Turbidity Sensors

Sensing and Control

Turbidity Sensors

Turbidity sensors can be described by three basic types: transmissive, scattering, and ratio. Each of the three configurations has its own advantages and disadvantages. For the design engineer, each represents a different option depending on the type and levels of turbidity to be measured. Each type also has unique output characteristics.

The different configurations increase in cost as the number and performance of required components — both electrical and mechanical — are increased. The selection of a sensor type should therefore be based on the acceptable cost for a given application and the level of performance required to achieve the desired results. All of these sensor types typically use visible light for a source, and therefore they must be shielded from stray or ambient light. Low cost LED's are often used as a light source because they offer a trade-off between cost and convenience versus performance. Infrared sources have also been used. Incandescent light sources are sometimes preferable because they omit different wavelengths from the source. This helps to reduce the sensitivity of the turbidity sensor to variations in particle size.

Transmissive sensor

The transmissive sensor configuration is low in cost, requiring only a light source and a detector. This sensor is usually placed so that it looks through the sides of a transparent tube through which the media under analysis flows. A example of the basic configuration is shown in Figure 3. While fairly easy to deploy, there are several critical design factors which can contribute to errors in the sensor output:

Intensity of the source
Sensitivity of the detector
Component alignment
Optical path
Sensor-to-sensor repeatability

The characteristic output of this configuration is a signal which decreases monotonically with increasing turbidity. This type of sensor has been used on several Japanese clothes washers as a rinse detector to determine if additional rinsing is necessary for a particular load.
Figure 3. Basic transmissive sensor configuration

Scattering sensor

Scattering sensors are also low in cost because they only require a light source and a detector. The scattering sensor differs from the transmissive sensor in that the alignment of the components is less critical. Various angles can be used to locate the detector depending on the desired sensitivity. Like the transmissive sensor, this sensor may be used to look through the sides of a transparent tube carrying the media under analysis, or it may be configured to look through the bottom of a vessel containing the media. Figure 4 illustrates a flow-tube application. Here too, several factors are critical to the sensor design:

Intensity of the source
Sensitivity of the detector
Optical path
Sensor-to-sensor repeatability

The characteristic output of this sensor is an increased signal for increased turbidity which demonstrates high sensitivity to low turbidity conditions. Geometry is important because as the turbidity of the media increases, a point can be reached where the scattering phenomenon is overcome by the absorption of the media. This event causes foldback in the output.
Figure 4. Basic scattering sensor operation

Ratio sensor

The ratio sensor shows the best overall performance by combining the better low sensitivity characteristics of a scattering sensor with the high turbidity capabilities of a transmissive sensor. The ratio sensor also benefits from the reduced sensitivity to common mode effects. A ratio may be determined from the simultaneous outputs of the direct and scattered sensor readings. As illustrated in Figure 5, the ratio helps to eliminate degrading effects from temperature, source intensity, and minor aberrations to the optical path because the two readings are similarly affected by these variables, so the effects of these variables are canceled. This sensor type yields a wide dynamic range but does require more components and is therefore typically higher in cost than the others. Again, several factors are critical to the sensor design :

Intensity of the source
Difference in sensitivity of the detectors
Optical path
Sensor-to-sensor repeatability

The characteristic output of the ratio sensor is a monotonically increased signal for increased turbidity if used within the design range. Sensor output is typically specified by a table showing the relationship between sensor output and NTUs.
Figure 5. Basic Ratio sensor configuration

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