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William Stone
Engl 317
Section of Final Report RD
Definitions
Bandwidth: the range of frequency (wavelength) that a device works in.
Nephelometry: A method of measuring turbidity by shining a directional light of a particular
wavelength into water and measuring how much light is scattered at 90 degrees from the
light source path.
NTU: nephelometric turbidity unit, the standard unit for measuring turbidity.
Spectral Output: the characteristic curve of a devices response magnitude versus frequency on
the electromagnetic spectrum.
Turbidity: a measure of how cloudy water is. Relates to total suspended solids.
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Turbidimeter Part Characteristics
The turbidimeter can be broken into a simplified block diagram as such.
A light source of some sort shines into the fluid sample, which reflects some light onto the light
receiver. This signal is passed through an analog to digital converter, which is then fed into an
input of the microcontroller. The microcontroller will translate this signal into an output reading
in NTU, which will then be outputted to an LCD for display on the meter and out to a digital to
analog converter. The output from the digital to analog converter will be fed into some circuitry
to convert the signal to a 4 to 20 milliamp current loop for communication with the plant
environment. Each block translates directly to a part of the whole system and can be analyzed
singularly.
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Light Source
Option A – Tungsten Bulb
Until recently, the tungsten bulb was the only light source used in turbidimeters.
Typically the bulb must burn at a temperature in the range of 2200 to 3000 degrees Kelvin.
Burning at this temperature bulbs give a spectral output that peaks at about 1300nm and provide
the necessary energy input between 400 and 1000nm. Bulbs require a fair amount of energy, as
most of their output is heat rather than light. Since the output bandwidth of a bulb is very large,
the response of a system that is bulb driven is mostly dependant on the light receiver.
The main problem with using a tungsten bulb for nephelometry is making the light
directional. In order to minimize stray light which could adversely affect measurements the light
from a bulb must be passed through a set of baffles and lenses in order to focus it in a single
path. In doing so the characteristics of the spectral output are changed, and must be tuned
according to the desired wavelength. This adds complexity to fabrication and troubleshooting.
Option B – Laser Diode
(schematic symbol for a light emitting diode)
(brief section on diode theory?)
The second most common light source used in nephelometery is a laser diode. There are
several distinct advantages to using a laser as opposed to a bulb. Laser diodes can be made with
many different output wavelengths and a small variance in output bandwidth. They typically
range from output in the middle of the visible light spectrum (green) to infrared light (like a
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television remote). Many laser diodes have an output bandwidth in the neighborhood of 60nm or
so, so they are much more focused than a bulb.
The spectral output of a laser diode is something of a mixed blessing for nephelometry.
The specifiable peak output wavelength is good, as it allows for specific light response
characteristics such as color interference to be controlled. The narrow bandwidth is good in that
it aids that control, but bad in that it requires the light circuit to be tuned more carefully. Laser
diodes produce a beam of intense directed light, and so do not require any lenses or baffles,
which removes some complexity in design work. Diodes also consume much less power since
they are producing mostly light and very little heat.
Light Receiver
Option A – Cadmium Sulfide Photoconductor
The most common method of receiving the reflected light in older turbidimters is the
cadmium sulfide (CdS) photoconductor. CdS crystals act as light sensitive semiconductor
material. When the crystal is in the dark, an applied voltage produces very little current,
equating to the crystal having a high resistance. Applying light to the crystal excites valence
electrons allowing for better conduction, equating to a lower electrical resistance. This change in
electrical resistance due to light can readily be turned into a usable signal relating to how much
light is present on the crystal.
CdS crystals are not ideally suited for application in nephelometry. They have a
relatively narrow spectral response, between 400 and 800nm with a peak at about 600nm. The
characteristic curves relating resistance to light intensity on the device is heavily dependant on
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temperature. This is particularly undesirable when working with 5000 degree Fahrenheit light
bulbs.
Option B – Photodiode used in photoconductive mode
Photodiodes come in several different flavors. A myriad of different semiconductor
materials can be used to make a P-N junction that will emit light when current is passed through
or that will generate voltage when exposed to light. Rather than measure the voltage generated
by light exposure for nephelometry, it is best to employ a different method of utilizing the diode.
A reverse voltage is applied to the diode, and subsequent current generation is measured. This
gives the best usage of the diodes properties for light sensing.
Silicon photodiodes have the best characteristic curves for use in nephelometry. They
have a spectral response between 250 and 1100 nm with a peak around 800nm or so. Figure 81
shows the different responses of an Si photodiode with a CdS crystal photoconductor.
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The Si photodiode has roughly the same output magnitude as the CdS photoconductor at 600nm,
but the photodiode does not have the sharp drop off in response at higher frequencies. This
makes it ideally suited for use in turbidity sensing as fluid coloring will not affect the output as
much.
Option C – A photodiode Array
The third option only differs form the second in that it would use more than one diode in
sensing the scattered light. Figure 11 illustrates the direction of light scattered by particles as a
function of particle size.
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By employing photodiodes in various places around the sample fluid, some of the other
directions of scattered light could also be captured and measured.
Using more than one photodiode would certainly be beneficial in allowing for more
accurate readings, but would also add another order of magnitude to the complexity of the
design. Each diode would need a channel of circuitry to provide for signal amplification, analog
to digital conversion, and lines of code in the microprocessor to analyze incoming signals. This
makes the design much more difficult to troubleshoot and maintain.
Option D – Charge Coupled Device
The fourth and final option is to use a charge coupled device at 90 degrees from the light
beam to detect incident light. A charge coupled device acts like an array of photodiodes in and
of itself. It allows a digital picture of the reflected light to be taken at any time. CCD chips are
used in digital cameras for just this purpose. They are more suited for taking pictures than
sensing a given amount of light, and so the signal processing involved would be rather difficult.
There is likely a significant future in using CCD chips in light scatter technology in water
treatment; the applications are likely far beyond the scope of this project. By sensing different
areas of scattered light around the fluid sample it would be possible to do some rough
calculations on particulate size. This would give a powerful feedback signal to the plant operator
as to treatment processes. This would include various edge and pattern detection software and a
much greater knowledge of light scattering physics and water treatment chemistry than any
member of my team would be able to gain in the given time frame.
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References
1: Hach Company. (1998). Turbidity Science. Learning Library Page. Retrieved October 1,
2005 from http://www.hach.com/fmmimghach?/CODE:L7061549|1//true.