The ISO standard for turbidity measurement uses infrared wavelength of light that is outside the visible spectrum. The primary benefit of the ISO method over the EPA method is the reduction of color interference. The EPA method uses a tungsten lamp that produces light that contains all of the visible wavelengths of light that we see as white. A solution that is colored will be adsorbed by a complementary wavelength of light which will affect the turbidity reading. Since the ISO method is outside the visible wavelength of light the color in the sample does not interfere with the measurement. The USEPA prefers the tungsten lamp since it provides for better accuracy at low ranges and drinking water should not be colored. The requirements of an optical turbidity meter for the measurement of diffuse radiation used in low range measurements (i.e. drinking water) expressed as formazin nephelometric units (FNU) according to ISO 7027:1999 are:

Measurement of diffuse radiation for water with low turbidity (0 FNU to 40 FNU)

• The wavelength of the incident radiation shall be 860 nm.

• The spectral bandwidth of the incident radiation shall be less than or equal to 60 nm.

• There shall be no divergence from parallelism of the incident radiation and any convergence shall not exceed 1.5o.

• The measuring angle, theta, between the optical axis of the incident radiation and that of the diffused radiation shall be 90o ±2.5o.

• The aperture angle should be between 20o and 30o in the water sample.

The HI93703 meets and exceeds the meter criteria speci˜ed by ISO 7027.

Principle of Operation

The light beam that passes through the sample is scattered in all directions. The intensity and pattern of the scattered light is a‰ffected by many variables, such as wavelength of the incident light, particle size and shape, refractive index, and color. The optical system of the HI93703 includes a LED, and a scattered light detector (90°).

The lower detection limit of a turbidimeter is determined by stray light that is detected by the sensors but not caused by light scattering from suspended particles. The optical system of the HI93703 is designed to have very low stray light, providing highly accurate results for low turbidity samples.