Intermediat Electronic Measurand Transduction Signal Mechanism FIGURE 56.1 Sensor block diagram. Active sensors require input power to accomplish transduction. Many sensors employ multiple transduction mechanisms in order to produce an electronic output in response to the measurand. sensor block diagram identifying the measurand and associated input signal, the primary and intermediate transduction mechanisms, and the electronic output signal. Active sensors require an external power source in order to produce a usable output signal, e.g., the piezoresistor. Table 56. 1 is a 6x 6 matrix of the more commonly employed physical and chemical transduction mechanisms. Many of the effects listed are described in more detail in this handbook(see Chapters 53-58) In choosing a particular sensor for a given application, there are many factors to be considered These deciding factors or specifications can be divided into three major categories: environmental factors, economic factors, and the sensor characteristics. The most commonly encountered factors are listed in Table 56.2, although not all of these factors may be pertinent to a particular application. Most of the environmental factors determine the packaging of the sensor, with packaging meaning the encapsulation or insulation which provides protection and isolation and the input/output leads or connections and cabling. The economic factors determine the type of manufacturing and materials used in the sensor and to some extent the quality of the materials(with respect to lifetime). For example, a very expensive sensor may be cost effective if it is used repeatedly or for very long periods of time. On the other hand, a disposable sensor, such as is desired in many medical applications, should be inexpensive. The sensor characteristics of the sensor are usually the specifications of primary concern. The most important parameters are sensitivity, stability, and repeatability. Normally, a sensor is only useful if all three of these parameters are tightly specified for a given range of measurand and time of operation. For example, a highly sensitive device is not useful if its output signal drifts greatly during the measurement time and the data obtained is not reliable if the measurement is not repeatable. Other output characteristics, such as selectivity and linearity, can often be compensated for by using additional, independent sensor input or with ignal conditioning circuits. In fact, most sensors have a response to temperature, since most tranducing effects are temperature dependent. Sensors are most often classified by the type of measurand, i.e., physical, chemical, or biological. This is a much simpler means of classification than by transduction mechanism or output signal (e.g, digital or analog) since many sensors use multiple transduction mechanisms and the output signal can always be processed, conditioned, or converted by a circuit so as to cloud the definition of output. a description of each class and examples are given in the following sections. The last section introduces microsensors and gives some examples TABLE 56.2 Environmental Factors Economic Factors Sensor Characteristics Humidity effects Availability Susceptibil y to Em interferences Power consumption Frequency response c 2000 by CRC Press LLC© 2000 by CRC Press LLC sensor block diagram identifying the measurand and associated input signal, the primary and intermediate transduction mechanisms, and the electronic output signal. Active sensors require an external power source in order to produce a usable output signal, e.g., the piezoresistor. Table 56.1 is a 6 ¥ 6 matrix of the more commonly employed physical and chemical transduction mechanisms. Many of the effects listed are described in more detail in this handbook (see Chapters 53–58). In choosing a particular sensor for a given application, there are many factors to be considered. These deciding factors or specifications can be divided into three major categories: environmental factors, economic factors, and the sensor characteristics. The most commonly encountered factors are listed in Table 56.2, although not all of these factors may be pertinent to a particular application. Most of the environmental factors determine the packaging of the sensor, with packaging meaning the encapsulation or insulation which provides protection and isolation and the input/output leads or connections and cabling. The economic factors determine the type of manufacturing and materials used in the sensor and to some extent the quality of the materials (with respect to lifetime). For example, a very expensive sensor may be cost effective if it is used repeatedly or for very long periods of time. On the other hand, a disposable sensor, such as is desired in many medical applications, should be inexpensive. The sensor characteristics of the sensor are usually the specifications of primary concern. The most important parameters are sensitivity, stability, and repeatability. Normally, a sensor is only useful if all three of these parameters are tightly specified for a given range of measurand and time of operation. For example, a highly sensitive device is not useful if its output signal drifts greatly during the measurement time and the data obtained is not reliable if the measurement is not repeatable. Other output characteristics, such as selectivity and linearity, can often be compensated for by using additional, independent sensor input or with signal conditioning circuits. In fact, most sensors have a response to temperature, since most tranducing effects are temperature dependent. Sensors are most often classified by the type of measurand, i.e., physical, chemical, or biological. This is a much simpler means of classification than by transduction mechanism or output signal (e.g., digital or analog), since many sensors use multiple transduction mechanisms and the output signal can always be processed, conditioned, or converted by a circuit so as to cloud the definition of output. A description of each class and examples are given in the following sections. The last section introduces microsensors and gives some examples. FIGURE 56.1 Sensor block diagram. Active sensors require input power to accomplish transduction. Many sensors employ multiple transduction mechanisms in order to produce an electronic output in response to the measurand. TABLE 56.2 Environmental Factors Economic Factors Sensor Characteristics Temperature range Cost Sensitivity Humidity effects Availability Range Corrosion Lifetime Stability Size Repeatability Overrange protection Linearity Susceptibility to EM interferences Error Ruggedness Response time Power consumption Frequency response Self-test capability