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Process Turbidity Analyzers |
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There are two basic types of photometers used for process turbidity measurements. Those based on light absorbance and those based on light scattering.
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| Absorption Photometers: |
Absorption based photometers typically incorporate a focused beam of light and project this beam through the process media while working much the same way as a laboratory spectrophotometer. Process scale absorption based photometers, are set at a fixed wavelength or bandwidth, rather than having the ability to select the wavelength.
Three-dimensional solids will exhibit absorbance at virtually all frequencies of light, however at short wavelength Infrared or Near Infrared (NIR), solids contributing to turbidity can be isolated from visible color as well as other dissolved constituents.
This bandwidth is called the "Herschel" range, after Sir William Frederick Herschel, and is identified as just beyond red light in the 700 - 1100 nanometer range. This bandwidth is unique in that it is virtually void of any dissolved constituents capable of absorbing light of these frequencies.
The attenuation of the NIR light transmittance is then used as an indicator of the solid's concentration. The percentage of light transmittance can be plotted against the concentration, however the relationship is not linear. Absorbance is then defined as the negative log 10 of the transmittance of the light. This relationship is linear as expressed by Lambert-Beer's Law.
In the case of solids concentration (turbidity), Lambert-Beer will hold true in most circumstances, with properly applied instruments. In addition, measurements in the low NIR typically are not influenced by temperature, making the method fairly simple and straightforward.
Absorption based photometers are typically used for higher concentrations of process turbidity in excess of 0.5 grams per liter. The measuring path length or OPL is then set to work within the effective linear range of the instrument.
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| Scattering Photometers: |
Light scattering photometers come in a wide variety of configurations, and are used in an even wider variety of applications. The term light scatter collectively includes both the scatter of light waves off of particles in the media, as well as reflection off of particulate in the media.
These types of measurements are not limited to on-line or in-line instrument design, or particulate in fluid media for that matter. However in terms of process turbidity, as discussed herein, there are four main types of scattered light photometers used for turbidity in liquid media assessment.
Back scatter systems typically offer an attractively broad application range, but require disproportional complexity of the evaluating processor unit because these systems use the low intensity part of scattered light.
Side scatter methods have been traditionally used to assess finished product clarity and can be very effective in doing so, provided proper filtration has been achieved, stray light is suppressed, and no abnormal particulate is present. A majority of turbidity units of measure used today are based on 90°, side scatter measurement techniques. This method is widely used in many different industries.
Forward scatter angle systems are the preferred methods for actual process control as they provide the most particle size sensitivity, which is required to effectively monitor any separation or filtration process, or to detect abnormal turbidity conditions. Forward scatter measurement results also correlate very well to the actual concentration of particulate, making quantification of the turbidity causing material simple and straightforward.
In some circumstances, multiple angles of detection (forward and side scatter) are both useful and necessary for a complete turbidity profile of the product, particularly when the final product quality is the objective. Process scale scattered light photometers are typically used for more precise, low level turbidity assessments below 0.5 grams per liter, whether it's process control, turbidity detection or quality assurance.
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| Summary |
Back Scatter is defined as less than 90°, toward the light source
Forward scatter is defined as less than 90° away from, or in the same general direction as the light source
Reflective systems vary considerably depending on the manufacturer, their individual instrument design, and intended purpose
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Turbidity Measurement Units |
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