Turbidity is measured by detecting and quantifying the scattering of light in water (solution), which can be measured in many ways, using visual methods and instrumental methods. Visual methods are more suitable for samples with high turbidity. Instrumental methods can be used on samples with both high and low levels of turbidity.
Two visual methods are the Secchi Disk method [2] and the Jackson Candle method. The Secchi Disk method is often used in natural waters. A black and white Secchi Disk is lowered into the water until it can no longer be seen. It is then raised until it can be seen again.
The average of these two distances is known as the ”Secchi Depth”.
The Jackson Candle method uses a long glass tube over a standard candle. Water is added or removed from the tube until the candle flame becomes indistinct. The depth of the water measured with a calibrated scale is reported as Jackson Turbidity Units (JTU). The lowest turbidity that can be determined with these methods is about 25 NTU.
There are two common methods for instruments to measure tur-bidity. Instruments can measure the attenuation of a light beam pass-ing through a sample and they measure the scattered light from a light beam passing through a sample. In the attenuation method, the intensity of a light beam passing through a turbidity sample is compared with the intensity passing through a turbidity-free sample at 180◦ from the light source. This method is good for highly turbid samples. The most common instrument for measuring scattering light in water samples is a nephelometer. A nephelometer measures light scattered usually at 90◦ to the light beam. Light scattered at others angles may also be measured, but the 90◦ angle defines a nephelomet-ric measurement.
3 Review of available turbidity sen-sors in the market
The aim of this project is to find a new turbidity sensor working principle to improve the accuracy of the devices available in the current different companies in the market.
The turbidity sensor market has been studied, through eighteen devices which belong to thirteen companies. Lamotte, Global Water, Hanna Instruments, Horiba, Wedgewood Analytical, ISI, ATI, En-dress + Hausser, Kobold, Usfilter, Hach Hydrolab, YSI, Technes and Tintometer.
In the following data chart have been compiled the accuracy and turbidity range of each eighteen devices doing easy to realize about values existing.
Company Model Accuracy Range
LAMOTTE 2020 e/i ±2 below 100NTU and
±3 above 100 NTU
0/2000 NTU
C124 ±3 below 40NTU and
±5 above 40 NTU
0/40 NTU
10/1000 NTU
HORIBA U-20 XD ±5 800 NTU
WEDGEWOOD AN-ALYTICAL
670 + TF10 ±1 20 NTU
WEDGEWOOD AN-ALYTICAL
870-872 ±2 1000 NTU
ISI 31 + 223 ±1 1000 NTU
ATI A15/76 ±5 in 40/400 scale and
±10 in 400/4000 scale
4/400 4/4000
KOBOLD LAT-N1 ±1 0/20/40/200/
400/1000 NTU
USFILTER TMS 561 ±2 below 40NTU and
±5 above 40 NTU
0/100/1000 NTU
HACH HYDROLAB
MS5-DS5-DS5X
±5 0/3000 NTU
YSI 600 OMS- 6136 ±5 0/1000 NTU
TECMES TS 289 ±3 0/100/250/500/
1000/2000 NTU TINTOMETER Turbidirect ±2 below 500NTU and
±3 above 500 NTU
0/1100 NTU
TINTOMETER DRT 15-CE ±1 below 10NTU,
±2 from 0 NTU to 100NTU and ±5 above 100 NTU
0/10/100/1000 NTU
4 Results
From the study we have taken the following conclusions:
The usual accuracy in the turbidity sensors available in the market is around 3%. This accuracy depends of the turbidity range being higher when the turbidity range decrease. Taking care this data, four devices have been chosen to continue the study. Kobold LAT-N1, Wedgewood Analytical 670+TF10, Tintometer DRT 15-CE and ISI 31+223. The data sheets of these turbidity sensors are available in the appendix 1. All these devices are highly accurate, arranging 1%
of accuray.
The Kobold LAT-N1 is a special device to measure turbidity in pipes. It is not able to measure turbidity in a sample.
The Wedgewood Analytical 670+TF10 sensor has a low turbidity range 20 NTU. This range is not enough in many applications.
By the same way Tintometer DRT 15-CE just arrange accuracy of 1% measuring turbidity below of 10 NTU and in a higher range, its accuracy decrease.
The final choice has been, the ISI 31+223, which has 1% accuracy in a big range 1000 NTU.
5 Properties of the ISI 31 + 223
The ISI 31 + 223 turbidity measuring instrument is a nephelometer.
A nephelometer is an instrument for measuring concentration of sus-pended particulates in a liquid or gas colloid. It does so by employing a light beam (source beam) and a light detector set to one side, 90◦ of the source beam. The particle density is then a function of the light reflected into the detector from the particles. How much light reflects for a given density of particules is dependent upon properties of the particles such as their shape, color, and reflectivity. Therefore, establishing a working correlation between turbidity and suspended solids (a more useful, but typically more difficult quantification of particulates) must be established independently for each situation. A more popular term for this instrument in water quality testing is a turbidimeter.
Figure 1: Working principle of the ISI 31 + 223. Nephelometric measure.
To improve this device, it is necessary to know the physical laws which are acting in a nephelometer. The nephelometer is an optical device, the different optical laws acting in the light scattering will be studied. Moreover, the relations between these laws and the particles suspended in a liquid should be found. After this study of the optical laws, a device with better accuracy will be able to be designed.
6 Scattering theory
The turbidity sensor working method are based in the scattering the-ories which give scattered light intensity in relation with other param-eters like the wavelength, angle between the scattered light and the emitted light, distance, . . .
We use two scattering theories (Rayleigh’s and Mie’s) which are ex-plained in the following lines.
6.1 Rayleigh’s scattering theory
Is the scattering of light by particles much smaller than the wavelength of the light λ. It occurs when light travels in transparent solids, liquids and gases.
The intensity of the light of wavelenght λ scattered in any direction making an angle θ with the incident light I0, is directly proportional to (1+cos2θ) and inversely proportional to λ4. The latter point is noteworthy in that it shows how much greater the scattering of the short wavelengt is. The intensity of light scattered Imeasured formula is given by:
Imeasured= I08π4N α2
λ4R2 (1 + cos2θ) (1) I0 is the intensity light beam emitted by the lamp source
Imeasured is the intensity light scattered
α is the polarizability1 (e.g. water polarizability value is 1.47A3) N is the number of scatters (equivalent to concentration of particles) λ is the wavelength of the light beam
θ is the scattering angle
R is the distance between where the light is emitted to where the light is measured
1Polarizability is the relative tendency of the electron cloud of an atom to be distorbed from each normal shape by the pressence of a nearby ion or dipole. The electronic po-larizability α is defined as the ratio of the induced dipole moment P of an atom to the electric field E that produces this dipole moment (P=αE). Polarizability has the SI units of Cm2V−1 but is more often expressed as polarizability volume with units of cm3 or in angstrom cubed A3= 10−24cm3