A parallel beam of light in a transparent liquid propagation, if the liquid does not have any suspended particles, the beam in a straight line will not change direction; if there are suspended particles in the liquid, regardless of whether the particles are transparent or not, the beam will change direction when encountering particles, the formation of each upward scattering, the more particles that is, the higher the turbidity, the greater the role of the beam of light scattering. But the liquid particles and particles will certainly exist between the gap, there must be part of the light beam from the liquid between the gap between the particles transmitted out, this part of the light we call transmitted light.

If the liquid chromaticity is zero, then theoretically the sum of scattered light and transmitted light is equal to the incident light, but in fact, the colour rendering material in the liquid absorbs part of the incident light. Instrumental turbidity measurements can be divided into three categories based on the measurement of the intensity of light emitted from different sources: scattered light measurement, transmitted light measurement and scattered light transmission measurement.

1. Scattered Light Measurement

The turbidity sensor is designed according to the principle of scattered light measurement. The principle of measurement is based on the following relationship between the turbidity S, the incident light intensity I0, the scattered light intensity 1, the instrumental constant k, the cuvette optical range length b, and the particles C in the sample for a low concentration sample: .

S=-Ig(I/ l0)=kbC

When light is injected into the water sample, the particles constituting the turbidity scatter the light, and the intensity of the scattered light is proportional to the turbidity of the water sample. According to the standard IS07027- 1984 design of portable turbidity meter, the measurement is with the incident light into 90 degrees of the intensity of the scattered light, the use of tungsten light source, LED emission of high-intensity 880nm wavelength near-infrared light, will be placed in the perpendicular position of the photoelectric sensor with the emitted light, the microcomputer for data processing, can be carried out to self-check and read out the value of the water samples NTU, the detection range of 0-4000NTU.

2. Transmitted light measurement

The spectrophotometer is designed according to the principle of transmitted light measurement and the wavelength of the incident light is selected as 680 nm. The principle of measurement is based on the Lambert-Beer law: mathematical expression

A=lg(1/T)=kbC

A is the absorbance, T is the transmittance ratio, which is the ratio of the transmitted light intensity to the incident light intensity. k is the molar absorption coefficient. It is related to the nature of the absorbing substance and the wavelength of the incident light . C is the concentration of the light-absorbing substance b is the thickness of the absorbing layer. Physical significance is when a beam of parallel monochromatic light perpendicular through a uniform non-scattering light-absorbing material, its absorbance A and light-absorbing material concentration C and the thickness of the absorbing layer b is proportional.

3. Visual Nephelometric Turbidimetry

This is relatively primitive but in some special conditions is also an effective method of turbidity measurement. The human eye as a detector, with turbidity standard solution diluted into a dense gradient series, from which the turbidity of the water samples to be tested to detect the two series close to the turbidity of the water samples, thus estimating the turbidity of the water samples. This method does not require complex instruments, does not cost a lot of money to buy equipment, for individuals who want to engage in scientific research but have limited financial resources, there are more areas of application.

4. Turbidity Online Monitoring and Turbidimetric Titration Method

With more and more automated and intelligent detection technology, the market has appeared on-line turbidity analyzer, the instrument adopts dual-beam infrared and scattered light photometer detection technology, the technology of scattered light - transmitted light measurement as the principle, based on the simultaneous measurement of the intensity of scattered light and transmitted light, according to the scattered light and transmitted light or the ratio of the two and the size of the determination of turbidity, so that the turbidity measured by the water samples of the chromaticity impact Smaller. It has the advantage of monitoring the turbidity of filter backwash water, raw water and sedimentation tank water, as well as the turbidity of circulating cooling water in industrial production process in waterworks.