Turbidity sensors
Contents
Introduction
Definition
Turbidity is defined as the reduction of transparency of a liquid caused by the presence of undissolved suspended matter.[1] The origin of the particles found in seawater can be mineral (such as clay and silts) or organic (such as particulate organic matter or living organisms like plankton). Turbidity is not, however, a direct measure of suspended particles in water, but a measure of the scattering effect such particles have on light.
Light Scattering different from Particle Concentration
A water sample colored by dissolved substances is an homogeneous system that only attenuates the radiation that goes through it. A water sample that contains undissolved particles attenuates the incident light and, in addition, the particles scatter the light unequally in all directions. The scattering of the radiation towards the front generated by the particles, changes the attenuation in such way that the relative spectral attenuation coefficient [math] \mu (\lambda)[/math] is the sum of the spectral diffusion coefficients [math] s (\lambda)[/math] and the spectral absorption coefficient [math] \alpha (\lambda)[/math].
To obtain the spectral diffusion coefficient [math] s (\lambda)[/math] alone, the spectral absorption coefficient [math] \alpha (\lambda)[/math] must be known. To determine the spectral absorption coefficient of the dissolved substances, in some cases the undissolved particles may be filtered, although this my cause interference. Thus, it necessary to compare the results of the turbidity measurements with a standard scale. The intensity of the scattered radiation depends on the wavelength of the incident radiation, the measuring angle, and on the shape, optical characteristics and size distribution of the suspended particles. In measurements of the attenuation of the transmitted radiation, the measured values depend on the aperture angle [math] \Omega_\theta [/math]. The angle [math] \theta [/math] is formed by the direction of the incident radiation and the direction of the measured scattered radiation, as shown in figure 1. [2]
Measurements of Turbidity throughout history
Secchi Disk
The first record of turbidity being approached in a scientific way, is attributed to the head of the Papal Navy in 1865, Commander Cialdi. Commander Cialdi was interested in the transparency of the sea and the visibility of its floor (for navigational purposes) and during his research he had read that a captain had reported seeing a plate in a net at a depth of 40 m. Commander Cialdi commenced his investigation on the sea visibility and immediately tried several different disc sizes and colors. Later, he hired the services of Professor Pietro Angelo Secchi, an Italian scholar and priest. Together they published Sur la Transparence de la Mer.(On the transparency of the Sea) where they described the development of a white disk connected to a pole or a cable to measure the transparency of the sea. Secchi made further observations on the influence of the boat's shadow, the surface reflection of the light, the clearness of the sky, and the height at which the observer stands[3]. While his method has been used ever since and it is still used today as a qualitative measure of the oceans' turbidity, it has never been standardized so its accuracy is very limited.
Jackson candle turbidimeter
The first attempt to measure turbidity in a standardized fashion was made by Whipple and Jackson in 1900. Whipple and Jackson developed a standard suspension fluid using 1,000 parts per million (ppm) of diatomaceous earth in distilled water to use as a scale and a turbidimeter (Jackson candle turbidimeter). The method consists in placing a lit candle under a flat-bottomed tube in which the water in poured until the image of the candles fades unto a glare when observed from above. Visual image extinction occurred when the intensity of the scattered light equaled that of transmitted light. The height of the water would be read against the ppm-silica scale and units were given in Jackson turbidity units (JTU). Needless to say that this method is prone to errors too, as the diatomaceous earth varies in its composition and the reading is subjected to the observer. In addition to the problems mentioned above, the wavelength of the candle light is not as effectively back-scattered by small size particles while the dark particles absorb so much more light than they scatter, that it is impossible to obtain a reading. Other methods were developed following the principle of the Jackson candle turbidimeter with improved light sources, but the bias induced by the observer was still a limitation. The development of the photoelectric detector brought much better precision under certain conditions. The photoelectric detector measured the attenuation of transmitted light through a fixed-volume sample and it was sensible to very small changes in light intensity. Still, the photoelectric detector could not read changes in high or extremely low turbidity.
Nephelometers
The solution for this problem finally came in the 1970's when the nephelometric turbidimeter, or nephelometer, was developed. The name comes from nepheloid layer which in turn comes from the Greek word nephos meaning cloud. This instrument determines turbidity by the light scattered at an angle of 90° from the incident beam. A 90° detection angle is considered to be the least sensitive to variations in particle size.
Calibration standards
Formazin
In 1926, Kingsbury and Clark discovered formazin, a suspension that is created by the polymerization of hexamethylenetetramine and hydrazine sulfate in water. This new suspension improved greatly the consistency in standards formulation. Measuring units were renamed to formazin turbidity units (FTU). Several techniques were developed throughout the years with different light sources and scale systems, but in the end they were all still dependent on the observer's subjectivity.
StablCal®
Causes and Impacts of turbidity in Marine Ecosystems
High turbidity has a number of detrimental effects on aquatic ecosystems: decrease in light penetration (limiting plant growth), fish movements and the ability of predatory fish and birds to see their prey. High turbidity means high concentration of suspended solids, which can harm fish and other aquatic fauna. These suspended solids in the process of settling down to the ocean bottom have a choking effect on bottom dwelling organisms and aquatic habitats.[5]
See also
References
- ↑ ISO 7027, Water Quality, International Standard,1999
- ↑ ISO 7027, Water Quality, International Standard,1999
- ↑ Tyler, John E. , 1968, THE SECCHI DISC, LIMNOLOGY AND OCEANOGRAPHY, vol.8, no. 1
- ↑ http://www.optek.com/Turbidity_Measurement_Units.asp
- ↑ Saba Mylvaganam, Torgeir Jakobsen, 1998, TURBIDITY SENSOR FOR UNDERWATER APPLICATIONS Sensor Design and System Performance with Calibration Results, OCEANS '98 Conference Proceedings, Vol.1, 158-161
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